hajimehoshi-ebiten/audio/audio.go

// Copyright 2015 Hajime Hoshi
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package audio provides audio players.
//
// This package must be used within an Ebitengine game; it does not work standalone, as the
// audio device is initialized only after the game starts running. To play audio outside an
// Ebitengine game, use Oto ([github.com/ebitengine/oto/v3]) directly.
//
// The stream format must be 16-bit little endian or 32-bit float little endian, and 2 channels. The format is as follows:
//
//	[data]      = [sample 1] [sample 2] [sample 3] ...
//	[sample *]  = [channel 1] [channel 2]
//	[channel *] = [byte 1] [byte 2] ...
//
// A channel is 2 bytes in the 16-bit integer format, and 4 bytes in the 32-bit float format.
//
// An audio context (audio.Context object) has a sample rate you can specify
// and all streams you want to play must have the same sample rate.
//
// An audio context can generate 'players' (audio.Player objects),
// and you can play sound by calling Play function of players.
// When multiple players play, mixing is automatically done.
// Note that too many players may cause distortion.
//
// For the simplest example to play sound, see the wav package in the examples.
package audio

import (
	"bytes"
	"errors"
	"fmt"
	"io"
	"reflect"
	"runtime"
	"sync"
	"time"

	"github.com/hajimehoshi/ebiten/v2/audio/internal/convert"
	"github.com/hajimehoshi/ebiten/v2/internal/hook"
)

const (
	channelCount           = 2
	bitDepthInBytesInt16   = 2
	bitDepthInBytesFloat32 = 4
)

// A Context represents a current state of audio.
//
// At most one Context object can exist in one process.
// This means only one constant sample rate is valid in your one application.
//
// For a typical usage example, see examples/wav/main.go.
type Context struct {
	playerFactory *playerFactory

	sampleRate int
	err        error
	ready      bool

	playingPlayers map[*playerImpl]struct{}

	m sync.Mutex
	// cond is signalled under m when a player is added to playingPlayers or when the context is
	// disposed. The goroutine updating the players waits on it while playingPlayers is empty.
	cond *sync.Cond

	// disposed reports that the goroutine updating the players must exit. A context lives until the
	// process exits, so this is set only when a test discards the context.
	disposed bool

	// updater tracks the goroutine updating the players, so that a test discarding the context can wait
	// for the goroutine to exit.
	updater sync.WaitGroup
}

var (
	theContext     *Context
	theContextLock sync.Mutex
)

// NewContext creates a new audio context with the given sample rate.
//
// sampleRate specifies the number of samples that should be played during one second.
// Usual numbers are 44100 or 48000. One context has only one sample rate. You cannot play multiple audio
// sources with different sample rates at the same time.
//
// NewContext panics when sampleRate is not positive or an audio context is already created.
func NewContext(sampleRate int) *Context {
	if sampleRate <= 0 {
		panic(fmt.Sprintf("audio: sample rate must be positive but was %d", sampleRate))
	}

	theContextLock.Lock()
	defer theContextLock.Unlock()

	if theContext != nil {
		panic("audio: context is already created")
	}

	c := &Context{
		sampleRate:     sampleRate,
		playerFactory:  newPlayerFactory(sampleRate),
		playingPlayers: map[*playerImpl]struct{}{},
	}
	c.cond = sync.NewCond(&c.m)
	theContext = c

	h := getHook()
	h.OnSuspendAudio(func() error {
		if err := c.playerFactory.suspend(); err != nil {
			return err
		}
		if err := c.onSuspend(); err != nil {
			return err
		}
		return nil
	})
	h.OnResumeAudio(func() error {
		if err := c.playerFactory.resume(); err != nil {
			return err
		}
		if err := c.onResume(); err != nil {
			return err
		}
		return nil
	})

	h.AppendHookOnBeforeUpdateWithVMGuestInfo(func(vmGuest bool) error {
		var err error
		theContextLock.Lock()
		if theContext != nil {
			err = theContext.error()
		}
		theContextLock.Unlock()
		if err != nil {
			return err
		}

