Initial patterm project

internal/projectkey/projectkey.go

@@ -0,0 +1,32 @@
+// 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
+}