merge_vanilla.bash

#!/usr/bin/env bash
set -euo pipefail

if [ "$#" -ne 3 ]; then
  echo "Usage: $0 /path/to/vanilla_old /path/to/vanilla_new /path/to/mod"
  exit 1
fi

VOLD="$1"
VNEW="$2"
MOD="$3"

VOLD="$(realpath "$VOLD")"
VNEW="$(realpath "$VNEW")"
MOD="$(realpath "$MOD")"

relpath_under() {
  local base="$1"
  local path="$2"

  case "$path" in
    "$base"/*) printf '%s\n' "${path#"$base"/}" ;;
    *) printf '%s\n' "$path" ;;
  esac
}

# safety
#TIMESTAMP=$(date +%Y%m%d%H%M%S)
#BACKUP="${MOD}_backup_${TIMESTAMP}"
#echo "Creating backup of mod -> ${BACKUP}"
#mkdir -p "$BACKUP"
#cp -a "$MOD/." "$BACKUP/"

# Find files that are Modified or Renamed between vanilla_old and vanilla_new.
# We exclude purely added files in new vanilla (we don't want to create new files in mod).
# Use NUL-delimited output so paths containing spaces are handled correctly.
files=()
while IFS= read -r -d '' status; do
  if [[ "$status" == R* ]]; then
    IFS= read -r -d '' _old_path
    IFS= read -r -d '' new_path
    files+=("$(relpath_under "$VNEW" "$new_path")")
  else
    IFS= read -r -d '' old_path
    files+=("$(relpath_under "$VOLD" "$old_path")")
  fi
done < <(git diff --no-index --name-status -z --diff-filter=MR -- "$VOLD" "$VNEW" || true)

if [ "${#files[@]}" -eq 0 ]; then
  echo "No modified/renamed files between vanilla versions (or git not available). Exiting."
  exit 0
fi

echo "Files changed in vanilla (filtered to M/R):"
printf '%s\n' "${files[@]}"

conflicts=()

# Process each file: if it exists in mod, try a 3-way merge: ours=mod, base=vanilla_old, theirs=vanilla_new
for f in "${files[@]}"; do
  # normalize path separators
  f="$(printf '%s' "$f" | sed 's#\\#/#g')"
  modfile="$MOD/$f"
  basefile="$VOLD/$f"
  newfile="$VNEW/$f"
  
  if [ -f "$modfile" ]; then
    echo "Merging: $f (present in mod) ..."
    # ensure base and new exist
    if [ ! -f "$basefile" ] || [ ! -f "$newfile" ]; then
      echo "  Warning: base or new missing for $f — skipping automatic 3-way merge."
      continue
    fi

    # Use git-merge-file for three-way merge (inplace).
    # It modifies the 'modfile' and leaves conflict markers if conflicts occur.
    # We'll work on a temp copy and then replace the modfile on success (or with conflict markers)
    tmp="$modfile.merge_tmp"
    mode="$(stat -c '%a' "$modfile")"
    cp -a "$modfile" "$tmp"
    # git-merge-file: OURS BASE THEIRS
    git merge-file -p "$tmp" "$basefile" "$newfile" > "$tmp.merged" || true
    mv "$tmp.merged" "$modfile"
    chmod "$mode" "$modfile"
    rm -f "$tmp"
    # Report if conflict markers are present
    if grep -q '^<<<<<<< ' "$modfile"; then
      echo "  Conflict markers left in $modfile — please resolve manually."
      conflicts+=("$modfile")
    else
      echo "  Merged successfully: $modfile"
    fi

  else
    echo "Skipping $f — not present in mod (we don't add new vanilla files)."
  fi
done

if [ "${#conflicts[@]}" -gt 0 ]; then
  echo "Merge completed with conflicts:"
  printf '  %s\n' "${conflicts[@]}"
  echo "Resolve conflict markers, then commit."
  exit 1
fi

echo "Merge completed without conflicts."
echo "Review changes in $MOD, then commit."