		// Create the audio device here, on the first update after the UI backend is
		// initialized, rather than when a player is first touched. Deferring the creation
		// keeps the device from being created before the environment is known (#969, #970,
		// #2715, #3438). This also initializes the device when there is no player and the
		// program waits for IsReady() to be true.
		ready, err := c.playerFactory.initContextIfNeeded(vmGuest)
		if err != nil {
			return err
		}
		if ready != nil {
			go func() {
				<-ready
				c.setReady()
			}()
		}
		return nil
	})

	// In the current Ebitengine implementation, update might not be called when the window is in the background (#3154).
	// In this case, an audio player's position is not updated correctly with AppendHookOnBeforeUpdateWithVMGuestInfo.
	// Use a distinct goroutine to update the player states.
	c.updater.Go(func() {
		for {
			// Block while there is no player to update, so that an idle context does not wake
			// up periodically. The sweep itself can remove the last player, so the check is
			// repeated after every sweep.
			if !c.waitForPlayingPlayers() {
				return
			}
			// A failure is local to the player which reported it. Record it and keep updating
			// the other players (#3647).
			if err := c.updatePlayers(); err != nil {
				c.setError(err)
			}
			time.Sleep(time.Second / 100)
		}
	})

	return c
}

// CurrentContext returns the current context or nil if there is no context.
func CurrentContext() *Context {
	theContextLock.Lock()
	c := theContext
	theContextLock.Unlock()
	return c
}

func (c *Context) setError(err error) {
	c.m.Lock()
	defer c.m.Unlock()
	c.err = errors.Join(c.err, err)
}

func (c *Context) error() error {
	c.m.Lock()
	defer c.m.Unlock()
	if c.err != nil {
		return fmt.Errorf("audio: context error: %w", c.err)
	}
	if err := c.playerFactory.error(); err != nil {
		return fmt.Errorf("audio: context error: %w", err)
	}
	return nil
}

func (c *Context) setReady() {
	c.m.Lock()
	c.ready = true
	c.m.Unlock()
}

// waitForPlayingPlayers blocks until at least one player is registered as playing, and reports
// whether the players should be updated. It returns false once the context is disposed.
func (c *Context) waitForPlayingPlayers() bool {
	c.m.Lock()
	defer c.m.Unlock()
	for len(c.playingPlayers) == 0 && !c.disposed {
		c.cond.Wait()
	}
	return !c.disposed
}

func (c *Context) addPlayingPlayer(p *playerImpl) {
	c.m.Lock()
	defer c.m.Unlock()
	c.playingPlayers[p] = struct{}{}
	c.cond.Signal()

	// An uncomparable ident is out of the duplication check (#3039).
	ident := p.sourceIdent()
	if !reflect.ValueOf(ident).Comparable() {
		return
	}

	// Check the source duplication. The players other than p have been checked against each
	// other when they were added, so it is enough to compare p with them.
	for playing := range c.playingPlayers {
		if playing == p {
			continue
		}
		playingIdent := playing.sourceIdent()
		if !reflect.ValueOf(playingIdent).Comparable() {
			continue
		}
		if playingIdent == ident {
			c.err = errors.Join(c.err, errors.New("audio: the same source must not be used by multiple Player objects"))
			return
		}
	}
}

func (c *Context) removePlayingPlayer(p *playerImpl) {
	c.m.Lock()
	delete(c.playingPlayers, p)
	c.m.Unlock()
}

func (c *Context) onSuspend() error {
	// A Context must not call playerImpl's functions with a lock, or this causes a deadlock (#2737).
	// Copy the playerImpls and iterate them without a lock.
	var players []*playerImpl
	c.m.Lock()
	players = make([]*playerImpl, 0, len(c.playingPlayers))
	for p := range c.playingPlayers {
		players = append(players, p)
	}
	c.m.Unlock()

	for _, p := range players {
		if err := p.Err(); err != nil {
			return err
		}
		p.onContextSuspended()
	}

	return nil
}

func (c *Context) onResume() error {
	// A Context must not call playerImpl's functions with a lock, or this causes a deadlock (#2737).
	// Copy the playerImpls and iterate them without a lock.
	var players []*playerImpl
	c.m.Lock()
	players = make([]*playerImpl, 0, len(c.playingPlayers))
	for p := range c.playingPlayers {
		players = append(players, p)
	}
	c.m.Unlock()

	for _, p := range players {
		if err := p.Err(); err != nil {
			return err
		}
		p.onContextResumed()
	}

	return nil
}

func (c *Context) updatePlayers() error {
	// A Context must not call playerImpl's functions with a lock, or this causes a deadlock (#2737).
	// Copy the playerImpls and iterate them without a lock.
	var players []*playerImpl
	c.m.Lock()
	players = make([]*playerImpl, 0, len(c.playingPlayers))
	for p := range c.playingPlayers {
		players = append(players, p)
	}
	c.m.Unlock()

