style: add comments to the public methods and members in the root package.

song.go

@@ -4,6 +4,12 @@ import (
 	"errors"
 )
 
+// Song includes a Score(the arrangement of notes in the song in one or more
+// tracks) and a Patch (the list of one or more instruments). Additionally, BPM
+// and RowsPerBeat fields set how fast the song should be played. Currently, BPM
+// is an integer as it offers already quite much granularity for controlling the
+// playback speed, but this could be changed to a floating point in future if
+// finer adjustments are necessary.
 type Song struct {
 	BPM         int
 	RowsPerBeat int
@@ -11,16 +17,20 @@ type Song struct {
 	Patch       Patch
 }
 
+// Copy makes a deep copy of a Score.
 func (s *Song) Copy() Song {
 	return Song{BPM: s.BPM, RowsPerBeat: s.RowsPerBeat, Score: s.Score.Copy(), Patch: s.Patch.Copy()}
 }
 
+// Assuming 44100 Hz playback speed, return the number of samples of each row of
+// the song.
 func (s *Song) SamplesPerRow() int {
 	return 44100 * 60 / (s.BPM * s.RowsPerBeat)
 }
 
-// TBD: Where shall we put methods that work on pure domain types and have no dependencies
-// e.g. Validate here
+// Validate checks if the Song looks like a valid song: BPM > 0, one or more
+// tracks, score uses less than or equal number of voices than patch. Not used
+// much so we could probably get rid of this function.
 func (s *Song) Validate() error {
 	if s.BPM < 1 {
 		return errors.New("BPM should be > 0")