// Package projectkey turns a project directory into the stable key
// patterm uses to name its scratchpad directory under $XDG_DATA_HOME.
// SPEC §3.
//
// Two invocations from the same realpath must produce the same key.
// The key is only used as a directory name on disk — there is no
// daemon to look up.
package projectkey

import (
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"path/filepath"
)

// Key derives the 16-char hex key from the realpath of dir.
func Key(dir string) (string, error) {
	abs, err := filepath.Abs(dir)
	if err != nil {
		return "", fmt.Errorf("projectkey: abs %q: %w", dir, err)
	}
	resolved, err := filepath.EvalSymlinks(abs)
	if err != nil {
		// Directory may not exist yet; fall back to the absolute path.
		// The key stays stable; downstream code will fail later when it
		// tries to chdir or write into the dir.
		resolved = abs
	}
	sum := sha256.Sum256([]byte(resolved))
	return hex.EncodeToString(sum[:8]), nil
}