#! /usr/bin/env python2

# Copyright 2012-2013 Michael Haggerty <mhagger@alum.mit.edu>
#
# This file is part of git-imerge.
#
# git-imerge is free software: you can redistribute it and/or
# modify it under the terms of the GNU General Public License as
# published by the Free Software Foundation, either version 2 of the
# License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful, but
# WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see
# <http://www.gnu.org/licenses/>.

r"""Git incremental merge

Perform the merge between two branches incrementally.  If conflicts
are encountered, figure out exactly which pairs of commits conflict,
and present the user with one pairwise conflict at a time for
resolution.

git-imerge has two primary design goals:

* Reduce the pain of resolving merge conflicts to its unavoidable
  minimum, by finding and presenting the smallest possible conflicts:
  those between the changes introduced by one commit from each branch.

* Allow a merge to be saved, tested, interrupted, published, and
  collaborated on while it is in progress.

The best thing to read to get started is "git-imerge: A Practical
Introduction" [1].  The theory and benefits of incremental merging are
described in minute detail in a series of blog posts [2], as are the
benefits of retaining history when doing a rebase [3].

Multiple incremental merges can be in progress at the same time.  Each
incremental merge has a name, and its progress is recorded in the Git
repository as references under 'refs/imerge/NAME'.  The current state
of an incremental merge can (crudely) be visualized using the
"diagram" command.

An incremental merge can be interrupted and resumed arbitrarily, or
even pushed to a server to allow somebody else to work on it.

When an incremental merge is finished, you can discard the
intermediate merge commits and create a simpler history to record
permanently in your project repository using either the "finish" or
"simplify" command.  The incremental merge can be simplified in one of
three ways:

  * merge
      keep only a simple merge of the second branch into the first
      branch, discarding all intermediate merges.  The result is
      similar to what you would get from

          git checkout BRANCH1
          git merge BRANCH2

  * rebase
      keep the versions of the commits from the second branch rebased
      onto the first branch.  The result is similar to what you would
      get from

          git checkout BRANCH2
          git rebase BRANCH1

  * rebase-with-history
      like rebase, except that each of the rebased commits is recorded
      as a merge from its original version to its rebased predecessor.
      This is a style of rebasing that does not discard the old
      version of the branch, and allows an already-published branch to
      be rebased.  See [3] for more information.

Simple operation:

For basic operation, you only need to know three git-imerge commands.
To merge BRANCH into MASTER or rebase BRANCH onto MASTER,

    git checkout MASTER
    git-imerge start --name=NAME --goal=GOAL --first-parent BRANCH
    while not done:
        <fix conflict presented to you>
        git commit
        git-imerge continue
    git-imerge finish

where

    NAME is the name for this merge (and also the default name of the
        branch to which the results will be saved)

    GOAL describes how you want to simplify the results:

        "merge" for a simple merge

        "rebase" for a simple rebase

        "rebase-with-history" for a rebase that retains history.  This
        is equivalent to merging the commits from BRANCH into MASTER,
        one commit at a time. In other words, it transforms this::

            o---o---o---o          MASTER
                 \
                  A---B---C---D    BRANCH

        into this::

            o---o---o---o---A'--B'--C'--D'    MASTER
                 \         /   /   /   /
                  --------A---B---C---D       BRANCH

        This is like a rebase, except that it retains the history of
        individual merges.  See [3] for more information.


[1] http://softwareswirl.blogspot.com/2013/05/git-imerge-practical-introduction.html

[2] http://softwareswirl.blogspot.com/2012/12/the-conflict-frontier-of-nightmare-merge.html
    http://softwareswirl.blogspot.com/2012/12/mapping-merge-conflict-frontier.html
    http://softwareswirl.blogspot.com/2012/12/real-world-conflict-diagrams.html
    http://softwareswirl.blogspot.com/2013/05/git-incremental-merge.html
    http://softwareswirl.blogspot.com/2013/05/one-merge-to-rule-them-all.html
    http://softwareswirl.blogspot.com/2013/05/incremental-merge-vs-direct-merge-vs.html

[3] http://softwareswirl.blogspot.com/2009/04/truce-in-merge-vs-rebase-war.html
    http://softwareswirl.blogspot.com/2009/08/upstream-rebase-just-works-if-history.html
    http://softwareswirl.blogspot.com/2009/08/rebase-with-history-implementation.html

"""

import sys
import re
import subprocess
import itertools
import functools
import argparse
from cStringIO import StringIO
import json
import os

# CalledProcessError, check_call, and check_output were not in the
# original Python 2.4 subprocess library, so implement it here if
# necessary (implementations are taken from the Python 2.7 library):
try:
    from subprocess import CalledProcessError
except ImportError:
    class CalledProcessError(Exception):
        def __init__(self, returncode, cmd, output=None):
            self.returncode = returncode
            self.cmd = cmd
            self.output = output
        def __str__(self):
            return "Command '%s' returned non-zero exit status %d" % (self.cmd, self.returncode)

try:
    from subprocess import check_call
except ImportError:
    def check_call(*popenargs, **kwargs):
        retcode = subprocess.call(*popenargs, **kwargs)
        if retcode:
            cmd = kwargs.get("args")
            if cmd is None:
                cmd = popenargs[0]
            raise CalledProcessError(retcode, cmd)
        return 0

try:
    from subprocess import check_output
except ImportError:
    def check_output(*popenargs, **kwargs):
        if 'stdout' in kwargs:
            raise ValueError('stdout argument not allowed, it will be overridden.')
        process = subprocess.Popen(stdout=subprocess.PIPE, *popenargs, **kwargs)
        output, unused_err = process.communicate()
        retcode = process.poll()
        if retcode:
            cmd = kwargs.get("args")
            if cmd is None:
                cmd = popenargs[0]
            # We don't store output in the CalledProcessError because
            # the "output" keyword parameter was not supported in
            # Python 2.6:
            raise CalledProcessError(retcode, cmd)
        return output


STATE_VERSION = (1, 0, 0)

ZEROS = '0' * 40

ALLOWED_GOALS = [
    #'full',
    'rebase-with-history',
    'rebase',
    'merge',
    ]
DEFAULT_GOAL = 'merge'


class Failure(Exception):
    """An exception that indicates a normal failure of the script.

    Failures are reported at top level via sys.exit(str(e)) rather
    than via a Python stack dump."""

    @classmethod
    def wrap(klass, f):
        """Wrap a function inside a try...except that catches this error.

        If the exception is thrown, call sys.exit().  This function
        can be used as a decorator."""

        @functools.wraps(f)
        def wrapper(*args, **kw):
            try:
                return f(*args, **kw)
            except klass, e:
                sys.exit(str(e))

        return wrapper

    pass


class AnsiColor:
    BLACK = '\033[0;30m'
    RED = '\033[0;31m'
    GREEN = '\033[0;32m'
    YELLOW = '\033[0;33m'
    BLUE = '\033[0;34m'
    MAGENTA = '\033[0;35m'
    CYAN = '\033[0;36m'
    B_GRAY = '\033[0;37m'
    D_GRAY = '\033[1;30m'
    B_RED = '\033[1;31m'
    B_GREEN = '\033[1;32m'
    B_YELLOW = '\033[1;33m'
    B_BLUE = '\033[1;34m'
    B_MAGENTA = '\033[1;35m'
    B_CYAN = '\033[1;36m'
    WHITE = '\033[1;37m'
    END = '\033[0m'

    @classmethod
    def disable(cls):
        cls.BLACK = ''
        cls.RED = ''
        cls.GREEN = ''
        cls.YELLOW = ''
        cls.BLUE = ''
        cls.MAGENTA = ''
        cls.CYAN = ''
        cls.B_GRAY = ''
        cls.D_GRAY = ''
        cls.B_RED = ''
        cls.B_GREEN = ''
        cls.B_YELLOW = ''
        cls.B_BLUE = ''
        cls.B_MAGENTA = ''
        cls.B_CYAN = ''
        cls.WHITE = ''
        cls.END = ''


def iter_neighbors(iterable):
    """For an iterable (x0, x1, x2, ...) generate [(x0,x1), (x1,x2), ...]."""

    i = iter(iterable)

    try:
        last = i.next()
    except StopIteration:
        return

    for x in i:
        yield (last, x)
        last = x


def find_first_false(f, lo, hi):
    """Return the smallest i in lo <= i < hi for which f(i) returns False using bisection.

    If there is no such i, return hi.

    """

    # Loop invariant: f(i) returns True for i < lo; f(i) returns False
    # for i >= hi.

    while lo < hi:
        mid = (lo + hi) // 2
        if f(mid):
            lo = mid + 1
        else:
            hi = mid

    return lo


def call_silently(cmd):
    try:
        NULL = open('/dev/null', 'w')
    except IOError:
        NULL = subprocess.PIPE

    p = subprocess.Popen(
        cmd, stdout=NULL, stderr=subprocess.PIPE,
        )
    (out,err) = p.communicate()
    retcode = p.wait()
    if retcode:
        raise CalledProcessError(retcode, cmd)


class UncleanWorkTreeError(Failure):
    pass


def require_clean_work_tree(action):
    """Verify that the current tree is clean.

    The code is a Python translation of the git-sh-setup(1) function
    of the same name."""

    process = subprocess.Popen(
        ['git', 'rev-parse', '--verify', 'HEAD'],
        stdout=subprocess.PIPE, stderr=subprocess.PIPE,
        )
    _unused, err = process.communicate()
    retcode = process.poll()
    if retcode:
        raise UncleanWorkTreeError(err.rstrip())

    process = subprocess.Popen(
        ['git', 'update-index', '-q', '--ignore-submodules', '--refresh'],
        stdout=subprocess.PIPE, stderr=subprocess.PIPE,
        )
    out, err = process.communicate()
    retcode = process.poll()
    if retcode:
        raise UncleanWorkTreeError(err.rstrip() or out.rstrip())

    error = []
    try:
        check_call(['git', 'diff-files', '--quiet', '--ignore-submodules'])
    except CalledProcessError:
        error.append('Cannot %s: You have unstaged changes.' % (action,))

    try:
        check_call([
            'git', 'diff-index', '--cached', '--quiet',
            '--ignore-submodules', 'HEAD', '--',
            ])
    except CalledProcessError:
        if not error:
            error.append('Cannot %s: Your index contains uncommitted changes.' % (action,))
        else:
            error.append('Additionally, your index contains uncommitted changes.')

    if error:
        raise UncleanWorkTreeError('\n'.join(error))


def rev_parse(arg):
    return check_output(['git', 'rev-parse', '--verify', '--quiet', arg]).strip()


def rev_list(*args):
    return [
        l.strip()
        for l in check_output(['git', 'rev-list'] + list(args),).splitlines()
        ]


def get_type(arg):
    """Return the type of a git object ('commit', 'tree', 'blob', or 'tag')."""

    return check_output(['git', 'cat-file', '-t', arg]).strip()


def get_tree(arg):
    return rev_parse('%s^{tree}' % (arg,))


BRANCH_PREFIX = 'refs/heads/'

def checkout(refname):
    if refname.startswith(BRANCH_PREFIX):
        target = refname[len(BRANCH_PREFIX):]
    else:
        target = '%s^0' % (refname,)
    check_call(['git', 'checkout', target])


def get_commit_sha1(arg):
    """Convert arg into a SHA1 and verify that it refers to a commit.

