#!/bin/bash
# This script is meant to be run as a GIT_SEQUENCE_EDITOR 
# when running `git rebase -i`. The idea being that it will
# cross-reference commit logs and history with the target branch and 
# drop any changes, squashed or otherwise, that exist on that branch

FILE=${1}

# Find the branch we're rebasing onto

ONTO=$(awk '/^# Rebase/{ print $5 }' ${FILE})

# Create tempfile to rewrite the todo list
random=$( (whoami ; hostname ; date; cat $1) | git hash-object --stdin)
dest="$1.tmp.${random}"

trap 'rm -f "${dest}"' EXIT

# For each commit...
while read -r line; do
    if [[ "$line" =~ ^#.* ]]; then
      echo $line >> ${dest}
      continue
    fi
    commit=$(echo -n $line | awk '{ print $2 }') 
    if [ -z "${commit}" ]; then
      continue
    fi
    # ...see if there's a Change-Id in it....
    changeid=$(git log --format=%B -n 1 ${commit} | awk '/Change-Id/{ print $2 }')
    if [ -n "${changeid}" ]; then
      # ...and match it against the history of the target branch
      gitlog=$(git log --grep "${changeid}" ${ONTO})
      if [ -n "${gitlog}" ] ; then
        echo $line "# Has Change-Id ${changeid}" | sed 's/pick/drop/g' >> ${dest}
        continue
      fi
    fi
    echo $line >> ${dest}
done < ${FILE}


# Apply the changes from the tempfile

if ! mv "${dest}" "$1" ; then
  echo "cannot mv ${dest} to $1"
  exit 1
fi

# Proceed with the editor as per usual

REAL_EDITOR="$(git var GIT_EDITOR)"

if [ -n "$REAL_EDITOR" ]; then
  $REAL_EDITOR "$@"
fi