Added a SongList and Song_ types. An underscore is temporary and is because there is an another Song type in an icecast.go file.

internal/radio/song.go

@@ -0,0 +1,70 @@
+package radio
+
+import (
+	"encoding/json"
+	"sync"
+	"time"
+)
+
+// Song_ stores artist and title of a song, a timestamp of when it started, and
+// a maximum number of listeners.
+type Song_ struct {
+	Artist       string
+	Title        string
+	MaxListeners int
+	StartAt      time.Time
+}
+
+func (s *Song_) ArtistTitle() string {
+	return s.Artist + " - " + s.Title
+}
+
+func (s *Song_) MarshalJSON() ([]byte, error) {
+	return json.Marshal(&struct {
+		Artist       string `json:"artist"`
+		Title        string `json:"title"`
+		MaxListeners int    `json:"listeners"`
+		StartAt      string `json:"start_at"`
+	}{
+		Artist:       s.Artist,
+		Title:        s.Title,
+		MaxListeners: s.MaxListeners,
+		StartAt:      s.StartAt.UTC().Format(time.RFC3339)})
+}
+
+type SongList struct {
+	mut        sync.Mutex
+	current    Song_
+	lastSongs  []Song_
+	listMaxLen int
+}
+
+func NewSongList(length int) *SongList {
+	return &SongList{listMaxLen: length, lastSongs: make([]Song_, 0, length)}
+}
+
+// Add a new song that is currently playing and update a list.
+func (sl *SongList) Add(newSong Song_) {
+	sl.mut.Lock()
+	defer sl.mut.Unlock()
+	if len(sl.lastSongs) == sl.listMaxLen {
+		sl.lastSongs = append(sl.lastSongs[1:], sl.current)
+	} else {
+		sl.lastSongs = append(sl.lastSongs, sl.current)
+	}
+	sl.current = newSong
+}
+
+// Current returns a current playing song.
+func (sl *SongList) Current() *Song_ {
+	sl.mut.Lock()
+	defer sl.mut.Unlock()
+	return &sl.current
+}
+
+// List returns a list of lastly played songs.
+func (sl *SongList) List() []Song_ {
+	sl.mut.Lock()
+	defer sl.mut.Unlock()
+	return sl.lastSongs
+}