    If not, raise ValueError."""

    try:
        return rev_parse('%s^{commit}' % (arg,))
    except CalledProcessError:
        raise ValueError('%r does not refer to a valid git commit' % (arg,))


def get_commit_parents(commit):
    """Return a list containing the parents of commit."""

    return check_output(
        ['git', 'log', '--no-walk', '--pretty=format:%P', commit]
        ).strip().split()


def get_log_message(commit):
    contents = check_output([
            'git', 'cat-file', 'commit', commit
            ]).splitlines(True)
    contents = contents[contents.index('\n') + 1:]
    if contents and contents[-1][-1:] != '\n':
        contents.append('\n')
    return ''.join(contents)


def get_author_info(commit):
    a = check_output([
        'git', 'log', '-n1',
        '--format=%an%x00%ae%x00%ad', commit
        ]).strip().split('\x00')

    return {
        'GIT_AUTHOR_NAME': a[0],
        'GIT_AUTHOR_EMAIL': a[1],
        'GIT_AUTHOR_DATE': a[2],
        }


def commit_tree(tree, parents, msg, metadata=None):
    """Create a commit containing the specified tree.

    metadata can be author or committer information to be added to the
    environment; e.g., {'GIT_AUTHOR_NAME' : 'me'}.

    Return the SHA-1 of the new commit object."""

    cmd = ['git', 'commit-tree', tree]
    for parent in parents:
        cmd += ['-p', parent]

    if metadata is not None:
        env = os.environ.copy()
        env.update(metadata)
    else:
        env = os.environ

    process = subprocess.Popen(
        cmd, env=env, stdin=subprocess.PIPE, stdout=subprocess.PIPE,
        )
    out, err = process.communicate(msg)
    retcode = process.poll()

    if retcode:
        # We don't store the output in the CalledProcessError because
        # the "output" keyword parameter was not supported in Python
        # 2.6:
        raise CalledProcessError(retcode, cmd)

    return out.strip()


class TemporaryHead(object):
    """A context manager that records the current HEAD state then restores it.

    The message is used for the reflog."""

    def __enter__(self, message='imerge: restoring'):
        self.message = message
        try:
            self.head_name = check_output(['git', 'symbolic-ref', '--quiet', 'HEAD']).strip()
        except CalledProcessError:
            self.head_name = None
        self.orig_head = get_commit_sha1('HEAD')
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        if self.head_name:
            try:
                check_call([
                    'git', 'symbolic-ref',
                    '-m', self.message, 'HEAD',
                    self.head_name,
                    ])
            except Exception, e:
                raise Failure(
                    'Could not restore HEAD to %r!:    %s\n'
                    % (self.head_name, e.message,)
                    )
            else:
                try:
                    check_call(['git', 'reset', '--hard', self.orig_head])
                except Exception, e:
                    raise Failure(
                        'Could not restore HEAD to %r!:    %s\n'
                        % (self.orig_head, e.message,)
                        )
        return False


def reparent(commit, parent_sha1s):
    """Create a new commit object like commit, but with the specified parents.

    commit is the SHA1 of an existing commit and parent_sha1s is a
    list of SHA1s.  Create a new commit exactly like that one, except
    that it has the specified parent commits.  Return the SHA1 of the
    resulting commit object, which is already stored in the object
    database but is not yet referenced by anything."""

    old_commit = check_output(['git', 'cat-file', 'commit', commit])
    separator = old_commit.index('\n\n')
    headers = old_commit[:separator + 1].splitlines(True)
    rest = old_commit[separator + 1:]

    new_commit = StringIO()
    for i in range(len(headers)):
        line = headers[i]
        if line.startswith('tree '):
            new_commit.write(line)
            for parent_sha1 in parent_sha1s:
                new_commit.write('parent %s\n' % (parent_sha1,))
        elif line.startswith('parent '):
            # Discard old parents:
            pass
        else:
            new_commit.write(line)

    new_commit.write(rest)

    process = subprocess.Popen(
        ['git', 'hash-object', '-t', 'commit', '-w', '--stdin'],
        stdin=subprocess.PIPE, stdout=subprocess.PIPE,
        )
    out, err = process.communicate(new_commit.getvalue())
    retcode = process.poll()
    if retcode:
        raise Failure('Could not reparent commit %s' % (commit,))
    return out.strip()


class AutomaticMergeFailed(Exception):
    def __init__(self, commit1, commit2):
        Exception.__init__(
            self, 'Automatic merge of %s and %s failed' % (commit1, commit2,)
            )
        self.commit1, self.commit2 = commit1, commit2


def automerge(commit1, commit2):
    """Attempt an automatic merge of commit1 and commit2.

    Return the SHA1 of the resulting commit, or raise
    AutomaticMergeFailed on error.  This must be called with a clean
    worktree."""

    call_silently(['git', 'checkout', '-f', commit1])
    try:
        call_silently(['git', '-c', 'rerere.enabled=false', 'merge', commit2])
    except CalledProcessError:
        # We don't use "git merge --abort" here because it was only
        # added in git version 1.7.4.
        call_silently(['git', 'reset', '--merge'])
        raise AutomaticMergeFailed(commit1, commit2)
    else:
        return get_commit_sha1('HEAD')


class MergeRecord(object):
    # Bits for the flags field:

    # There is a saved successful auto merge:
    SAVED_AUTO = 0x01

    # An auto merge (which may have been unsuccessful) has been done:
    NEW_AUTO = 0x02

    # There is a saved successful manual merge:
    SAVED_MANUAL = 0x04

    # A manual merge (which may have been unsuccessful) has been done:
    NEW_MANUAL = 0x08

    # A merge that is currently blocking the merge frontier:
    BLOCKED = 0x10

    # Some useful bit combinations:
    SAVED = SAVED_AUTO | SAVED_MANUAL
    NEW = NEW_AUTO | NEW_MANUAL

    AUTO = SAVED_AUTO | NEW_AUTO
    MANUAL = SAVED_MANUAL | NEW_MANUAL

    ALLOWED_INITIAL_FLAGS = [
        SAVED_AUTO,
        SAVED_MANUAL,
        NEW_AUTO,
        NEW_MANUAL,
        ]

    def __init__(self, sha1=None, flags=0):
        # The currently believed correct merge, or None if it is
        # unknown or the best attempt was unsuccessful.
        self.sha1 = sha1

        if self.sha1 is None:
            if flags != 0:
                raise ValueError('Initial flags (%s) for sha1=None should be 0' % (flags,))
        elif flags not in self.ALLOWED_INITIAL_FLAGS:
            raise ValueError('Initial flags (%s) is invalid' % (flags,))

        # See bits above.
        self.flags = flags

    def record_merge(self, sha1, source):
        """Record a merge at this position.

        source must be SAVED_AUTO, SAVED_MANUAL, NEW_AUTO, or NEW_MANUAL."""

        if source == self.SAVED_AUTO:
            # SAVED_AUTO is recorded in any case, but only used if it
            # is the only info available.
            if self.flags & (self.MANUAL | self.NEW) == 0:
                self.sha1 = sha1
            self.flags |= source
        elif source == self.NEW_AUTO:
            # NEW_AUTO is silently ignored if any MANUAL value is
            # already recorded.
            if self.flags & self.MANUAL == 0:
                self.sha1 = sha1
                self.flags |= source
        elif source == self.SAVED_MANUAL:
            # SAVED_MANUAL is recorded in any case, but only used if
            # no NEW_MANUAL is available.
            if self.flags & self.NEW_MANUAL == 0:
                self.sha1 = sha1
            self.flags |= source
        elif source == self.NEW_MANUAL:
            # NEW_MANUAL is always used, and also causes NEW_AUTO to
            # be forgotten if present.
            self.sha1 = sha1
            self.flags = (self.flags | source) & ~self.NEW_AUTO
        else:
            raise ValueError('Undefined source: %s' % (source,))

    def record_blocked(self, blocked):
        if blocked:
            self.flags |= self.BLOCKED
        else:
            self.flags &= ~self.BLOCKED

    def is_known(self):
        return self.sha1 is not None

    def is_blocked(self):
        return self.flags & self.BLOCKED != 0

    def is_manual(self):
        return self.flags & self.MANUAL != 0

    def save(self, name, i1, i2):
        """If this record has changed, save it."""

        def set_ref(source):
            check_call([
                    'git', 'update-ref',
                    '-m', 'imerge %r: Record %s merge' % (name, source,),
                    'refs/imerge/%s/%s/%d-%d' % (name, source, i1, i2),
                    self.sha1,
                    ])

        def clear_ref(source):
            check_call([
                    'git', 'update-ref',
                    '-d', 'imerge %r: Remove obsolete %s merge' % (name, source,),
                    'refs/imerge/%s/%s/%d-%d' % (name, source, i1, i2),
                    ])

        if self.flags & self.MANUAL:
            if self.flags & self.AUTO:
                # Any MANUAL obsoletes any AUTO:
                if self.flags & self.SAVED_AUTO:
                    clear_ref('auto')

                self.flags &= ~self.AUTO

            if self.flags & self.NEW_MANUAL:
                # Convert NEW_MANUAL to SAVED_MANUAL.
                if self.sha1:
                    set_ref('manual')
                    self.flags |= self.SAVED_MANUAL
                elif self.flags & self.SAVED_MANUAL:
                    # Delete any existing SAVED_MANUAL:
                    clear_ref('manual')
                    self.flags &= ~self.SAVED_MANUAL
                self.flags &= ~self.NEW_MANUAL

        elif self.flags & self.NEW_AUTO:
            # Convert NEW_AUTO to SAVED_AUTO.
            if self.sha1:
                set_ref('auto')
                self.flags |= self.SAVED_AUTO
            elif self.flags & self.SAVED_AUTO:
                # Delete any existing SAVED_AUTO:
                clear_ref('auto')
                self.flags &= ~self.SAVED_AUTO
            self.flags &= ~self.NEW_AUTO


class UnexpectedMergeFailure(Exception):
    def __init__(self, msg, i1, i2):
        Exception.__init__(self, msg)
        self.i1, self.i2 = i1, i2


class BlockCompleteError(Exception):
    pass


class FrontierBlockedError(Exception):
    def __init__(self, msg, i1, i2):
        Exception.__init__(self, msg)
        self.i1 = i1
        self.i2 = i2


class NotABlockingCommitError(Exception):
    pass


class MergeFrontier(object):
    """Represents the merge frontier within a Block.

