#!/usr/bin/env zsh

# this is a git merge driver that magically fixes conflicts in the ChangeLog
# file (for simple additions at least). useful for back-porting
#
# to use, add this to your .git/config:
#
#   [merge "zsh-merge-changelog"]
#     name = merge the zsh changelog somewhat reasonably
#     driver = Util/zsh-merge-changelog %O %A %B %L %P
#
# and add this to your .git/info/attributes:
#
#   /ChangeLog merge=zsh-merge-changelog
#
# note: currently the ChangeLog is still updated even when the rest of the merge
# would be a no-op
#
# original script written by Mikachu

emulate -L zsh -o extended_glob -o warn_create_global

eputs() {
  [[ $1 == (-|--) ]] && shift
  print -rlu2 - "${ZSH_ARGZERO:t}: $1" "${@[2,-1]}"
}

main() {
  local tmp=${TMPDIR:-/tmp}/${ZSH_ARGZERO:t}.$$.$RANDOM
  local diff line ctx_after header data datb idx color
  local -a out orig

  diff=$( git diff --no-index -U200 -- $1 $3 )

  for line in ${${(f)diff}[5,-1]}; do
    case $line in
      @@*) ;;
      ' '*)
        (( $#out )) && ctx_after=1
        # commit header from context (entry added under existing header)
        (( ! $#out )) && [[ $line == ' '<->-<->-<->* ]] && header=$line[2,-1]
        ;;
      +*)
        (( ctx_after )) && {
          eputs 'unable to handle non-consecutive addition'
          return 1
        }
        [[ -n $header ]] && {
          out+=( $header '' )
          header=
        }
        out+=( "$line[2,-1]" )
        ;;
      -*)
        eputs 'unable to handle deletion'
        return 1
        ;;
      *)
        eputs 'unexpected line in diff:' $line
        return 1
        ;;
    esac
  done

  (( $#out )) || {
    eputs 'no changed lines found'
    return 1
  }
  [[ $out[1] == <->-<->-<->* && $out[2] == '' ]] || {
    eputs 'commit header not found'
    return 1
  }

  trap "rm -f -- ${(q+)tmp}" EXIT HUP INT

  orig=( "${(@f)"$( < $2 )"}" )

  data=${(M)orig[1]##<->-<->-<->}
  [[ -n $data ]] || {
    eputs 'failed to get latest date in original change log'
    return 1
  }
  datb=${(M)out[1]##<->-<->-<->}

  # new entry later than last -- just prepend
  if [[ $datb > $data ]]; then
    print -rl - "${(@)out}" "${(@)orig}" > $tmp || return

  # header already present in file -- re-use it. this always inserts above the
  # existing entries, so it might not be the same order it was in originally.
  # but it's fine
  elif [[ -n ${(M)orig:#$out[1]} ]]; then
    idx=$orig[(ie)$out[1]]
    print -rl - \
      "${(@)orig[1,idx]}" \
      "${(@)out[2,-1]}" \
      "${(@)orig[idx+2,-1]}" \
    > $tmp || return

  # insert in the middle somewhere
  else
    for (( idx = 1; idx <= $#orig; idx++ )); do
      data=${(M)orig[idx]##<->-<->-<->}
      [[ -n $data && $datb > $data ]] || continue
      print -rl - \
        "${(@)orig[1,idx-1]}" \
        "${(@)out}" \
        "${(@)orig[idx,-1]}" \
      > $tmp || return
      break
    done
  fi

  [[ -t 2 ]] && color=--color=always
  out=( "${(@f)"$( git diff --no-index $color -U4 -- $2 $tmp )"}" )
  out=( "${(@)out[3,-1]}" )
  out[1]=${out[1]/a\/[^[:cntrl:]]##/a/$5}
  out[2]=${out[2]/b\/[^[:cntrl:]]##/b/$5}
  eputs 'fixed ChangeLog entry:' "${(F)out}"

  [[ -n $ZMC_FAIL ]] && return 1 # for testing
  cp -- $tmp $2
}

main "$@"