clipper/backend/media/audio_progress.go

package media

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"io"
	"math"
)

type GetAudioProgress struct {
	PercentComplete float32
	Peaks           []int16
}

type GetAudioProgressReader interface {
	Read() (GetAudioProgress, error)
	Close() error
}

// fetchAudioProgressReader accepts a byte stream containing little endian
// signed int16s and, given a target number of bins, emits a stream of peaks
// corresponding to each channel of the audio data.
type fetchAudioProgressReader struct {
	byteOrder      binary.ByteOrder
	framesExpected int64
	channels       int
	framesPerBin   int

	samples         []int16
	currPeaks       []int16
	currCount       int
	framesProcessed int64
	progress        chan GetAudioProgress
	errorChan       chan error
}

// TODO: validate inputs, debugging is confusing otherwise
func newGetAudioProgressReader(byteOrder binary.ByteOrder, framesExpected int64, channels, numBins int) *fetchAudioProgressReader {
	return &fetchAudioProgressReader{
		byteOrder:      byteOrder,
		channels:       channels,
		framesExpected: framesExpected,
		framesPerBin:   int(math.Ceil(float64(framesExpected) / float64(numBins))),
		samples:        make([]int16, 8_192),
		currPeaks:      make([]int16, channels),
		progress:       make(chan GetAudioProgress),
		errorChan:      make(chan error, 1),
	}
}

func (w *fetchAudioProgressReader) Abort(err error) {
	w.errorChan <- err
}

func (w *fetchAudioProgressReader) Close() error {
	close(w.progress)
	return nil
}

func (w *fetchAudioProgressReader) Read() (GetAudioProgress, error) {
	for {
		select {
		case progress, ok := <-w.progress:
			if !ok {
				return GetAudioProgress{}, io.EOF
			}
			return progress, nil
		case err := <-w.errorChan:
			return GetAudioProgress{}, fmt.Errorf("error waiting for progress: %v", err)
		}
	}
}

func (w *fetchAudioProgressReader) Write(p []byte) (int, error) {
	// expand our target slice if it is of insufficient size:
	numSamples := len(p) / SizeOfInt16
	if len(w.samples) < numSamples {
		w.samples = append(w.samples, make([]int16, numSamples-len(w.samples))...)
	}

	samples := w.samples[:numSamples]

	if err := binary.Read(bytes.NewReader(p), w.byteOrder, samples); err != nil {
		return 0, fmt.Errorf("error parsing samples: %v", err)
	}

	for i := 0; i < len(samples); i += w.channels {
		for j := 0; j < w.channels; j++ {
			samp := samples[i+j]
			if samp < 0 {
				samp = -samp
			}
			if samp > w.currPeaks[j] {
				w.currPeaks[j] = samp
			}
		}
		w.currCount++
		if w.currCount == w.framesPerBin {
			w.nextBin()
		}
	}

	w.framesProcessed += int64(len(samples) / w.channels)

	return len(p), nil
}

func (w *fetchAudioProgressReader) nextBin() {
	var progress GetAudioProgress
	// TODO: avoid an allocation?
	progress.Peaks = append(progress.Peaks, w.currPeaks...)
	progress.PercentComplete = (float32(w.framesProcessed) / float32(w.framesExpected)) * 100.0

	w.progress <- progress

	w.currCount = 0
	for i := 0; i < len(w.currPeaks); i++ {
		w.currPeaks[i] = 0
	}
	w.framesProcessed++
}