    A MergeFrontier is represented by a list of SubBlocks, each of
    which is thought to be completely mergeable.  The list is kept in
    normalized form:

    * Only non-empty blocks are retained

    * Blocks are sorted from bottom left to upper right

    * No redundant blocks

    """

    @staticmethod
    def map_known_frontier(block):
        """Return the MergeFrontier describing existing successful merges in block.

        The return value only includes the part that is fully outlined
        and whose outline consists of rectangles reaching back to
        (0,0).

        A blocked commit is *not* considered to be within the
        frontier, even if a merge is registered for it.  Such merges
        must be explicitly unblocked."""

        # FIXME: This algorithm can take combinatorial time, for
        # example if there is a big block of merges that is a dead
        # end:
        #
        #     +++++++
        #     +?+++++
        #     +?+++++
        #     +?+++++
        #     +?*++++
        #
        # The problem is that the algorithm will explore all of the
        # ways of getting to commit *, and the number of paths grows
        # like a binomial coefficient.  The solution would be to
        # remember dead-ends and reject any curves that visit a point
        # to the right of a dead-end.
        #
        # For now we don't intend to allow a situation like this to be
        # created, so we ignore the problem.

        # A list (i1, i2, down) of points in the path so far.  down is
        # True iff the attempted step following this one was
        # downwards.
        path = []

        def create_frontier(path):
            blocks = []
            for ((i1old, i2old, downold), (i1new, i2new, downnew)) in iter_neighbors(path):
                if downold == True and downnew == False:
                    blocks.append(block[:i1new + 1, :i2new + 1])
            return MergeFrontier(block, blocks)

        # Loop invariants:
        #
        # * path is a valid path
        #
        # * (i1, i2) is in block but it not yet added to path
        #
        # * down is True if a step downwards from (i1, i2) has not yet
        #   been attempted
        (i1, i2) = (block.len1 - 1, 0)
        down = True
        while True:
            if down:
                if i2 == block.len2 - 1:
                    # Hit edge of block; can't move down:
                    down = False
                elif (i1, i2 + 1) in block and not block.is_blocked(i1, i2 + 1):
                    # Can move down
                    path.append((i1, i2, True))
                    i2 += 1
                else:
                    # Can't move down.
                    down = False
            else:
                if i1 == 0:
                    # Success!
                    path.append((i1, i2, False))
                    return create_frontier(path)
                elif (i1 - 1, i2) in block and not block.is_blocked(i1 - 1, i2):
                    # Can move left
                    path.append((i1, i2, False))
                    down = True
                    i1 -= 1
                else:
                    # There's no way to go forward; backtrack until we
                    # find a place where we can still try going left:
                    while True:
                        try:
                            (i1, i2, down) = path.pop()
                        except IndexError:
                            # This shouldn't happen because, in the
                            # worst case, there is a valid path across
                            # the top edge of the merge diagram.
                            raise RuntimeError('Block is improperly formed!')
                        if down:
                            down = False
                            break

    @staticmethod
    def compute_by_bisection(block):
        """Return a MergeFrontier instance for block.

        We make the following assumptions (using Python subscript
        notation):

        0. All of the merges in block[1:,0] and block[0,1:] are
           already known.  (This is an invariant of the Block class.)

        1. If a direct merge can be done between block[i1-1,0] and
           block[0,i2-1], then all of the pairwise merges in
           block[1:i1, 1:i2] can also be done.

        2. If a direct merge fails between block[i1-1,0] and
           block[0,i2-1], then all of the pairwise merges in
           block[i1-1:,i2-1:] would also fail.

        Under these assumptions, the merge frontier is a stepstair
        pattern going from the bottom-left to the top-right, and
        bisection can be used to find the transition between mergeable
        and conflicting in any row or column.

        Of course these assumptions are not rigorously true, so the
        MergeFrontier returned by this function is only an
        approximation of the real merge diagram.  We check for and
        correct such inconsistencies later."""

        # Given that these diagrams typically have few blocks, check
        # the end of a range first to see if the whole range can be
        # determined, and fall back to bisection otherwise.  We
        # determine the frontier block by block, starting in the lower
        # left.

        if block.len1 <= 1 or block.len2 <= 1 or block.is_blocked(1, 1):
            return MergeFrontier(block, [])

        if not block.is_mergeable(1, 1):
            # There are no mergeable blocks in block; therefore,
            # block[1,1] must itself be unmergeable.  Record that
            # fact:
            block[1,1].record_blocked(True)
            return MergeFrontier(block, [])

        blocks = []

        # At this point, we know that there is at least one mergeable
        # commit in the first column.  Find the height of the success
        # block in column 1:
        i1 = 1
        i2 = find_first_false(
            lambda i: block.is_mergeable(i1, i),
            1, block.len2,
            )

        # Now we know that (1,i2-1) is mergeable but (1,i2) is not;
        # e.g., (i1, i2) is like 'A' (or maybe 'B') in the following
        # diagram (where '*' means mergeable, 'x' means not mergeable,
        # and '?' means indeterminate) and that the merge under 'A' is
        # not mergeable:
        #
        #          i1
        #
        #        0123456
        #      0 *******
        #      1 **?????
        #  i2  2 **?????
        #      3 **?????
        #      4 *Axxxxx
        #      5 *xxxxxx
        #         B

        while True:
            if i2 == 1:
                break

            # At this point in the loop, we know that any blocks to
            # the left of 'A' have already been recorded, (i1, i2-1)
            # is mergeable but (i1, i2) is not; e.g., we are at a
            # place like 'A' in the following diagram:
            #
            #          i1
            #
            #        0123456
            #      0 **|****
            #      1 **|*???
            #  i2  2 **|*???
            #      3 **|Axxx
            #      4 --+xxxx
            #      5 *xxxxxx
            #
            # This implies that (i1, i2) is the first unmergeable
            # commit in a blocker block (though blocker blocks are not
            # recorded explicitly).  It also implies that a mergeable
            # block has its last mergeable commit somewhere in row
            # i2-1; find its width.
            if block.is_mergeable(block.len1 - 1, i2 - 1):
                i1 = block.len1
                blocks.append(block[:i1,:i2])
                break
            else:
                i1 = find_first_false(
                    lambda i: block.is_mergeable(i, i2 - 1),
                    i1 + 1, block.len1 - 1,
                    )
                blocks.append(block[:i1,:i2])

            # At this point in the loop, (i1-1, i2-1) is mergeable but
            # (i1, i2-1) is not; e.g., 'A' in the following diagram:
            #
            #          i1
            #
            #        0123456
            #      0 **|*|**
            #      1 **|*|??
            #  i2  2 --+-+xx
            #      3 **|xxAx
            #      4 --+xxxx
            #      5 *xxxxxx
            #
            # The block ending at (i1-1,i2-1) has just been recorded.
            # Now find the height of the conflict rectangle at column
            # i1 and fill it in:
            if block.is_mergeable(i1, 1):
                i2 = find_first_false(
                    lambda i: block.is_mergeable(i1, i),
                    2, i2 - 1,
                    )
            else:
                break

        return MergeFrontier(block, blocks)

    def __init__(self, block, blocks=None):
        self.block = block
        blocks = list(self._iter_non_empty_blocks(blocks or []))
        blocks.sort(key=lambda block: block.len1)
        self.blocks = list(self._iter_non_redundant_blocks(blocks))

    def __iter__(self):
        """Iterate over blocks from bottom left to upper right."""

        return iter(self.blocks)

    def __nonzero__(self):
        """Return True iff this frontier has no completed parts."""

        return bool(self.blocks)

    def is_complete(self):
        """Return True iff the frontier covers the whole block."""

        return (
            len(self.blocks) == 1
            and self.blocks[0].len1 == self.block.len1
            and self.blocks[0].len2 == self.block.len2
            )

    # Additional codes used in the 2D array returned from create_diagram()
    FRONTIER_WITHIN = 0x10
    FRONTIER_RIGHT_EDGE = 0x20
    FRONTIER_BOTTOM_EDGE = 0x40
    FRONTIER_MASK = 0x70

    @classmethod
    def default_formatter(cls, node, legend=None):
        def color(node, within):
            if within:
                return AnsiColor.B_GREEN + node + AnsiColor.END
            else:
                return AnsiColor.B_RED + node + AnsiColor.END

        if legend is None:
            legend = ["?", "*", ".", "#", "@", "-", "|", "+"]
        merge = node & ~cls.FRONTIER_MASK
        within = merge == Block.MERGE_MANUAL or (node & cls.FRONTIER_WITHIN)
        skip = [Block.MERGE_MANUAL, Block.MERGE_BLOCKED, Block.MERGE_UNBLOCKED]
        if merge not in skip:
            vertex = (cls.FRONTIER_BOTTOM_EDGE | cls.FRONTIER_RIGHT_EDGE)
            edge_status = node & vertex
            if edge_status == vertex:
                return color(legend[-1], within)
            elif edge_status == cls.FRONTIER_RIGHT_EDGE:
                return color(legend[-2], within)
            elif edge_status == cls.FRONTIER_BOTTOM_EDGE:
                return color(legend[-3], within)
        return color(legend[merge], within)

    def create_diagram(self):
        """Generate a diagram of this frontier.

        The returned diagram is a nested list of integers forming a 2D array,
        representing the merge frontier embedded in the diagram of commits
        returned from Block.create_diagram().

