#!/usr/bin/env bash
# Required env vars: (none)
# Optional env vars: PREV_SHA (if set, generates an incremental diff from that commit)
set -euo pipefail

git fetch origin main --depth=100

if [ -n "${PREV_SHA:-}" ] && git cat-file -e "${PREV_SHA}^{commit}" 2>/dev/null; then
  OLD_BASE=$(git merge-base origin/main "$PREV_SHA")
  NEW_BASE=$(git merge-base origin/main HEAD)
  if [ "$OLD_BASE" = "$NEW_BASE" ]; then
    echo "Generating incremental diff from ${PREV_SHA} to HEAD"
    git diff "${PREV_SHA}..HEAD" > pr.diff
    echo "incremental=true" >> "$GITHUB_OUTPUT"
  else
    echo "PR base changed (merge/rebase from main) — generating full diff"
    git diff origin/main...HEAD > pr.diff
    echo "incremental=false" >> "$GITHUB_OUTPUT"
  fi
else
  echo "Generating full diff from origin/main to HEAD"
  git diff origin/main...HEAD > pr.diff
  echo "incremental=false" >> "$GITHUB_OUTPUT"
fi

DIFF_SIZE=$(wc -c < pr.diff)
echo "Diff size: ${DIFF_SIZE} bytes"

if [ "$DIFF_SIZE" -gt 12288 ]; then
  head -c 12288 pr.diff > pr_truncated.diff
  printf '\n\n[... diff truncated due to size ...]' >> pr_truncated.diff
  mv pr_truncated.diff pr.diff
  echo "truncated=true" >> "$GITHUB_OUTPUT"
fi