wip on autotagging

backend/autotag/groupkey.go

@@ -0,0 +1,47 @@
+// Package autotag provides the shared primitives used by the
+// MusicBrainz autotagger: deterministic album-group keys, scoring,
+// and (in later phases) MB orchestration.
+package autotag
+
+import (
+	"crypto/sha1" //nolint:gosec // non-crypto deterministic grouping key.
+	"encoding/hex"
+	"path/filepath"
+	"strconv"
+	"strings"
+)
+
+// GroupKey returns a deterministic album-group identifier for the
+// file at filePath belonging to libraryID, carrying the given disc
+// number.
+//
+// The key is a lower-case hex SHA-1 over
+//
+//	libraryID || 0 || normalized_parent_dir || 0 || disc_number
+//
+// where the parent directory is lower-cased.  The folder is taken
+// as the album boundary — including the album tag string would
+// fragment albums whose tracks carry slightly different tags
+// (`Abbey Road` vs `Abbey Road (Remastered 2009)`, etc.).  The
+// album name is still surfaced in `tagging_items.album_name` for
+// the review UI; it just doesn't decide grouping.
+//
+// Using SHA-1 matches the codebase's existing non-crypto
+// deterministic-key convention; collision risk at album-group
+// cardinality is irrelevant.
+func GroupKey(
+	libraryID int64,
+	filePath string,
+	discNumber int,
+) string {
+	parentDir := strings.ToLower(filepath.Dir(filePath))
+
+	h := sha1.New() //nolint:gosec // see package doc — grouping only.
+	h.Write([]byte(strconv.FormatInt(libraryID, 10)))
+	h.Write([]byte{0})
+	h.Write([]byte(parentDir))
+	h.Write([]byte{0})
+	h.Write([]byte(strconv.Itoa(discNumber)))
+
+	return hex.EncodeToString(h.Sum(nil))
+}