	// In the current implementation, players cannot remove themselves from the context when the underlying player stops by itself.
	// The underlying player can become paused after finishing playback,
	// but there is no way to notify this to players so far.
	// Instead, let's check the states proactively every frame.
	//
	// A failing player must not stop the sweep: the other players still need their positions
	// updated and their pending plays started (#3647). Collect the errors and go on.
	var errs []error
	for _, p := range players {
		if err := p.Err(); err != nil {
			errs = append(errs, err)
			// A player whose device player failed does not play again. Drop it from the playing
			// players so that its error is reported once, not on every sweep.
			c.removePlayingPlayer(p)
			continue
		}
		// Start playing if Play was requested before the audio device was created.
		if err := p.startIfPending(); err != nil {
			errs = append(errs, err)
			continue
		}
		p.updatePosition()
		// The player itself decides whether it is removed, so that a player restarted between
		// the check and the removal is not dropped from the context.
		p.removeFromContextIfNotPlaying()
	}

	return errors.Join(errs...)
}

// IsReady returns a boolean value indicating whether the audio is ready or not.
//
// On some browsers, user interaction like click or pressing keys is required to start audio.
func (c *Context) IsReady() bool {
	c.m.Lock()
	defer c.m.Unlock()
	return c.ready
}

// SampleRate returns the sample rate.
func (c *Context) SampleRate() int {
	return c.sampleRate
}

// Player is an audio player which has one stream.
//
// Even when all references to a Player object are gone,
// the object is not GCed until the player finishes playing.
// This means that if a Player plays an infinite stream,
// the object is never GCed unless [Player.Pause] or [Player.PauseAndStopReading] is called.
type Player struct {
	p       *playerImpl
	cleanup runtime.Cleanup
}

// NewPlayer creates a new player with the given stream.
//
// src's format must be linear PCM (signed 16-bit little endian, 2 channel stereo)
// without a header (e.g. RIFF header).
// The sample rate must be same as that of the audio context.
//
// The player is seekable when src is io.Seeker.
// Attempt to seek the player that is not io.Seeker causes panic.
//
// Note that the given src can't be shared with other Player objects.
//
// NewPlayer does not touch src. The player asks src for its current position when it
// first uses src, at [Player.Play] or [Player.SetPosition].
//
// A Player doesn't close src even if src implements io.Closer.
// Closing the source is src owner's responsibility.
//
// For new code, NewPlayerF32 is preferable to NewPlayer, since Ebitengine will treat only 32bit float audio internally in the future.
//
// A Player for 16bit integer must be used with 16bit integer version of audio APIs, like vorbis.DecodeWithoutResampling or audio.NewInfiniteLoop, not vorbis.DecodeF32 or audio.NewInfiniteLoopF32.
func (c *Context) NewPlayer(src io.Reader) (*Player, error) {
	_, seekable := src.(io.Seeker)
	f32Src := convert.NewFloat32BytesReaderFromInt16BytesReader(src)
	pi, err := c.playerFactory.newPlayer(c, f32Src, seekable, src, bitDepthInBytesFloat32)
	if err != nil {
		return nil, err
	}

	p := &Player{p: pi}

	p.cleanup = runtime.AddCleanup(p, (*playerImpl).finalize, p.p)

