#!/usr/bin/env bash
# SessionStart hook: fast-forward current branch to origin default branch.
# Non-destructive: fetch always; ff-only merge only when branch has no own
# commits (covers stale main AND fresh worktree branched off stale main).
# Feature branches with own work are left untouched (just fetched).
set -u

# Must be inside a work tree.
git rev-parse --is-inside-work-tree >/dev/null 2>&1 || exit 0

branch=$(git rev-parse --abbrev-ref HEAD 2>/dev/null)
[ "$branch" = "HEAD" ] && { echo "git-autoupdate: detached HEAD, skip"; exit 0; }

# Dirty tree -> fetch only, never move HEAD.
if [ -n "$(git status --porcelain 2>/dev/null)" ]; then
  git fetch --quiet --all --prune 2>/dev/null
  echo "git-autoupdate: working tree dirty, fetched only (no update)"
  exit 0
fi

git fetch --quiet origin --prune 2>/dev/null || { echo "git-autoupdate: fetch failed"; exit 0; }

# Resolve origin default branch (e.g. origin/main).
base=$(git symbolic-ref --quiet refs/remotes/origin/HEAD 2>/dev/null | sed 's|^refs/remotes/||')
if [ -z "$base" ]; then
  git remote set-head origin -a >/dev/null 2>&1
  base=$(git symbolic-ref --quiet refs/remotes/origin/HEAD 2>/dev/null | sed 's|^refs/remotes/||')
fi
[ -z "$base" ] && base="origin/main"
git rev-parse --verify --quiet "$base" >/dev/null 2>&1 || { echo "git-autoupdate: no $base"; exit 0; }

ahead=$(git rev-list --count "$base"..HEAD 2>/dev/null)
behind=$(git rev-list --count HEAD.."$base" 2>/dev/null)

if [ "${ahead:-0}" -gt 0 ]; then
  echo "git-autoupdate: '$branch' has $ahead own commit(s); fetched, not moved (base $base fresh)"
  exit 0
fi

if [ "${behind:-0}" -eq 0 ]; then
  echo "git-autoupdate: '$branch' already up to date with $base"
  exit 0
fi

if git merge --ff-only "$base" >/dev/null 2>&1; then
  echo "git-autoupdate: '$branch' fast-forwarded to $base (+$behind)"
else
  echo "git-autoupdate: '$branch' could not ff to $base"
fi
exit 0