#!/bin/sh
#-----------------------------------------------------------------------
#
#  git-fork-files -- Make local forks of files while preserving history
#
# 2019-07-02 (marcelk) Initial version as shell script
#
#-----------------------------------------------------------------------

# Based on:
#   https://stackoverflow.com/questions/16937359/git-copy-file-preserving-history
#
#  Why this works
#
#  You will end up with a revision history like this:
#
#   ORIG_HEAD
#     /    \
# SAVED  ALTERNATE
#     \    /
#     MERGED
#       |
#    RESTORED

# Check if new files don't exist already
for F in "$@"; do
  if [ ! -f "$F" ]
  then
    echo 1>&2 "$0: $F isn't a file"
    exit 10
  fi
  if [ -f "$F".forked ]
  then
    echo 1>&2 "$0: $F.forked already exists"
    exit 10
  fi
  if [ -f "$F".forked.$$ ]
  then
    echo 1>&2 "$0: $F.forked.$$ already exists"
    exit 10
  fi
done

# git mv foo bar
for F in "$@"; do
  git mv "$F" "$F".forked
done

# git commit -n
message="(git-fork-files) Creating:"
for F in "$@"; do
  message="$message $F.forked"
done
git commit -n -m "$message"
echo

# SAVED=`git rev-parse HEAD`
saved=$(git rev-parse HEAD)

# git reset --hard HEAD^
git reset --hard HEAD^

# git mv foo foo-magic
for F in "$@"; do
  git mv "$F" "$F".forked.$$
done

# git commit -n
git commit -n -m "(git-fork-files) Saving: $*"
echo

# git merge $SAVED # This will generate conflicts
git merge "$saved"

# git commit -a -n # Trivially resolved like this
git commit -a -n -m "(git-fork-files) Merge and resolve conflict"
echo

# git mv foo-magic foo
for F in "$@"; do
  git mv "$F".forked.$$ "$F"
done

echo "New files:"
for F in "$@"; do
  echo \"$F.forked\"
done

# git commit -n
echo
echo "Rename the new files and then commit all with:"
echo git commit -n -m \""Forking: $*"\"