	return p, nil
}

// NewPlayerF32 creates a new player with the given stream.
//
// src's format must be linear PCM (32bit float, little endian, 2 channel stereo)
// without a header (e.g. RIFF header).
// The sample rate must be same as that of the audio context.
//
// The player is seekable when src is io.Seeker.
// Attempt to seek the player that is not io.Seeker causes panic.
//
// Note that the given src can't be shared with other Player objects.
//
// NewPlayerF32 does not touch src. The player asks src for its current position when it
// first uses src, at [Player.Play] or [Player.SetPosition].
//
// A Player doesn't close src even if src implements io.Closer.
// Closing the source is src owner's responsibility.
//
// For new code, NewPlayerF32 is preferable to NewPlayer, since Ebitengine will treat only 32bit float audio internally in the future.
//
// A Player for 32bit float must be used with 32bit float version of audio APIs, like vorbis.DecodeF32 or audio.NewInfiniteLoopF32, not vorbis.DecodeWithoutResampling or audio.NewInfiniteLoop.
func (c *Context) NewPlayerF32(src io.Reader) (*Player, error) {
	_, seekable := src.(io.Seeker)
	pi, err := c.playerFactory.newPlayer(c, src, seekable, src, bitDepthInBytesFloat32)
	if err != nil {
		return nil, err
	}

	p := &Player{p: pi}
	p.cleanup = runtime.AddCleanup(p, (*playerImpl).finalize, p.p)

	return p, nil
}

// NewPlayer creates a new player with the given stream.
//
// Deprecated: as of v2.2. Use (*Context).NewPlayer instead.
func NewPlayer(context *Context, src io.Reader) (*Player, error) {
	return context.NewPlayer(src)
}

// NewPlayerFromBytes creates a new player with the given bytes.
//
// As opposed to NewPlayer, you don't have to care if src is already used by another player or not.
// src can be shared by multiple players.
//
// The format of src should be same as noted at NewPlayer.
func (c *Context) NewPlayerFromBytes(src []byte) *Player {
	p, err := c.NewPlayer(bytes.NewReader(src))
	if err != nil {
		// Errors should never happen.
		panic(fmt.Sprintf("audio: NewPlayerFromBytes failed: %v", err))
	}
	return p
}

// NewPlayerF32FromBytes creates a new player with the given bytes.
//
// As opposed to NewPlayerF32, you don't have to care if src is already used by another player or not.
// src can be shared by multiple players.
//
// The format of src should be same as noted at NewPlayerF32.
func (c *Context) NewPlayerF32FromBytes(src []byte) *Player {
	p, err := c.NewPlayerF32(bytes.NewReader(src))
	if err != nil {
		// Errors should never happen.
		panic(fmt.Sprintf("audio: NewPlayerF32FromBytes failed: %v", err))
	}
	return p
}

// NewPlayerFromBytes creates a new player with the given bytes.
//
// Deprecated: as of v2.2. Use (*Context).NewPlayerFromBytes instead.
func NewPlayerFromBytes(context *Context, src []byte) *Player {
	return context.NewPlayerFromBytes(src)
}

// finalize closes the player when its Player wrapper is no longer reachable.
//
// A Player becomes unreachable as early as the moment one of its methods reads the playerImpl out
// of it, so finalize can run while that method is still in flight. Every Player method keeps its
// receiver alive with runtime.KeepAlive until the delegated call returns, so that finalize does
// not close the player under it.
func (p *playerImpl) finalize() {
	if !p.IsPlaying() {
		_ = p.Close()
	}
}

// Close closes the stream.
//
// When Close is called, the stream owned by the player is NOT closed,
// even if the stream implements io.Closer.
//
// Close returns error when the player is already closed.
//
// Deprecated: as of v2.10. Use [Player.PauseAndStopReading] instead.
func (p *Player) Close() error {
	defer runtime.KeepAlive(p)
	return p.p.Close()
}

// Play plays the stream.
func (p *Player) Play() {
	defer runtime.KeepAlive(p)
	p.p.Play()
}

// IsPlaying returns a boolean indicating whether the player is playing.
func (p *Player) IsPlaying() bool {
	defer runtime.KeepAlive(p)
	return p.p.IsPlaying()
}

// Rewind rewinds the current position to the start.
//
// The passed source to NewPlayer must be io.Seeker, or Rewind panics.
// Rewind does not panic before Play is called, as it needs no source access
// to rewind a player that has not read the source yet.
//
// Rewind returns error when seeking the source stream returns error.
func (p *Player) Rewind() error {
	defer runtime.KeepAlive(p)
	return p.p.Rewind()
}

