song: zero fields for variants

Once again, helps remove code duplication and be more future-proof in
case any more variants are uncovered.

Signed-off-by: Yonah <contrib@gensokyo.uk>

song.go

@@ -13,7 +13,6 @@ import (
 type SongResponse Response[Song]
 
 // Song holds the metadata of a song.
-// Fields marked with omitempty are only populated when the IsFull method returns true.
 type Song struct {
 	CID        StringInt `json:"cid"`
 	Name       string    `json:"name"`
@@ -25,9 +24,53 @@ type Song struct {
 	Artists    []string  `json:"artists"`
 }
 
-// IsFull returns whether the metadata held by [Song] is considered full (originating from a [SongResponse]).
+const (
+	// SongVariantCurrent copies the current variant as-is.
+	SongVariantCurrent = iota
+	// SongVariantFull leaves all fields intact in the copy.
+	// This variant is returned by /api/song/%d.
+	SongVariantFull
+	// SongVariantBase zeroes [Song.SourceURL], [Song.LyricURL], [Song.MvURL], [Song.MvCoverURL].
+	// This variant is included in /api/songs.
+	SongVariantBase
+)
+
+// IsFull returns whether [Song] is considered to be its [SongVariantFull] variant.
 func (s *Song) IsFull() bool { return s.SourceURL != "" }
 
+// Copy makes a copy of [Song].
+// For a [Song] where the IsFull method returns true, Copy zeroes fields to convert the copy into other variants.
+// For [Song] where IsFull returns false, any variant other than [SongVariantCurrent] is undefined.
+func (s *Song) Copy(variant int) *Song {
+	if s == nil {
+		return nil
+	}
+
+	v := *s
+	if variant == SongVariantCurrent {
+		return &v
+	}
+	if !s.IsFull() {
+		return nil
+	}
+
+	switch variant {
+	case SongVariantFull:
+		break
+
+	case SongVariantBase:
+		s.SourceURL = ""
+		s.LyricURL = ""
+		s.MvURL = ""
+		s.MvCoverURL = ""
+
+	default:
+		return nil
+	}
+
+	return s
+}
+
 // Enrich populates the remaining fields of a non-full [Song].
 func (s *Song) Enrich(ctx context.Context, n Net) error {
 	if s == nil || s.IsFull() {

song_test.go

@@ -22,6 +22,66 @@ func TestSong(t *testing.T) {
 		LyricURL:  "https://web.hycdn.cn/siren/lyric/20240709/4a10c70629b68a187fdbef4a27bd32d8.lrc",
 		Artists:   []string{"塞壬唱片-MSR"},
 	}}, songJSON)
+
+	t.Run("copy", func(t *testing.T) {
+		testCases := []struct {
+			name    string
+			a       *Song
+			variant int
+			want    *Song
+		}{
+			{"nil", nil, SongVariantBase, nil},
+			{"current", new(Song), SongVariantCurrent, new(Song)},
+			{"full guard", new(Song), SongVariantFull, nil},
+
+			{"current full", &Song{
+				SourceURL: "\x00",
+			}, SongVariantCurrent, &Song{
+				SourceURL: "\x00",
+			}},
+
+			{"oob", &Song{
+				SourceURL: "\x00",
+			}, 0xbad, nil},
+
+			{"full", &Song{
+				CID:       48794,
+				Name:      "Warm and Small Light",
+				AlbumCID:  6660,
+				SourceURL: "https://res01.hycdn.cn/04ce5de54bb52eb85008644d541d40fa/68CA0442/siren/audio/20240709/a7f650238eaefc9c30a9627d7f78d819.wav",
+				LyricURL:  "https://web.hycdn.cn/siren/lyric/20240709/4a10c70629b68a187fdbef4a27bd32d8.lrc",
+				Artists:   []string{"塞壬唱片-MSR"},
+			}, SongVariantFull, &Song{
+				CID:       48794,
+				Name:      "Warm and Small Light",
+				AlbumCID:  6660,
+				SourceURL: "https://res01.hycdn.cn/04ce5de54bb52eb85008644d541d40fa/68CA0442/siren/audio/20240709/a7f650238eaefc9c30a9627d7f78d819.wav",
+				LyricURL:  "https://web.hycdn.cn/siren/lyric/20240709/4a10c70629b68a187fdbef4a27bd32d8.lrc",
+				Artists:   []string{"塞壬唱片-MSR"},
+			}},
+
+			{"base", &Song{
+				CID:       48794,
+				Name:      "Warm and Small Light",
+				AlbumCID:  6660,
+				SourceURL: "https://res01.hycdn.cn/04ce5de54bb52eb85008644d541d40fa/68CA0442/siren/audio/20240709/a7f650238eaefc9c30a9627d7f78d819.wav",
+				LyricURL:  "https://web.hycdn.cn/siren/lyric/20240709/4a10c70629b68a187fdbef4a27bd32d8.lrc",
+				Artists:   []string{"塞壬唱片-MSR"},
+			}, SongVariantBase, &Song{
+				CID:      48794,
+				Name:     "Warm and Small Light",
+				AlbumCID: 6660,
+				Artists:  []string{"塞壬唱片-MSR"},
+			}},
+		}
+		for _, tc := range testCases {
+			t.Run(tc.name, func(t *testing.T) {
+				if got := tc.a.Copy(tc.variant); !reflect.DeepEqual(got, tc.want) {
+					t.Errorf("Copy: %#v, want %#v", got, tc.want)
+				}
+			})
+		}
+	})
 }
 
 func TestSongEnrich(t *testing.T) {