        At each node in the returned diagram is an integer whose value is a
        bitwise-or of existing MERGE_* constant from Block.create_diagram()
        and zero or more of the FRONTIER_* constants defined in this class."""

        diagram = self.block.create_diagram()
        for block in self:
            for i1 in range(block.len1):
                for i2 in range(block.len2):
                    v = self.FRONTIER_WITHIN
                    if i1 == block.len1 - 1:
                        v |= self.FRONTIER_RIGHT_EDGE
                    if i2 == block.len2 - 1:
                        v |= self.FRONTIER_BOTTOM_EDGE
                    diagram[i1][i2] |= v
        return diagram

    def format_diagram(self, formatter=None, diagram=None):
        if formatter is None:
            formatter = self.default_formatter
        if diagram is None:
            diagram = self.create_diagram()
        return [
            [formatter(diagram[i1][i2]) for i2 in range(self.block.len2)]
            for i1 in range(self.block.len1)]

    def write(self, f):
        """Write this frontier to file-like object f."""
        diagram = self.format_diagram()
        for i2 in range(self.block.len2):
            for i1 in range(self.block.len1):
                f.write(diagram[i1][i2])
            f.write('\n')

    def write_html(self, f, name, cssfile="imerge.css", abbrev_sha1=7):
        class_map = {
            Block.MERGE_UNKNOWN: "merge_unknown",
            Block.MERGE_MANUAL: "merge_manual",
            Block.MERGE_AUTOMATIC: "merge_automatic",
            Block.MERGE_BLOCKED: "merge_blocked",
            Block.MERGE_UNBLOCKED: "merge_unblocked",
            self.FRONTIER_WITHIN: "frontier_within",
            self.FRONTIER_RIGHT_EDGE: "frontier_right_edge",
            self.FRONTIER_BOTTOM_EDGE: "frontier_bottom_edge",
            }

        def map_to_classes(node):
            merge = node & ~self.FRONTIER_MASK
            ret = [class_map[merge]]
            for bit in [self.FRONTIER_WITHIN, self.FRONTIER_RIGHT_EDGE,
                        self.FRONTIER_BOTTOM_EDGE]:
                if node & bit:
                    ret.append(class_map[bit])
            return ret

        f.write("""\
<html>
<head>
<title>git-imerge: %s</title>
<link rel="stylesheet" href="%s" type="text/css" />
</head>
<body>
<table id="imerge">
""" % (name, cssfile))

        diagram = self.create_diagram()
        for i2 in range(self.block.len2):
            f.write("  <tr>\n")
            for i1 in range(self.block.len1):
                classes = map_to_classes(diagram[i1][i2])
                record = self.block.get_value(i1, i2)
                sha1 = record.sha1 or ""
                td_id = record.sha1 and ' id="%s"' % (record.sha1) or ''
                td_class = classes and ' class="' + " ".join(classes) + '"' or ''
                f.write("    <td%s%s>%.*s</td>\n" % (
                        td_id, td_class, abbrev_sha1, sha1))
            f.write("  </tr>\n")
        f.write("</table>\n</body>\n</html>\n")

    @staticmethod
    def _iter_non_empty_blocks(blocks):
        for block in blocks:
            if block.len1 > 1 and block.len2 > 1:
                yield block

    @staticmethod
    def _iter_non_redundant_blocks(blocks):
        def contains(block1, block2):
            """Return true if block1 contains block2."""

            return block1.len1 >= block2.len1 and block1.len2 >= block2.len2

        i = iter(blocks)
        try:
            last = i.next()
        except StopIteration:
            return

        for block in i:
            if contains(last, block):
                pass
            elif contains(block, last):
                last = block
            else:
                yield last
                last = block

        yield last

    def remove_failure(self, i1, i2):
        """Refine the merge frontier given that the specified merge fails."""

        newblocks = []
        shrunk_block = False

        for block in self.blocks:
            if i1 < block.len1 and i2 < block.len2:
                if i1 > 1:
                    newblocks.append(block[:i1, :])
                if i2 > 1:
                    newblocks.append(block[:, :i2])
                shrunk_block = True
            else:
                newblocks.append(block)

        if shrunk_block:
            self.blocks = list(self._iter_non_redundant_blocks(newblocks))

    def partition(self, block):
        """Return two MergeFrontier instances partitioned by block.

        Return (frontier1, frontier2), where each frontier is limited
        to each side of the argument.

        block must be contained in this MergeFrontier and already be
        outlined."""

        # Remember that the new blocks have to include the outlined
        # edge of the partitioning block to satisfy the invariant that
        # the left and upper edge of a block has to be known.

        left = []
        right = []
        for b in self.blocks:
            if b.len1 == block.len1 and b.len2 == block.len2:
                # That's the block we're partitioning on; just skip it.
                pass
            elif b.len1 < block.len1 and b.len2 > block.len2:
                left.append(b[:, block.len2 - 1:])
            elif b.len1 > block.len1 and b.len2 < block.len2:
                right.append(b[block.len1 - 1:, :])
            else:
                raise ValueError(
                    'MergeFrontier partitioned with inappropriate block'
                    )
        return (
            MergeFrontier(self.block[:block.len1, block.len2 - 1:], left),
            MergeFrontier(self.block[block.len1 - 1:, :block.len2], right),
            )

    def iter_blocker_blocks(self):
        """Iterate over the blocks on the far side of this frontier.

        This must only be called for an outlined frontier."""

        if not self:
            yield self.block
            return

        blockruns = []
        if self.blocks[0].len2 < self.block.len2:
            blockruns.append([self.block[0,:]])
        blockruns.append(self)
        if self.blocks[-1].len1 < self.block.len1:
            blockruns.append([self.block[:,0]])

        for block1, block2 in iter_neighbors(itertools.chain(*blockruns)):
            yield self.block[block1.len1 - 1:block2.len1, block2.len2 - 1: block1.len2]

    def get_affected_blocker_block(self, i1, i2):
        """Return the blocker block that a successful merge (i1,i2) would unblock.

        If there is no such block, raise NotABlockingCommitError."""

        for block in self.iter_blocker_blocks():
            try:
                (block_i1, block_i2) = block.convert_original_indexes(i1, i2)
            except IndexError:
                pass
            else:
                if (block_i1, block_i2) == (1,1):
                    # That's the one we need to improve this block:
                    return block
                else:
                    # An index pair can only be in a single blocker
                    # block, which we've already found:
                    raise NotABlockingCommitError(
                        'Commit %d-%d was not blocking the frontier.'
                        % self.block.get_original_indexes(i1, i2)
                        )
        else:
            raise NotABlockingCommitError(
                'Commit %d-%d was not on the frontier.'
                % self.block.get_original_indexes(i1, i2)
                )

    def auto_expand(self):
        """Try pushing out one of the blocks on this frontier.

        Raise BlockCompleteError if the whole block has already been
        solved.  Raise FrontierBlockedError if the frontier is blocked
        everywhere.  This method does *not* update self; if it returns
        successfully you should recompute the frontier from
        scratch."""

        blocks = list(self.iter_blocker_blocks())
        if not blocks:
            raise BlockCompleteError('The block is already complete')
        # Try blocks from biggest to smallest:
        blocks.sort(key=lambda block: block.get_area(), reverse=True)
        for block in blocks:
            if block.auto_outline_frontier():
                return
        else:
            # None of the blocks could be expanded.  Suggest that the
            # caller do a manual merge of the commit that is blocking
            # the *smallest* blocker block.
            i1, i2 = blocks[-1].get_original_indexes(1, 1)
            raise FrontierBlockedError(
                'Frontier blocked; suggest manual merge of %d-%d' % (i1, i2),
                i1, i2
                )


class NoManualMergeError(Exception):
    pass


class ManualMergeUnusableError(Exception):
    def __init__(self, msg, commit):
        Exception.__init__(self, 'Commit %s is not usable; %s' % (commit, msg))
        self.commit = commit


class CommitNotFoundError(Exception):
    def __init__(self, commit):
        Exception.__init__(
            self,
            'Commit %s was not found among the known merge commits' % (commit,),
            )
        self.commit = commit


class Block(object):
    """A rectangular range of commits, indexed by (i1,i2).

    The commits block[0,1:] and block[1:,0] are always all known.
    block[0,0] may or may not be known; it is usually unneeded (except
    maybe implicitly).

    Members:

        len1, len2 -- the dimensions of the block.

    """

    def __init__(self, len1, len2):
        self.len1 = len1
        self.len2 = len2

    def get_area(self):
        """Return the area of this block, ignoring the known edges."""

        return (self.len1 - 1) * (self.len2 - 1)

    def _check_indexes(self, i1, i2):
        if not (0 <= i1 < self.len1):
            raise IndexError('first index (%s) is out of range 0:%d' % (i1, self.len1,))
        if not (0 <= i2 < self.len2):
            raise IndexError('second index (%s) is out of range 0:%d' % (i2, self.len2,))

    def _normalize_indexes(self, index):
        """Return a pair of non-negative integers (i1, i2)."""

        try:
            (i1, i2) = index
        except TypeError:
            raise IndexError('Block indexing requires exactly two indexes')

        if i1 < 0:
            i1 += self.len1
        if i2 < 0:
            i2 += self.len2

        self._check_indexes(i1, i2)
        return (i1, i2)

    def get_original_indexes(self, i1, i2):
        """Return the original indexes corresponding to (i1,i2) in this block.

        This function supports negative indexes."""

        return self._normalize_indexes((i1, i2))

    def convert_original_indexes(self, i1, i2):
        """Return the indexes in this block corresponding to original indexes (i1,i2).

        raise IndexError if they are not within this block.  This
        method does not support negative indices."""

        return (i1, i2)

    def _set_value(self, i1, i2, value):
        """Set the MergeRecord for integer indexes (i1, i2).

        i1 and i2 must be non-negative."""

        raise NotImplementedError()

    def get_value(self, i1, i2):
        """Return the MergeRecord for integer indexes (i1, i2).

        i1 and i2 must be non-negative."""

        raise NotImplementedError()

    def __getitem__(self, index):
        """Return the MergeRecord at (i1, i2) (requires two indexes).

        If i1 and i2 are integers but the merge is unknown, return
        None.  If either index is a slice, return a SubBlock."""

        try:
            (i1, i2) = index
        except TypeError:
            raise IndexError('Block indexing requires exactly two indexes')
        if isinstance(i1, slice) or isinstance(i2, slice):
            return SubBlock(self, i1, i2)
        else:
            return self.get_value(*self._normalize_indexes((i1, i2)))

    def __contains__(self, index):
        return self[index].is_known()

    def is_blocked(self, i1, i2):
        """Return True iff the specified commit is blocked."""

        (i1, i2) = self._normalize_indexes((i1, i2))
        return self[i1, i2].is_blocked()

    def is_mergeable(self, i1, i2):
        """Determine whether (i1,i2) can be merged automatically.

        If we already have a merge record for (i1,i2), return True.
        Otherwise, attempt a merge (discarding the result)."""

        (i1, i2) = self._normalize_indexes((i1, i2))
        if (i1, i2) in self:
            return True
        else:
            sys.stderr.write(
                'Attempting automerge of %d-%d...' % self.get_original_indexes(i1, i2)
                )
            try:
                automerge(self[i1, 0].sha1, self[0, i2].sha1)
                sys.stderr.write('success.\n')
                return True
            except AutomaticMergeFailed:
                sys.stderr.write('failure.\n')
                return False

    def auto_outline(self):
        """Complete the outline of this Block.

        raise UnexpectedMergeFailure if automerging fails."""