// SetPosition sets the position with the given offset.
//
// The passed source to NewPlayer must be io.Seeker, or SetPosition panics.
// A zero offset does not panic before Play is called, as it needs no source
// access to reset a player that has not read the source yet.
//
// SetPosition returns error when seeking the source stream returns an error.
// SetPosition returns an error for a negative offset.
func (p *Player) SetPosition(offset time.Duration) error {
	defer runtime.KeepAlive(p)
	return p.p.SetPosition(offset)
}

// Seek seeks the position with the given offset.
//
// Deprecated: as of v2.6. Use SetPosition instead.
func (p *Player) Seek(offset time.Duration) error {
	return p.SetPosition(offset)
}

// Pause pauses the playing.
func (p *Player) Pause() {
	defer runtime.KeepAlive(p)
	p.p.Pause()
}

// PauseAndStopReading pauses the playing and stops reading the source.
// After PauseAndStopReading returns, this player does not read the source until [Player.Play],
// [Player.SetPosition], or [Player.Rewind] is called, so the source can be closed safely.
// The buffered data is kept, and [Player.Play] resumes the playing without a gap.
// PauseAndStopReading blocks until an ongoing read from the source finishes, if any.
func (p *Player) PauseAndStopReading() {
	defer runtime.KeepAlive(p)
	p.p.PauseAndStopReading()
}

// Position returns the current position in time.
//
// As long as the player continues to play, Position's returning value is increased monotonically,
// even though the source stream loops and its position goes back.
func (p *Player) Position() time.Duration {
	defer runtime.KeepAlive(p)
	return p.p.Position()
}

// Current returns the current position in time.
//
// Deprecated: as of v2.6. Use Position instead.
func (p *Player) Current() time.Duration {
	return p.Position()
}

// Volume returns the current volume of this player, which is 0 or larger.
func (p *Player) Volume() float64 {
	defer runtime.KeepAlive(p)
	return p.p.Volume()
}

// SetVolume sets the volume of this player, which must be in the range of [0, math.MaxFloat32].
// A volume larger than 1 amplifies the sound and might cause clipping.
// A value out of the range, including NaN, is treated as 0.
func (p *Player) SetVolume(volume float64) {
	defer runtime.KeepAlive(p)
	p.p.SetVolume(volume)
}

// SetBufferSize adjusts the buffer size of the player.
// If a nonpositive value or a value too large to represent is specified, the default buffer size is used.
// A small buffer size is useful if you want to play a real-time PCM for example.
// Note that the audio quality might be affected if you modify the buffer size.
func (p *Player) SetBufferSize(bufferSize time.Duration) {
	defer runtime.KeepAlive(p)
	p.p.SetBufferSize(bufferSize)
}

type hooker interface {
	OnSuspendAudio(f func() error)
	OnResumeAudio(f func() error)
	AppendHookOnBeforeUpdateWithVMGuestInfo(f func(vmGuest bool) error)
}

var hookerForTesting hooker

func getHook() hooker {
	if hookerForTesting != nil {
		return hookerForTesting
	}
	return &hookerImpl{}
}

type hookerImpl struct{}

func (h *hookerImpl) OnSuspendAudio(f func() error) {
	hook.OnSuspendAudio(f)
}

func (h *hookerImpl) OnResumeAudio(f func() error) {
	hook.OnResumeAudio(f)
}

func (h *hookerImpl) AppendHookOnBeforeUpdateWithVMGuestInfo(f func(vmGuest bool) error) {
	hook.AppendHookOnBeforeUpdateWithVMGuestInfo(f)
}

// ResampleReader converts the sample rate of the given signed 16bit integer, little-endian, 2 channels (stereo) stream.
// length is the length of the source stream in bytes. 0 indicates the length is unknown.
// from is the original sample rate.
// to is the target sample rate.
// ResampleReader panics if from or to is not positive.
//
// If the source ends before length bytes, the remainder of the result is silence.
//
// If the original sample rate equals the new one, ResampleReader returns source as it is and does not apply length.
// When the rates differ, the returned value's Read returns only whole samples (4 bytes each), and its Seek rounds a position in
// the middle of a sample down to a sample boundary.
// For a non-empty buffer shorter than one sample, Read returns [io.ErrShortBuffer], or [io.EOF] once the stream has ended.
//
// The returned value implements io.Seeker when the source implements io.Seeker.
// The returned value might implement io.Seeker even when the source doesn't implement io.Seeker, but
// there is no guarantee that the Seek function works correctly.
func ResampleReader(source io.Reader, length int64, from, to int) io.Reader {
	validateResamplingSampleRates(from, to)
	if from == to {
		return source
	}
	return convert.NewResampling(source, resamplingSourceLength(length), from, to, bitDepthInBytesInt16)
}

