#!/usr/bin/env bash
# gstack-wtree — print a working-tree CONTENT fingerprint (a git tree hash).
#
# Builds a temp index, stages the full working tree into it (`git add -A`, so
# .gitignore'd scratch stays out and UNTRACKED source is included), and prints
# `git write-tree` of that index. Properties that make this the right
# staleness fingerprint, vs `git rev-parse HEAD^{tree}`:
#
#   - Committing identical content does NOT change the fingerprint, so a
#     record made on a dirty tree stays valid after the exact same content is
#     committed (the /ship Step 5 -> Step 16 case).
#   - Untracked new source files DO change the fingerprint, so "tests passed"
#     can't stay FRESH after a new file appears.
#   - Rebase/amend/squash that preserve content do not change it.
#
# Performance: the temp index is seeded by COPYING the real index (git writes
# it atomically via rename, so the copy is a consistent snapshot). That
# preserves the stat cache, so `git add -A` only re-hashes files whose stat
# changed — measured 40x faster than a `read-tree HEAD` seed, which zeroes
# stat data and forces a full re-hash of every tracked file. Both seeds
# produce the identical write-tree hash. Fallback: `read-tree HEAD` when the
# index copy is unavailable (fresh repo, exotic index).
#
# The real repo index is never touched. Staged blobs land in the object store
# as unreachable objects and get gc'd like stash churn (note: this means the
# CONTENT of untracked, non-ignored files enters .git/objects until gc — the
# same property `git stash -u` has). Exit 1 outside a git repo or in a repo
# with no commits — callers treat that as "no fingerprint".
set -euo pipefail

TOP=$(git rev-parse --show-toplevel 2>/dev/null) || exit 1
# Resolve the REAL index path BEFORE exporting GIT_INDEX_FILE — with the env
# var set, `git rev-parse --git-path index` returns the temp index itself and
# the stat-cache seed silently self-copies into a dead fast path.
REAL_INDEX=$(git -C "$TOP" rev-parse --git-path index 2>/dev/null || true)
TMPIDX=$(mktemp "${TMPDIR:-/tmp}/gstack-wtree-XXXXXX")
trap 'rm -f "$TMPIDX"' EXIT
export GIT_INDEX_FILE="$TMPIDX"
# Resolve relative --git-path output against the repo root.
case "$REAL_INDEX" in
  ""|/*) ;;
  *) REAL_INDEX="$TOP/$REAL_INDEX" ;;
esac
if [ -n "$REAL_INDEX" ] && [ -f "$REAL_INDEX" ] && cp "$REAL_INDEX" "$TMPIDX" 2>/dev/null; then
  : # stat-cache-preserving seed
else
  git -C "$TOP" read-tree HEAD 2>/dev/null || exit 1
fi
git -C "$TOP" add -A 2>/dev/null || exit 1
git -C "$TOP" write-tree 2>/dev/null