        # Check that all of the merges go through before recording any
        # of them permanently.
        merges = []

        def do_merge(i1, commit1, i2, commit2, msg='Autofilling %d-%d...', record=True):
            if (i1, i2) in self:
                return self[i1,i2].sha1
            try:
                sys.stderr.write(msg % self.get_original_indexes(i1, i2))
                merge = automerge(commit1, commit2)
                sys.stderr.write('success.\n')
            except AutomaticMergeFailed, e:
                sys.stderr.write('unexpected conflict.  Backtracking...\n')
                raise UnexpectedMergeFailure(str(e), i1, i2)
            if record:
                merges.append((i1, i2, merge))
            return merge

        i2 = self.len2 - 1
        left = self[0, i2].sha1
        for i1 in range(1, self.len1 - 1):
            left = do_merge(i1, self[i1,0].sha1, i2, left)

        i1 = self.len1 - 1
        above = self[i1, 0].sha1
        for i2 in range(1, self.len2 - 1):
            above = do_merge(i1, above, i2, self[0,i2].sha1)

        # We will compare two ways of doing the final "vertex" merge:
        # as a continuation of the bottom edge, or as a continuation
        # of the right edge.  We only accept it if both approaches
        # succeed and give identical trees.
        i1, i2 = self.len1 - 1, self.len2 - 1
        vertex_v1 = do_merge(
            i1, self[i1,0].sha1, i2, left,
            msg='Autofilling %d-%d (first way)...',
            record=False,
            )
        vertex_v2 = do_merge(
            i1, above, i2, self[0,i2].sha1,
            msg='Autofilling %d-%d (second way)...',
            record=False,
            )
        if get_tree(vertex_v1) == get_tree(vertex_v2):
            sys.stderr.write(
                'The two ways of autofilling %d-%d agree.\n'
                % self.get_original_indexes(i1, i2)
                )
        else:
            sys.stderr.write(
                'The two ways of autofilling %d-%d do not agree.  Backtracking...\n'
                % self.get_original_indexes(i1, i2)
                )
            raise UnexpectedMergeFailure('Inconsistent vertex merges', i1, i2)

        # Everything is OK.  Now reparent the actual vertex merge to
        # have above and left as its parents:
        merges.append((i1, i2, reparent(vertex_v1, [above, left])))

        # Done!  Now we can record the results:
        sys.stderr.write('Recording autofilled block %s.\n' % (self,))
        for (i1, i2, merge) in merges:
            self[i1, i2].record_merge(merge, MergeRecord.NEW_AUTO)

    def auto_outline_frontier(self, merge_frontier=None):
        """Try to outline the merge frontier of this block.

        Return True iff some progress was made."""

        if merge_frontier is None:
            merge_frontier = MergeFrontier.compute_by_bisection(self)

        if not merge_frontier:
            # Nothing to do.
            return False

        best_block = max(merge_frontier, key=lambda block: block.get_area())

        try:
            best_block.auto_outline()
        except UnexpectedMergeFailure, e:
            # One of the merges that we expected to succeed in
            # fact failed.
            merge_frontier.remove_failure(e.i1, e.i2)
            return self.auto_outline_frontier(merge_frontier)
        else:
            f1, f2 = merge_frontier.partition(best_block)
            if f1:
                f1.block.auto_outline_frontier(f1)
            if f2:
                f2.block.auto_outline_frontier(f2)
            return True

    # The codes in the 2D array returned from create_diagram()
    MERGE_UNKNOWN = 0
    MERGE_MANUAL = 1
    MERGE_AUTOMATIC = 2
    MERGE_BLOCKED = 3
    MERGE_UNBLOCKED = 4

    # A map {(is_known(), manual, is_blocked()) : integer constant}
    MergeState = {
        (False, False, False) : MERGE_UNKNOWN,
        (False, False, True) : MERGE_BLOCKED,
        (True, False, True) : MERGE_UNBLOCKED,
        (True, True, True) : MERGE_UNBLOCKED,
        (True, False, False) : MERGE_AUTOMATIC,
        (True, True, False) : MERGE_MANUAL,
        }

    def create_diagram(self):
        """Generate a diagram of this Block.

        The returned diagram, is a nested list of integers forming a 2D array,
        where the integer at diagram[i1][i2] is one of MERGE_UNKNOWN,
        MERGE_MANUAL, MERGE_AUTOMATIC, MERGE_BLOCKED, or MERGE_UNBLOCKED,
        representing the state of the commit at (i1, i2)."""

        diagram = [[None for i2 in range(self.len2)] for i1 in range(self.len1)]

        for i2 in range(self.len2):
            for i1 in range(self.len1):
                rec = self.get_value(i1, i2)
                c = self.MergeState[
                    rec.is_known(), rec.is_manual(), rec.is_blocked()]
                diagram[i1][i2] = c

        return diagram

    def format_diagram(self, legend=None, diagram=None):
        if legend is None:
            legend = [
                AnsiColor.D_GRAY + "?" + AnsiColor.END,
                AnsiColor.B_GREEN + "*" + AnsiColor.END,
                AnsiColor.B_GREEN + "." + AnsiColor.END,
                AnsiColor.B_RED + "#" + AnsiColor.END,
                AnsiColor.B_YELLOW + "@" + AnsiColor.END,
                ]
        if diagram is None:
            diagram = self.create_diagram()
        return [
            [legend[diagram[i1][i2]] for i2 in range(self.len2)]
            for i1 in range(self.len1)]

    def write(self, f, legend=None, sep='', linesep='\n'):
        diagram = self.format_diagram(legend)
        for i2 in range(self.len2):
            f.write(sep.join(diagram[i1][i2] for i1 in range(self.len1)) + linesep)

    def writeppm(self, f):
        f.write('P3\n')
        f.write('%d %d 255\n' % (self.len1, self.len2,))
        legend = ['127 127 0', '0 255 0', '0 127 0', '255 0 0', '127 0 0']
        self.write(f, legend, sep='  ')


class SubBlock(Block):
    @staticmethod
    def _convert_to_slice(i, len):
        """Return (start, len) for the specified index.

        i may be an integer or a slice with step equal to 1."""

        if isinstance(i, int):
            if i < 0:
                i += len
            i = slice(i, i + 1)
        elif isinstance(i, slice):
            if i.step is not None and i.step != 1:
                raise ValueError('Index has a non-zero step size')
        else:
            raise ValueError('Index cannot be converted to a slice')

        (start, stop, step) = i.indices(len)
        return (start, stop - start)

    def __init__(self, block, slice1, slice2):
        (start1, len1) = self._convert_to_slice(slice1, block.len1)
        (start2, len2) = self._convert_to_slice(slice2, block.len2)
        Block.__init__(self, len1, len2)
        if isinstance(block, SubBlock):
            # Peel away one level of indirection:
            self._block = block._block
            self._start1 = start1 + block._start1
            self._start2 = start2 + block._start2
        else:
            self._block = block
            self._start1 = start1
            self._start2 = start2

    def get_original_indexes(self, i1, i2):
        i1, i2 = self._normalize_indexes((i1, i2))
        return self._block.get_original_indexes(i1 + self._start1, i2 + self._start2)

    def convert_original_indexes(self, i1, i2):
        (i1, i2) = self._block.convert_original_indexes(i1, i2)
        if not (
            self._start1 <= i1 < self._start1 + self.len1
            and self._start2 <= i2 < self._start2 + self.len2
            ):
            raise IndexError('Indexes are not within block')
        return (i1 - self._start1, i2 - self._start2)

    def _set_value(self, i1, i2, sha1, flags):
        self._check_indexes(i1, i2)
        self._block._set_value(i1 + self._start1, i2 + self._start2, sha1, flags)

    def get_value(self, i1, i2):
        self._check_indexes(i1, i2)
        return self._block.get_value(i1 + self._start1, i2 + self._start2)

    def __str__(self):
        return '%s[%d:%d,%d:%d]' % (
            self._block,
            self._start1, self._start1 + self.len1,
            self._start2, self._start2 + self.len2,
            )


class MergeState(Block):
    SOURCE_TABLE = {
        'auto' : MergeRecord.SAVED_AUTO,
        'manual' : MergeRecord.SAVED_MANUAL,
        }

    MERGE_STATE_RE = re.compile(
        r"""
        ^
        refs\/imerge\/
        (?P<name>[^\/]+)
        \/state
        $
        """,
        re.VERBOSE,
        )

    @staticmethod
    def iter_existing_names():
        """Iterate over the names of existing MergeStates in this repo."""

        for line in check_output(['git', 'for-each-ref', 'refs/imerge',]).splitlines():
            (sha1, type, refname) = line.split()
            if type == 'blob':
                m = MergeState.MERGE_STATE_RE.match(refname)
                if m:
                    yield m.group('name')

    @staticmethod
    def get_scratch_refname(name):
        return 'refs/heads/imerge/%s' % (name,)

    @staticmethod
    def _read_state(name, sha1):
        state_string = check_output(['git', 'cat-file', 'blob', sha1])
        state = json.loads(state_string)

        version = tuple(map(int, state['version'].split('.')))
        if version[0] != STATE_VERSION[0] or version > STATE_VERSION:
            raise Failure(
                'The format of imerge %s (%s) is not compatible with this script version.'
                % (name, state['version'],)
                )
        state['version'] = version
        return state

    @staticmethod
    def check_exists(name):
        """Verify that a MergeState with the given name exists.

        Just check for the existence, readability, and compatible
        version of the "state" reference.  If the reference doesn't
        exist, just return False.  If it exists but is unusable for
        some other reason, raise an exception."""

        try:
            call_silently(
                ['git', 'check-ref-format', 'refs/imerge/%s' % (name,)]
                )
        except CalledProcessError:
            raise Failure('Name %r is not a valid refname component!' % (name,))

        state_refname = 'refs/imerge/%s/state' % (name,)
        for line in check_output(['git', 'for-each-ref', state_refname]).splitlines():
            (sha1, type, refname) = line.split()
            if refname == state_refname and type == 'blob':
                MergeState._read_state(name, sha1)
                # If that didn't throw an exception:
                return True
        else:
            return False

    @staticmethod
    def set_default_name(name):
        """Set the default merge to the specified one.

        name can be None to cause the default to be cleared."""

        if name is None:
            try:
                check_call(['git', 'config', '--unset', 'imerge.default'])
            except CalledProcessError, e:
                if e.returncode == 5:
                    # Value was not set
                    pass
                else:
                    raise
        else:
            check_call(['git', 'config', 'imerge.default', name])

    @staticmethod
    def get_default_name():
        """Get the name of the default merge, or None if none is currently set."""

        try:
            return check_output(['git', 'config', 'imerge.default']).rstrip()
        except CalledProcessError:
            return None

    @staticmethod
    def _check_no_merges(commits):
        multiparent_commits = [
            commit
            for commit in commits
            if len(get_commit_parents(commit)) > 1
            ]
        if multiparent_commits:
            raise Failure(
                'The following commits on the to-be-merged branch are merge commits:\n'
                '    %s\n'
                '--goal=\'rebase\' is not yet supported for branches that include merges.\n'
                % ('\n    '.join(multiparent_commits),)
                )

    @staticmethod
    def initialize(name, goal, tip1, tip2):
        """Create and return a new MergeState object."""

