flacgo/meta/cuesheet.go

package meta

import (
	"bytes"
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"io/ioutil"
)

// A CueSheet describes how tracks are laid out within a FLAC stream.
//
// ref: https://www.xiph.org/flac/format.html#metadata_block_cuesheet
type CueSheet struct {
	// Media catalog number.
	MCN string
	// Number of lead-in samples. This field only has meaning for CD-DA cue
	// sheets; for other uses it should be 0. Refer to the spec for additional
	// information.
	NLeadInSamples uint64
	// Specifies if the cue sheet corresponds to a Compact Disc.
	IsCompactDisc bool
	// One or more tracks. The last track of a cue sheet is always the lead-out
	// track.
	Tracks []CueSheetTrack
}

// parseCueSheet reads and parses the body of a CueSheet metadata block.
func (block *Block) parseCueSheet() error {
	// Parse cue sheet.
	// 128 bytes: MCN.
	buf, err := readBytes(block.lr, 128)
	if err != nil {
		return unexpected(err)
	}
	cs := &CueSheet{
		MCN: stringFromSZ(buf),
	}
	block.Body = cs

	// 64 bits: NLeadInSamples.
	if err = binary.Read(block.lr, binary.BigEndian, &cs.NLeadInSamples); err != nil {
		return unexpected(err)
	}

	// 1 bit: IsCompactDisc.
	var x uint8
	if err := binary.Read(block.lr, binary.BigEndian, &x); err != nil {
		return unexpected(err)
	}
	// mask = 10000000
	if x&0x80 != 0 {
		cs.IsCompactDisc = true
	}

	// 7 bits and 258 bytes: reserved.
	// mask = 01111111
	if x&0x7F != 0 {
		return ErrInvalidPadding
	}
	lr := io.LimitReader(block.lr, 258)
	zr := zeros{r: lr}
	if _, err := io.Copy(ioutil.Discard, zr); err != nil {
		return err
	}

	// Parse cue sheet tracks.
	// 8 bits: (number of tracks)
	if err := binary.Read(block.lr, binary.BigEndian, &x); err != nil {
		return unexpected(err)
	}
	if x < 1 {
		return errors.New("meta.Block.parseCueSheet: at least one track required")
	}
	if cs.IsCompactDisc && x > 100 {
		return fmt.Errorf("meta.Block.parseCueSheet: number of CD-DA tracks (%d) exceeds 100", x)
	}
	cs.Tracks = make([]CueSheetTrack, x)
	// Each track number within a cue sheet must be unique; use uniq to keep
	// track.
	uniq := make(map[uint8]struct{})
	for i := range cs.Tracks {
		if err := block.parseTrack(cs, i, uniq); err != nil {
			return err
		}
	}

	return nil
}

// parseTrack parses the i:th cue sheet track, and ensures that its track number
// is unique.
func (block *Block) parseTrack(cs *CueSheet, i int, uniq map[uint8]struct{}) error {
	track := &cs.Tracks[i]
	// 64 bits: Offset.
	if err := binary.Read(block.lr, binary.BigEndian, &track.Offset); err != nil {
		return unexpected(err)
	}
	if cs.IsCompactDisc && track.Offset%588 != 0 {
		return fmt.Errorf("meta.Block.parseCueSheet: CD-DA track offset (%d) must be evenly divisible by 588", track.Offset)
	}

