Added a Playlist struct.

internal/radio/playlist.go

@@ -0,0 +1,56 @@
+package radio
+
+import (
+	"os"
+	"strings"
+	"sync"
+)
+
+// Playlist holds a list of paths to a song files.
+type Playlist struct {
+	mut      sync.Mutex
+	filePath string
+	playlist []string
+	cur      int
+	repeat   bool
+}
+
+// NewPlaylist returns an instance of a Playlist struct with a loaded playlist.
+// Returns an error if failed to load a playlist file.
+func NewPlaylist(filePath string, repeat bool) (*Playlist, error) {
+	p := &Playlist{filePath: filePath, repeat: repeat}
+	return p, p.Load()
+}
+
+// Next returns the next song to play. Returns an empty string if repeat is
+// false and the end of a playlist was reached.
+func (p *Playlist) Next() (song string) {
+	p.mut.Lock()
+	defer p.mut.Unlock()
+	if p.cur == len(p.playlist) {
+		// If the end of a playlist was reached and repeat is set to true,
+		// then go back to the head of it, thus repeating it. Return an empty
+		// string otherwise.
+		if p.repeat {
+			p.cur = 0
+		} else {
+			return ""
+		}
+	}
+	song = p.playlist[p.cur]
+	p.cur++
+	return
+}
+
+// Load reads a playlist file and fills.
+func (p *Playlist) Load() error {
+	data, err := os.ReadFile(p.filePath)
+	if err != nil {
+		return err
+	}
+	p.mut.Lock()
+	p.playlist = strings.Split(string(data), "\n")
+	p.cur = 0
+	p.mut.Unlock()
+	return nil
+}