        if '/' in name:
            raise Failure('Name %r must not include a slash character!' % (name,))

        try:
            call_silently(
                ['git', 'check-ref-format', 'refs/imerge/%s' % (name,)]
                )
        except CalledProcessError:
            raise Failure('Name %r is not a valid refname component!' % (name,))

        if check_output(['git', 'for-each-ref', 'refs/imerge/%s' % (name,)]):
            raise Failure('Name %r is already in use!' % (name,))

        merge_base = check_output(['git', 'merge-base', '--all', tip1, tip2]).splitlines()
        if not merge_base:
            raise Failure('%r and %r do not have a common merge base' % (tip1, tip2))
        if len(merge_base) > 1:
            raise Failure('%r and %r do not have a unique merge base' % (tip1, tip2))

        [merge_base] = merge_base

        ancestry_path1 = set(rev_list('--ancestry-path', '%s..%s' % (merge_base, tip1)))
        commits1 = [
            sha1
            for sha1 in rev_list('--first-parent', '%s..%s' % (merge_base, tip1))
            if sha1 in ancestry_path1
            ]
        commits1.reverse()
        if not commits1:
            raise Failure(
                'There are no commits on %r that are not already in %r' % (tip1, tip2)
                )

        ancestry_path2 = set(rev_list('--ancestry-path', '%s..%s' % (merge_base, tip2)))
        commits2 = [
            sha1
            for sha1 in rev_list('--first-parent', '%s..%s' % (merge_base, tip2))
            if sha1 in ancestry_path2
            ]
        commits2.reverse()
        if not commits2:
            raise Failure(
                'There are no commits on %r that are not already in %r' % (tip2, tip1)
                )

        if goal == 'rebase':
            MergeState._check_no_merges(commits2)

        return MergeState(name, goal, merge_base, commits1, commits2, MergeRecord.NEW_MANUAL)

    @staticmethod
    def read(name):
        merge_ref_re = re.compile(
            r"""
            ^
            refs\/imerge\/
            """ + re.escape(name) + r"""
            \/(?P<source>auto|manual)\/
            (?P<i1>0|[1-9][0-9]*)
            \-
            (?P<i2>0|[1-9][0-9]*)
            $
            """,
            re.VERBOSE,
            )

        state_ref_re = re.compile(
            r"""
            ^
            refs\/imerge\/
            """ + re.escape(name) + r"""
            \/state
            $
            """,
            re.VERBOSE,
            )

        state = None

        # A map {(i1, i2) : (sha1, source)}:
        merges = {}

        # refnames that were found but not understood:
        unexpected = []

        for line in check_output([
                'git', 'for-each-ref', 'refs/imerge/%s' % (name,)
                ]).splitlines():
            (sha1, type, refname) = line.split()
            m = merge_ref_re.match(refname)
            if m:
                if type != 'commit':
                    raise Failure('Reference %r is not a commit!' % (refname,))
                i1, i2 = int(m.group('i1')), int(m.group('i2'))
                source = MergeState.SOURCE_TABLE[m.group('source')]
                merges[i1, i2] = (sha1, source)
                continue

            m = state_ref_re.match(refname)
            if m:
                if type != 'blob':
                    raise Failure('Reference %r is not a blob!' % (refname,))
                state = MergeState._read_state(name, sha1)
                continue

            unexpected.append(refname)

        if state is None:
            raise Failure(
                'No state found; it should have been a blob reference at '
                '"refs/imerge/%s/state' % (name,)
                )

        goal = state['goal']
        if goal not in ALLOWED_GOALS:
            raise Failure('Goal %r, read from state, is not recognized.' % (goal,))

        blockers = state.get('blockers', [])

        if unexpected:
            raise Failure(
                'Unexpected reference(s) found in "refs/imerge/%s" namespace:\n    %s\n'
                % (name, '\n    '.join(unexpected),)
                )

        # Find merge_base, commits1, and commits2:
        (merge_base, source) = merges.pop((0, 0))
        if source != MergeRecord.SAVED_MANUAL:
            raise Failure('Merge base should be manual!')
        commits1 = []
        for i1 in itertools.count(1):
            try:
                (sha1, source) = merges.pop((i1, 0))
                if source != MergeRecord.SAVED_MANUAL:
                    raise Failure('Merge %d-0 should be manual!' % (i1,))
                commits1.append(sha1)
            except KeyError:
                break

        commits2 = []
        for i2 in itertools.count(1):
            try:
                (sha1, source) = merges.pop((0, i2))
                if source != MergeRecord.SAVED_MANUAL:
                    raise Failure('Merge (0,%d) should be manual!' % (i2,))
                commits2.append(sha1)
            except KeyError:
                break

        state = MergeState(name, goal, merge_base, commits1, commits2, MergeRecord.SAVED_MANUAL)

        # Now write the rest of the merges to state:
        for ((i1, i2), (sha1, source)) in merges.iteritems():
            if i1 == 0 and i2 >= state.len2:
                raise Failure('Merge 0-%d is missing!' % (state.len2,))
            if i1 >= state.len1 and i2 == 0:
                raise Failure('Merge %d-0 is missing!' % (state.len1,))
            if i1 >= state.len1 or i2 >= state.len2:
                raise Failure(
                    'Merge %d-%d is out of range [0:%d,0:%d]'
                    % (i1, i2, state.len1, state.len2)
                    )
            state[i1, i2].record_merge(sha1, source)

        # Record any blockers:
        for (i1,i2) in blockers:
            state[i1, i2].record_blocked(True)

        return state

    @staticmethod
    def remove(name):
        # If HEAD is the scratch refname, abort any in-progress
        # commits and detach HEAD:
        scratch_refname = MergeState.get_scratch_refname(name)
        try:
            head_refname = check_output(['git', 'symbolic-ref', '--quiet', 'HEAD']).strip()
        except CalledProcessError:
            head_refname = None
        if head_refname == scratch_refname:
            try:
                # We don't use "git merge --abort" here because it was
                # only added in git version 1.7.4.
                check_call(['git', 'reset', '--merge'])
            except CalledProcessError:
                pass
            # Detach head so that we can delete scratch_refname:
            check_call([
                    'git', 'update-ref', '--no-deref',
                    '-m', 'Detach HEAD from %s' % (scratch_refname,),
                    'HEAD', get_commit_sha1('HEAD'),
                    ])

        # Delete the scratch refname:
        check_call([
                'git', 'update-ref',
                '-m', 'imerge %s: remove scratch reference' % (name,),
                '-d', scratch_refname,
                ])

        # Remove any references referring to intermediate merges:
        for line in check_output([
                'git', 'for-each-ref', 'refs/imerge/%s' % (name,)
                ]).splitlines():
            (sha1, type, refname) = line.split()
            try:
                check_call([
                        'git', 'update-ref',
                        '-m', 'imerge: remove merge %r' % (name,),
                        '-d', refname,
                        ])
            except CalledProcessError, e:
                sys.stderr.write(
                    'Warning: error removing reference %r: %s' % (refname, e)
                    )

        # If this merge was the default, unset the default:
        if MergeState.get_default_name() == name:
            MergeState.set_default_name(None)

    def __init__(self, name, goal, merge_base, commits1, commits2, source):
        Block.__init__(self, len(commits1) + 1, len(commits2) + 1)
        self.name = name
        self.goal = goal

        # A simulated 2D array.  Values are None or MergeRecord instances.
        self._data = [[None] * self.len2 for i1 in range(self.len1)]

        self.get_value(0, 0).record_merge(merge_base, source)
        for (i1, commit) in enumerate(commits1, 1):
            self.get_value(i1, 0).record_merge(commit, source)
        for (i2, commit) in enumerate(commits2, 1):
            self.get_value(0, i2).record_merge(commit, source)

    def set_goal(self, goal):
        if goal not in ALLOWED_GOALS:
            raise ValueError('%r is not an allowed goal' % (goal,))

        if goal == 'rebase':
            self._check_no_merges(
                [self[0,i2].sha1 for i2 in range(1,self.len2)]
                )

        self.goal = goal

    def _set_value(self, i1, i2, value):
        self._data[i1][i2] = value

    def get_value(self, i1, i2):
        value = self._data[i1][i2]
        # Missing values spring to life on first access:
        if value is None:
            value = MergeRecord()
            self._data[i1][i2] = value
        return value

    def __contains__(self, index):
        # Avoid creating new MergeRecord objects here.
        (i1, i2) = self._normalize_indexes(index)
        value = self._data[i1][i2]
        return (value is not None) and value.is_known()

    def auto_complete_frontier(self):
        """Complete the frontier using automerges.

        If progress is blocked before the frontier is complete, raise
        a FrontierBlockedError.  Save the state as progress is
        made."""

        progress_made = False
        try:
            while True:
                frontier = MergeFrontier.map_known_frontier(self)
                frontier.auto_expand()
                self.save()
                progress_made = True
        except BlockCompleteError:
            return
        except FrontierBlockedError, e:
            self.save()
            if not progress_made:
                # Adjust the error message:
                raise FrontierBlockedError(
                    'No progress was possible; suggest manual merge of %d-%d'
                    % (e.i1, e.i2),
                    e.i1, e.i2,
                    )
            else:
                raise

    def find_index(self, commit):
        """Return (i1,i2) for the specified commit.

        Raise CommitNotFoundError if it is not known."""

        for i2 in range(0, self.len2):
            for i1 in range(0, self.len1):
                if (i1, i2) in self:
                    record = self[i1,i2]
                    if record.sha1 == commit:
                        return (i1, i2)
        raise CommitNotFoundError(commit)

    def incorporate_manual_merge(self, commit):
        """Record commit as a manual merge of its parents.