	// 8 bits: Num.
	if err := binary.Read(block.lr, binary.BigEndian, &track.Num); err != nil {
		return unexpected(err)
	}
	if _, ok := uniq[track.Num]; ok {
		return fmt.Errorf("meta.Block.parseCueSheet: duplicated track number %d", track.Num)
	}
	uniq[track.Num] = struct{}{}
	if track.Num == 0 {
		return errors.New("meta.Block.parseCueSheet: invalid track number (0)")
	}
	isLeadOut := i == len(cs.Tracks)-1
	if cs.IsCompactDisc {
		if !isLeadOut {
			if track.Num >= 100 {
				return fmt.Errorf("meta.Block.parseCueSheet: CD-DA track number (%d) exceeds 99", track.Num)
			}
		} else {
			if track.Num != 170 {
				return fmt.Errorf("meta.Block.parseCueSheet: invalid lead-out CD-DA track number; expected 170, got %d", track.Num)
			}
		}
	} else {
		if isLeadOut && track.Num != 255 {
			return fmt.Errorf("meta.Block.parseCueSheet: invalid lead-out track number; expected 255, got %d", track.Num)
		}
	}

	// 12 bytes: ISRC.
	buf, err := readBytes(block.lr, 12)
	if err != nil {
		return unexpected(err)
	}
	track.ISRC = stringFromSZ(buf)

	// 1 bit: IsAudio.
	var x uint8
	if err = binary.Read(block.lr, binary.BigEndian, &x); err != nil {
		return unexpected(err)
	}
	// mask = 10000000
	if x&0x80 == 0 {
		track.IsAudio = true
	}

	// 1 bit: HasPreEmphasis.
	// mask = 01000000
	if x&0x40 != 0 {
		track.HasPreEmphasis = true
	}

	// 6 bits and 13 bytes: reserved.
	// mask = 00111111
	if x&0x3F != 0 {
		return ErrInvalidPadding
	}
	lr := io.LimitReader(block.lr, 13)
	zr := zeros{r: lr}
	_, err = io.Copy(ioutil.Discard, zr)
	if err != nil {
		return err
	}

	// Parse indicies.
	// 8 bits: (number of indicies)
	if err = binary.Read(block.lr, binary.BigEndian, &x); err != nil {
		return unexpected(err)
	}
	if x < 1 {
		if !isLeadOut {
			return errors.New("meta.Block.parseCueSheet: at least one track index required")
		}
		// Lead-out track has no track indices to parse; return early.
		return nil
	}
	track.Indicies = make([]CueSheetTrackIndex, x)
	for i := range track.Indicies {
		index := &track.Indicies[i]
		// 64 bits: Offset.
		if err = binary.Read(block.lr, binary.BigEndian, &index.Offset); err != nil {
			return unexpected(err)
		}

		// 8 bits: Num.
		if err = binary.Read(block.lr, binary.BigEndian, &index.Num); err != nil {
			return unexpected(err)
		}

		// 3 bytes: reserved.
		lr = io.LimitReader(block.lr, 3)
		zr = zeros{r: lr}
		_, err = io.Copy(ioutil.Discard, zr)
		if err != nil {
			return err
		}
	}
	return nil
}

// stringFromSZ converts the provided byte slice to a string after terminating
// it at the first occurrence of a NULL character.
func stringFromSZ(buf []byte) string {
	pos := bytes.IndexByte(buf, 0)
	if pos == -1 {
		return string(buf)
	}
	return string(buf[:pos])
}

// CueSheetTrack contains the start offset of a track and other track specific
// metadata.
type CueSheetTrack struct {
	// Track offset in samples, relative to the beginning of the FLAC audio
	// stream.
	Offset uint64
	// Track number; never 0, always unique.
	Num uint8
	// International Standard Recording Code; empty string if not present.
	//
	// ref: http://isrc.ifpi.org/
	ISRC string
	// Specifies if the track contains audio or data.
	IsAudio bool
	// Specifies if the track has been recorded with pre-emphasis
	HasPreEmphasis bool
	// Every track has one or more track index points, except for the lead-out
	// track which has zero. Each index point specifies a position within the
	// track.
	Indicies []CueSheetTrackIndex
}

// A CueSheetTrackIndex specifies a position within a track.
type CueSheetTrackIndex struct {
	// Index point offset in samples, relative to the track offset.
	Offset uint64
	// Index point number; subsequently incrementing by 1 and always unique
	// within a track.
	Num uint8
}