song: struct for /api/song/%d

This also includes tests against a sample response from the
https://monster-siren.hypergryph.com/api/song/048794 endpoint.

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

song.go

@@ -0,0 +1,55 @@
+package monstersirenfetch
+
+import (
+	"bytes"
+	"encoding/json"
+)
+
+// SongResponse is the response of /api/song/%d.
+type SongResponse Response[Song]
+
+// Song holds the metadata of a song.
+type Song struct {
+	CID        StringInt `json:"cid"`
+	Name       string    `json:"name"`
+	AlbumCID   StringInt `json:"albumCid"`
+	SourceURL  string    `json:"sourceUrl,omitempty"`
+	LyricURL   string    `json:"lyricUrl,omitempty"`
+	MvURL      string    `json:"mvUrl,omitempty"`
+	MvCoverURL string    `json:"mvCoverUrl,omitempty"`
+	Artists    []string  `json:"artists"`
+}
+
+// IsFull returns whether the metadata held by [Song] is considered full (originating from a [SongResponse]).
+func (s *Song) IsFull() bool { return s.SourceURL != "" }
+
+// songDirect is [Song] without its MarshalJSON method.
+type songDirect Song
+
+// songNullable is [Song] with corresponding nullable string fields.
+type songNullable struct {
+	CID        StringInt      `json:"cid"`
+	Name       string         `json:"name"`
+	AlbumCID   StringInt      `json:"albumCid"`
+	SourceURL  string         `json:"sourceUrl"`
+	LyricURL   NullableString `json:"lyricUrl"`
+	MvURL      NullableString `json:"mvUrl"`
+	MvCoverURL NullableString `json:"mvCoverUrl"`
+	Artists    []string       `json:"artists"`
+}
+
+func (s *Song) MarshalJSON() (data []byte, err error) {
+	buf := new(bytes.Buffer)
+	e := json.NewEncoder(buf)
+	e.SetEscapeHTML(false)
+
+	if !s.IsFull() {
+		err = e.Encode((*songDirect)(s))
+		data = buf.Bytes()
+		return
+	}
+
+	return json.Marshal(&songNullable{s.CID, s.Name, s.AlbumCID, s.SourceURL,
+		NullableString(s.LyricURL), NullableString(s.MvURL), NullableString(s.MvCoverURL),
+		s.Artists})
+}

song_test.go

@@ -0,0 +1,23 @@
+package monstersirenfetch_test
+
+import (
+	_ "embed"
+	"testing"
+
+	. "git.gensokyo.uk/yonah/monstersirenfetch"
+)
+
+//go:embed testdata/song.json
+var songJSON []byte
+
+func TestSong(t *testing.T) {
+	checkJSONRoundTrip(t, SongResponse{Data: 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",
+		MvURL:     "", MvCoverURL: "",
+		Artists: []string{"塞壬唱片-MSR"},
+	}}, songJSON)
+}

songs.go

@@ -8,11 +8,3 @@ type SongsData = struct {
 	List     []Song `json:"list"`
 	Autoplay string `json:"autoplay"`
 }
-
-// Song represents the metadata of a song.
-type Song struct {
-	CID      StringInt `json:"cid"`
-	Name     string    `json:"name"`
-	AlbumCID StringInt `json:"albumCid"`
-	Artists  []string  `json:"artists"`
-}

testdata/song.json

@@ -0,0 +1 @@
+{"code":0,"msg":"","data":{"cid":"048794","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","mvUrl":null,"mvCoverUrl":null,"artists":["塞壬唱片-MSR"]}}