        Return the indexes (i1,i2) where it was recorded.  If the
        commit is not usable for some reason, raise
        ManualMergeUnusableError."""

        parents = get_commit_parents(commit)
        if len(parents) < 2:
            raise ManualMergeUnusableError('it is not a merge', commit)
        if len(parents) > 2:
            raise ManualMergeUnusableError('it is an octopus merge', commit)
        # Find the parents among our contents...
        try:
            (i1first, i2first) = self.find_index(parents[0])
            (i1second, i2second) = self.find_index(parents[1])
        except CommitNotFoundError:
            raise ManualMergeUnusableError(
                'its parents are not known merge commits', commit,
                )
        swapped = False
        if i1first < i1second:
            # Swap parents to make the parent from above the first parent:
            (i1first, i2first, i1second, i2second) = (i1second, i2second, i2first, i1first)
            swapped = True
        if i1first != i1second + 1 or i2first != i2second - 1:
            raise ManualMergeUnusableError(
                'it is not a pairwise merge of adjacent parents', commit,
                )
        if swapped:
            # Create a new merge with the parents in the conventional order:
            commit = reparent(commit, [parents[1], parents[0]])

        i1, i2 = i1first, i2second
        self[i1, i2].record_merge(commit, MergeRecord.NEW_MANUAL)
        return (i1, i2)

    @staticmethod
    def _is_ancestor(commit1, commit2):
        """Return True iff commit1 is an ancestor (or equal to) commit2."""

        if commit1 == commit2:
            return True
        else:
            return int(
                check_output([
                        'git', 'rev-list', '--count', '--ancestry-path',
                        '%s..%s' % (commit1, commit2,),
                        ]).strip()
                ) != 0

    @staticmethod
    def _set_refname(refname, commit, force=False):
        try:
            ref_oldval = get_commit_sha1(refname)
        except ValueError:
            # refname doesn't already exist; simply point it at commit:
            check_call(['git', 'update-ref', refname, commit])
            checkout(refname)
        else:
            # refname already exists.  This has two ramifications:
            # 1. HEAD might point at it
            # 2. We may only fast-forward it (unless force is set)
            try:
                head_refname = check_output(['git', 'symbolic-ref', '--quiet', 'HEAD']).strip()
            except CalledProcessError:
                head_refname = None

            if not force:
                if not MergeState._is_ancestor(ref_oldval, commit):
                    raise Failure(
                        '%s cannot be fast-forwarded to %s!' % (refname, commit)
                        )

            if head_refname == refname:
                check_call(['git', 'reset', '--hard', commit])
            else:
                check_call([
                        'git', 'update-ref',
                        '-m', 'imerge: recording final merge',
                        refname, commit,
                        ])
                checkout(refname)

    def simplify_to_rebase_with_history(self, refname, force=False):
        i1 = self.len1 - 1
        for i2 in range(1, self.len2):
            if not (i1, i2) in self:
                raise Failure(
                    'Cannot simplify to rebase-with-history because '
                    'merge %d-%d is not yet done'
                    % (i1, i2)
                    )

        commit = self[i1, 0].sha1
        for i2 in range(1, self.len2):
            orig = self[0, i2].sha1
            tree = get_tree(self[i1, i2].sha1)

            # Create a commit, copying the old log message:
            commit = commit_tree(tree, [commit, orig], msg=get_log_message(orig))

        self._set_refname(refname, commit, force=force)

    def simplify_to_rebase(self, refname, force=False):
        i1 = self.len1 - 1
        for i2 in range(1, self.len2):
            if not (i1, i2) in self:
                raise Failure(
                    'Cannot simplify to rebase because merge %d-%d is not yet done'
                    % (i1, i2)
                    )

        commit = self[i1, 0].sha1
        for i2 in range(1, self.len2):
            orig = self[0, i2].sha1
            tree = get_tree(self[i1, i2].sha1)
            authordata = get_author_info(orig)

            # Create a commit, copying the old log message and author info:
            commit = commit_tree(
                tree, [commit], msg=get_log_message(orig), metadata=authordata,
                )

        self._set_refname(refname, commit, force=force)

    def simplify_to_merge(self, refname, force=False):
        if not (-1, -1) in self:
            raise Failure(
                'Cannot simplify to merge because merge %d-%d is not yet done'
                % (self.len1 - 1, self.len2 - 1)
                )
        tree = get_tree(self[-1, -1].sha1)
        parents = [self[-1,0].sha1, self[0,-1].sha1]

        # Create a preliminary commit with a generic commit message:
        sha1 = commit_tree(
            tree, parents,
            msg='Merge commit %s into commit %s' % (parents[1], parents[0]),
            )

        self._set_refname(refname, sha1, force=force)

        # Now let the user edit the commit log message:
        check_call(['git', 'commit', '--amend'])

    def simplify(self, refname, force=False):
        """Simplify this MergeState and save the result to refname.

        The merge must be complete before calling this method."""

        if self.goal == 'rebase-with-history':
            self.simplify_to_rebase_with_history(refname, force=force)
        elif self.goal == 'rebase':
            self.simplify_to_rebase(refname, force=force)
        elif self.goal == 'merge':
            self.simplify_to_merge(refname, force=force)
        else:
            raise ValueError('Invalid value for goal (%r)' % (self.goal,))

    def save(self):
        """Write the current MergeState to the repository."""

        blockers = []
        for i2 in range(0, self.len2):
            for i1 in range(0, self.len1):
                record = self[i1,i2]
                if record.is_known():
                    record.save(self.name, i1, i2)
                if record.is_blocked():
                    blockers.append((i1, i2))

        state = dict(
            version='.'.join(map(str, STATE_VERSION)),
            goal=self.goal,
            blockers=blockers,
            )
        state_string = json.dumps(state, sort_keys=True) + '\n'

        cmd = ['git', 'hash-object', '-t', 'blob', '-w', '--stdin']
        p = subprocess.Popen(cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
        (out, err) = p.communicate(state_string)
        retcode = p.poll()
        if retcode:
            raise CalledProcessError(retcode, cmd)
        sha1 = out.strip()
        check_call([
                'git', 'update-ref',
                '-m', 'imerge %r: Record state' % (self.name,),
                'refs/imerge/%s/state' % (self.name,),
                sha1,
                ])

    def __str__(self):
        return 'MergeState(%r, goal=%r)' % (self.name, self.goal,)


def request_user_merge(merge_state, i1, i2):
    """Prepare the working tree for the user to do a manual merge.

    It is assumed that the merge above and to the left of (i1, i2) are
    already done."""

    above = merge_state[i1, i2 - 1]
    left = merge_state[i1 - 1, i2]
    if not above.is_known() or not left.is_known():
        raise RuntimeError('The parents of merge %d-%d are not ready' % (i1, i2))
    refname = MergeState.get_scratch_refname(merge_state.name)
    check_call([
            'git', 'update-ref',
            '-m', 'imerge %r: Prepare merge %d-%d' % (merge_state.name, i1, i2,),
            refname, above.sha1,
            ])
    checkout(refname)
    try:
        check_call([
                'git', 'merge', '--no-commit',
                '-m', 'Merge %d-%d of incremental merge \'%s\'' % (i1, i2, merge_state.name,),
                left.sha1,
                ])
    except CalledProcessError:
        # We expect an error (otherwise we would have automerged!)
        pass
    sys.stderr.write(
        '\n'
        'Original first commit:\n'
        )
    check_call(['git', 'log', '--no-walk', merge_state[i1,0].sha1])
    sys.stderr.write(
        '\n'
        'Original second commit:\n'
        )
    check_call(['git', 'log', '--no-walk', merge_state[0,i2].sha1])
    sys.stderr.write(
        '\n'
        'There was a conflict merging commit %d-%d, shown above.\n'
        'Please resolve the conflict, commit the result, then type\n'
        '\n'
        '    git-imerge continue\n'
        % (i1, i2)
        )

def incorporate_user_merge(merge_state):
    """If the user has done a merge for us, incorporate the results.

    If reference refs/heads/imerge/NAME exists, try to incorporate it
    into merge_state, delete the reference, and return (i1,i2)
    corresponding to the merge.  If the reference cannot be used,
    raise NoManualMergeError().  If the reference exists but cannot be
    used, raise a ManualMergeUnusableError.  This function must be
    called with a clean work tree."""

    refname = MergeState.get_scratch_refname(merge_state.name)
    try:
        commit = get_commit_sha1(refname)
    except ValueError:
        raise NoManualMergeError('There was no merge at %s!' % (refname,))

    merge_frontier = MergeFrontier.map_known_frontier(merge_state)

    # This might throw ManualMergeUnusableError:
    (i1, i2) = merge_state.incorporate_manual_merge(commit)

    try:
        headref = check_output(['git', 'symbolic-ref', '-q', 'HEAD']).strip()
    except CalledProcessError:
        pass
    else:
        if headref == refname:
            # Detach head so that we can delete refname.
            check_call([
                    'git', 'update-ref', '--no-deref',
                    '-m', 'Detach HEAD from %s' % (refname,),
                    'HEAD', commit,
                    ])

    check_call([
            'git', 'update-ref',
            '-m', 'imerge %s: remove scratch reference' % (merge_state.name,),
            '-d', refname,
            ])

    try:
        # This might throw NotABlockingCommitError:
        unblocked_block = merge_frontier.get_affected_blocker_block(i1, i2)
        unblocked_block[1,1].record_blocked(False)
        sys.stderr.write(
            'Merge has been recorded for merge %d-%d.\n'
            % unblocked_block.get_original_indexes(1, 1)
            )
    except NotABlockingCommitError:
        raise
    finally:
        merge_state.save()


def choose_merge_name(name, default_to_unique=True):
    # If a name was specified, try to use it and fail if not possible:
    if name is not None:
        if not MergeState.check_exists(name):
            raise Failure('There is no incremental merge called \'%s\'!' % (name,))
        MergeState.set_default_name(name)
        return name

    # A name was not specified.  Try to use the default name:
    default_name = MergeState.get_default_name()
    if default_name:
        if MergeState.check_exists(default_name):
            return default_name
        else:
            # There's no reason to keep the invalid default around:
            MergeState.set_default_name(None)
            raise Failure(
                'Warning: The default incremental merge \'%s\' has disappeared.\n'
                '(The setting imerge.default has been cleared.)\n'
                'Please select an incremental merge using --name'
                % (default_name,)
                )

    if default_to_unique:
        # If there is exactly one imerge, set it to be the default and use it.
        names = list(MergeState.iter_existing_names())
        if len(names) == 1 and MergeState.check_exists(names[0]):
            MergeState.set_default_name(names[0])
            return names[0]

    raise Failure('Please select an incremental merge using --name')


def read_merge_state(name=None):
    return MergeState.read(choose_merge_name(name))


