#!/usr/bin/python

# This will output a Git "grafts" file to use when a (set of) downstream branch(es)
# is to switch from merging with one upstream branch to another upstream branch, without
# keeping the full history of both upstream branches in the repository.
# If the upstream branches contain the same history information (semantically speaking),
# such as when they represent different conversions from an old CVS/SVN repository, then
# this script will find an alignment between them.
# The resulting grafts file will map past merges in the downstream branch to the new
# upstream branch, as if the old upstream branch never existed.
# If this works well, a "git filter-branch" should burn in the grafts so that they can
# be removed.  (This is a history-changing operation.)

# See also:
# http://thread.gmane.org/gmane.comp.version-control.git/136377


# recommended usage:
#./make-grafts.py | sort | uniq > .git/info/grafts

# set input file names
upstream1 = "em-log"
upstream2 = "em-new-log"
mergecommits = "aq-merges"


# produce files:
# git checkout upstream1
# git log --since=2009-01-01  --format="%H %f">../emc/em-log
# git checkout upstream2
# git log --since=2009-01-01  --format="%H %f">../emc/em-new-log
# export BRANCHES='topic/b1 topic/b2 john jane master'
# git log --since=2009-01-01 --author="David Reitter" --format="%H %P" $BRANCHES >../emc/aq-merges; git log --since=2009-01-01 --merges --format="%H %P" $BRANCHES >>../emc/aq-merges

#################

import re
import sys

r1msgs = {}
r1revids = {}
# checkout emacs
# git log --since=2009-01-01  --format="%H %f">../emc/em-log
file = open(upstream1, 'r')
for l in file:
    m =    re.match("([a-f0-9]*) (.*)", l)
    if m:
       r1msgs[m.group(1)] = m.group(2)
       r1revids[m.group(2)] = m.group(1)
file.close()


r2msgs = {}
r2revids = {}
# checkout emacs23
# git log --since=2009-01-01  --format="%H %f" >../emc/em-new-log
file = open(upstream2, 'r')
for l in file:
    m = re.match("([a-f0-9]*) (.*)", l)
    if m:
       r2msgs[m.group(1)] = m.group(2)
       r2revids[m.group(2)] = m.group(1)
file.close()

# checkout master
# export BRANCHES='23.1.undone  Aquamacs22  dr-after-merge  dr/dev  dr/experimental  dr/suedit master topic/NSAlertDialogs  topic/dialogs  topic/face-remapping  topic/mac-support  topic/menu-bar  topic/minibuffer  topic/option-key-remap  topic/printing  topic/python-mode  topic/reconf  topic/smart-spacing  topic/spelling  topic/tabbar  topic/tmm  topic/toolbar'
# git log --since=2009-01-01 --author="David Reitter" --merges --format="%H %P" $BRANCHES >../emc/aq-merges
# it's better to use all commits so we don't miss anything
# git log --since=2009-01-01 --author="David Reitter" --format="%H %P" $BRANCHES >../emc/aq-merges; git log --since=2009-01-01 --merges --format="%H %P" $BRANCHES >>../emc/aq-merges
def replace (revid):
   if revid in r1msgs:
      r1m = r1msgs[revid]
      if r1m in r2revids:
         r2r = r2revids[r1m]
         if r2r:
            return r2r
         else:
            print >> sys.stderr, "can't get mapping for revid "+revid+ r1m
            
      else:
         print >> sys.stderr, "can't find message of revid "+revid
   return revid
      

# checkout master
# 

file = open(mergecommits, 'r')
for l in file:
    revids = l.rstrip().split(" ")
    if revids:

        # test:
        # revids2 = revids
        # for r in revids[1:]:
        #     r2 = replace(r)
        #     if r != r2:
        #         revids2 = revids2 + [r2]
        # correct alternative
        revids2 = [revids[0]] + map(replace, revids[1:])
        if revids != revids2:
            # make graft entry to write the merge such that it
            # looks like the merge came from the other branch
            print " ".join(revids2)
        else:
            print >> sys.stderr, "no remappings found for parents of merge revid "+" ".join(revids)

file.close()