@Failure.wrap
def main(args):
    parser = argparse.ArgumentParser(
        description=__doc__,
        formatter_class=argparse.RawDescriptionHelpFormatter,
        )
    subparsers = parser.add_subparsers(dest='subcommand', help='sub-command')

    parser_start = subparsers.add_parser(
        'start',
        help=(
            'start a new incremental merge '
            '(equivalent to "init" followed by "continue")'
            ),
        )
    parser_start.add_argument(
        '--name', action='store', default=None,
        help='name to use for this incremental merge',
        )
    parser_start.add_argument(
        '--goal',
        action='store', default=DEFAULT_GOAL,
        choices=ALLOWED_GOALS,
        help='the goal of the incremental merge',
        )
    #parser_start.add_argument(
    #    '--conflicts', ...
    #    action='store', default='pairwise',
    #    choices=['pairwise', 'fewest'],
    #    help='what sort of conflicts will be presented to the user',
    #    )
    parser_start.add_argument(
        '--first-parent', action='store_true', default=None,
        help=(
            'handle only the first parent commits '
            '(this option is currently required)'
            ),
        )
    parser_start.add_argument(
        'branch', action='store',
        help='the tip of the branch to be merged into HEAD',
        )

    parser_continue = subparsers.add_parser(
        'continue',
        help=(
            'record the merge at branch imerge/NAME '
            'and start the next step of the merge '
            '(equivalent to "record" followed by "autofill" '
            'and then sets up the working copy with the next '
            'conflict that has to be resolved manually)'
            ),
        )
    parser_continue.add_argument(
        '--name', action='store', default=None,
        help='name of merge to continue',
        )

    parser_finish = subparsers.add_parser(
        'finish',
        help=(
            'simplify then remove a completed incremental merge '
            '(equivalent to "simplify" followed by "remove")'
            ),
        )
    parser_finish.add_argument(
        '--name', action='store', default=None,
        help='name of merge to finish',
        )
    parser_finish.add_argument(
        '--goal',
        action='store', default=None,
        choices=ALLOWED_GOALS,
        help=(
            'the type of simplification to be made '
            '(default is the value provided to "init" or "start")'
            ),
        )
    parser_finish.add_argument(
        '--branch',
        action='store', default=None,
        help=(
            'the name of the branch to which to store the result '
            '(default is the name of the merge).   If '
            'BRANCH already exists then it must be able to be '
            'fast-forwarded to the result unless the --force option is '
            'specified.'
            ),
        )
    parser_finish.add_argument(
        '--force',
        action='store_true', default=False,
        help='allow the target branch to be updated in a non-fast-forward manner',
        )

    parser_diagram = subparsers.add_parser(
        'diagram',
        help='display a diagram of the current state of a merge',
        )
    parser_diagram.add_argument(
        '--name', action='store', default=None,
        help='name of merge to diagram',
        )
    parser_diagram.add_argument(
        '--commits', action='store_true', default=False,
        help='show the merges that have been made so far',
        )
    parser_diagram.add_argument(
        '--frontier', action='store_true', default=False,
        help='show the current merge frontier',
        )
    parser_diagram.add_argument(
        '--html', action='store', default=None,
        help='generate HTML diagram showing the current merge frontier',
        )
    parser_diagram.add_argument(
        '--color', dest='color', action='store_true', default=None,
        help='draw diagram with colors',
        )
    parser_diagram.add_argument(
        '--no-color', dest='color', action='store_false',
        help='draw diagram without colors',
        )

    parser_list = subparsers.add_parser(
        'list',
        help=(
            'list the names of incremental merges that are currently in progress.  '
            'The active merge is shown with an asterisk next to it.'
            ),
        )

    parser_init = subparsers.add_parser(
        'init',
        help='initialize a new incremental merge',
        )
    parser_init.add_argument(
        '--name', action='store', default=None,
        help='name to use for this incremental merge',
        )
    parser_init.add_argument(
        '--goal',
        action='store', default=DEFAULT_GOAL,
        choices=ALLOWED_GOALS,
        help='the goal of the incremental merge',
        )
    #parser_init.add_argument(
    #    '--conflicts', ...
    #    action='store', default='pairwise',
    #    choices=['pairwise', 'fewest'],
    #    help='what sort of conflicts will be presented to the user',
    #    )
    parser_init.add_argument(
        '--first-parent', action='store_true', default=None,
        help=(
            'handle only the first parent commits '
            '(this option is currently required)'
            ),
        )
    parser_init.add_argument(
        'branch', action='store',
        help='the tip of the branch to be merged into HEAD',
        )

    parser_record = subparsers.add_parser(
        'record',
        help='record the merge at branch imerge/NAME',
        )
    parser_record.add_argument(
        '--name', action='store', default=None,
        help='name of merge to which the merge should be added',
        )

    parser_autofill = subparsers.add_parser(
        'autofill',
        help='autofill non-conflicting merges',
        )
    parser_autofill.add_argument(
        '--name', action='store', default=None,
        help='name of merge to autofill',
        )

    parser_simplify = subparsers.add_parser(
        'simplify',
        help=(
            'simplify a completed incremental merge by discarding unneeded '
            'intermediate merges and cleaning up the ancestry of the commits '
            'that are retained'
            ),
        )
    parser_simplify.add_argument(
        '--name', action='store', default=None,
        help='name of merge to simplify',
        )
    parser_simplify.add_argument(
        '--goal',
        action='store', default=None,
        choices=ALLOWED_GOALS,
        help=(
            'the type of simplification to be made '
            '(default is the value provided to "init" or "start")'
            ),
        )
    parser_simplify.add_argument(
        '--branch',
        action='store', default=None,
        help=(
            'the name of the branch to which to store the result '
            '(default is the name of the merge).   If '
            'BRANCH already exists then it must be able to be '
            'fast-forwarded to the result unless the --force option is '
            'specified.'
            ),
        )
    parser_simplify.add_argument(
        '--force',
        action='store_true', default=False,
        help='allow the target branch to be updated in a non-fast-forward manner',
        )

    parser_remove = subparsers.add_parser(
        'remove',
        help='irrevocably remove an incremental merge',
        )
    parser_remove.add_argument(
        '--name', action='store', default=None,
        help='name of incremental merge to remove',
        )

    parser_reparent = subparsers.add_parser(
        'reparent',
        help='change the parents of the HEAD commit',
        )
    parser_reparent.add_argument(
        'parents', nargs='*', help='[PARENT...]',
        )

    options = parser.parse_args(args)

    if options.subcommand == 'list':
        default_merge = MergeState.get_default_name()
        for name in MergeState.iter_existing_names():
            if name == default_merge:
                sys.stdout.write('* %s\n' % (name,))
            else:
                sys.stdout.write('  %s\n' % (name,))
    elif options.subcommand == 'init':
        require_clean_work_tree('proceed')

        if not options.first_parent:
            parser.error(
                'The --first-parent option is currently required for the "init" command'
                )
        if not options.name:
            parser.error(
                'Please specify the --name to be used for this incremental merge'
                )
        merge_state = MergeState.initialize(
            options.name, options.goal, 'HEAD', options.branch,
            )
        merge_state.save()
        MergeState.set_default_name(options.name)
    elif options.subcommand == 'start':
        require_clean_work_tree('proceed')

        if not options.first_parent:
            parser.error(
                'The --first-parent option is currently required for the "start" command'
                )
        if not options.name:
            parser.error(
                'Please specify the --name to be used for this incremental merge'
                )
        merge_state = MergeState.initialize(
            options.name, options.goal, 'HEAD', options.branch,
            )
        merge_state.save()
        MergeState.set_default_name(options.name)

        try:
            merge_state.auto_complete_frontier()
        except FrontierBlockedError, e:
            request_user_merge(merge_state, e.i1, e.i2)
        else:
            sys.stderr.write('Merge is complete!\n')
    elif options.subcommand == 'remove':
        MergeState.remove(choose_merge_name(options.name, default_to_unique=False))
    elif options.subcommand == 'continue':
        require_clean_work_tree('proceed')
        merge_state = read_merge_state(options.name)
        try:
            incorporate_user_merge(merge_state)
        except NoManualMergeError:
            pass
        except NotABlockingCommitError:
            raise Failure(str(e))
        except ManualMergeUnusableError, e:
            raise Failure(str(e))

        try:
            merge_state.auto_complete_frontier()
        except FrontierBlockedError, e:
            request_user_merge(merge_state, e.i1, e.i2)
        else:
            sys.stderr.write('Merge is complete!\n')
    elif options.subcommand == 'record':
        require_clean_work_tree('proceed')
        merge_state = read_merge_state(options.name)
        try:
            incorporate_user_merge(merge_state)
        except NoManualMergeError, e:
            raise Failure(str(e))
        except NotABlockingCommitError:
            raise Failure(str(e))
        except ManualMergeUnusableError, e:
            raise Failure(str(e))

        try:
            merge_state.auto_complete_frontier()
        except FrontierBlockedError, e:
            pass
        else:
            sys.stderr.write('Merge is complete!\n')
    elif options.subcommand == 'autofill':
        require_clean_work_tree('proceed')
        merge_state = read_merge_state(options.name)
        with TemporaryHead():
            try:
                merge_state.auto_complete_frontier()
            except FrontierBlockedError, e:
                raise Failure(str(e))
    elif options.subcommand == 'simplify':
        require_clean_work_tree('proceed')
        merge_state = read_merge_state(options.name)
        merge_frontier = MergeFrontier.map_known_frontier(merge_state)
        if not merge_frontier.is_complete():
            raise Failure('Merge %s is not yet complete!' % (merge_state.name,))
        refname = 'refs/heads/%s' % ((options.branch or merge_state.name),)
        if options.goal is not None:
            merge_state.set_goal(options.goal)
            merge_state.save()
        merge_state.simplify(refname, force=options.force)
    elif options.subcommand == 'finish':
        require_clean_work_tree('proceed')
        options.name = choose_merge_name(options.name, default_to_unique=False)
        merge_state = read_merge_state(options.name)
        merge_frontier = MergeFrontier.map_known_frontier(merge_state)
        if not merge_frontier.is_complete():
            raise Failure('Merge %s is not yet complete!' % (merge_state.name,))
        refname = 'refs/heads/%s' % ((options.branch or merge_state.name),)
        if options.goal is not None:
            merge_state.set_goal(options.goal)
            merge_state.save()
        merge_state.simplify(refname, force=options.force)
        MergeState.remove(options.name)
    elif options.subcommand == 'diagram':
        if not (options.commits or options.frontier):
            options.frontier = True
        if not (options.color or (options.color is None and sys.stdout.isatty())):
            AnsiColor.disable()

        merge_state = read_merge_state(options.name)
        if options.commits:
            merge_state.write(sys.stdout)
            sys.stdout.write('\n')
        if options.frontier:
            merge_frontier = MergeFrontier.map_known_frontier(merge_state)
            merge_frontier.write(sys.stdout)
            sys.stdout.write('\n')
        if options.html:
            merge_frontier = MergeFrontier.map_known_frontier(merge_state)
            html = open(options.html, "w")
            merge_frontier.write_html(html, merge_state.name)
            html.close()
        sys.stdout.write(
            'Key:\n'
            )
        if options.frontier:
            sys.stdout.write(
                '  |,-,+ = rectangles forming current merge frontier\n'
                )
        sys.stdout.write(
            '  * = merge done manually\n'
            '  . = merge done automatically\n'
            '  # = conflict that is currently blocking progress\n'
            '  @ = merge was blocked but has been resolved\n'
            '  ? = no merge recorded\n'
            '\n'
            )
    elif options.subcommand == 'reparent':
        try:
            commit_sha1 = get_commit_sha1('HEAD')
        except ValueError:
            sys.exit('HEAD is not a valid commit')

        try:
            parent_sha1s = map(get_commit_sha1, options.parents)
        except ValueError, e:
            sys.exit(e.message)

        sys.stdout.write('%s\n' % (reparent(commit_sha1, parent_sha1s),))
    else:
        parser.error('Unrecognized subcommand')


main(sys.argv[1:])