// ResampleReaderF32 converts the sample rate of the given 32bit float, little-endian, 2 channels (stereo) stream.
// length is the length of the source stream in bytes. 0 indicates the length is unknown.
// from is the original sample rate.
// to is the target sample rate.
// ResampleReaderF32 panics if from or to is not positive.
//
// If the source ends before length bytes, the remainder of the result is silence.
//
// If the original sample rate equals the new one, ResampleReaderF32 returns source as it is and does not apply length.
// When the rates differ, the returned value's Read returns only whole samples (8 bytes each), and its Seek rounds a position in
// the middle of a sample down to a sample boundary.
// For a non-empty buffer shorter than one sample, Read returns [io.ErrShortBuffer], or [io.EOF] once the stream has ended.
//
// The returned value implements io.Seeker when the source implements io.Seeker.
// The returned value might implement io.Seeker even when the source doesn't implement io.Seeker, but
// there is no guarantee that the Seek function works correctly.
func ResampleReaderF32(source io.Reader, length int64, from, to int) io.Reader {
	validateResamplingSampleRates(from, to)
	if from == to {
		return source
	}
	return convert.NewResampling(source, resamplingSourceLength(length), from, to, bitDepthInBytesFloat32)
}

// Resample converts the sample rate of the given signed 16bit integer, little-endian, 2 channels (stereo) stream.
// length is the length of the source stream in bytes. 0 indicates the length is unknown.
// from is the original sample rate.
// to is the target sample rate.
// Resample panics if from or to is not positive.
//
// If the source ends before length bytes, the remainder of the result is silence.
//
// If the original sample rate equals the new one, Resample returns source as it is and does not apply length.
// When the rates differ, the returned value's Read returns only whole samples (4 bytes each), and its Seek rounds a position in
// the middle of a sample down to a sample boundary.
// For a non-empty buffer shorter than one sample, Read returns [io.ErrShortBuffer], or [io.EOF] once the stream has ended.
//
// Deprecated: as of v2.9. Use ResampleReader instead.
func Resample(source io.ReadSeeker, length int64, from, to int) io.ReadSeeker {
	validateResamplingSampleRates(from, to)
	if from == to {
		return source
	}
	return convert.NewResampling(source, resamplingSourceLength(length), from, to, bitDepthInBytesInt16)
}

// ResampleF32 converts the sample rate of the given 32bit float, little-endian, 2 channels (stereo) stream.
// length is the length of the source stream in bytes. 0 indicates the length is unknown.
// from is the original sample rate.
// to is the target sample rate.
// ResampleF32 panics if from or to is not positive.
//
// If the source ends before length bytes, the remainder of the result is silence.
//
// If the original sample rate equals the new one, ResampleF32 returns source as it is and does not apply length.
// When the rates differ, the returned value's Read returns only whole samples (8 bytes each), and its Seek rounds a position in
// the middle of a sample down to a sample boundary.
// For a non-empty buffer shorter than one sample, Read returns [io.ErrShortBuffer], or [io.EOF] once the stream has ended.
//
// Deprecated: as of v2.9. Use ResampleReaderF32 instead.
func ResampleF32(source io.ReadSeeker, length int64, from, to int) io.ReadSeeker {
	validateResamplingSampleRates(from, to)
	if from == to {
		return source
	}
	return convert.NewResampling(source, resamplingSourceLength(length), from, to, bitDepthInBytesFloat32)
}

// resamplingSourceLength converts a length given to ResampleReader, ResampleReaderF32, Resample, or ResampleF32,
// where 0 indicates an unknown length, to a length for convert.NewResampling,
// where a negative value indicates an unknown length and 0 an empty stream.
//
// TODO: Let these functions take a negative value instead of 0 for an unknown length (#3395).
func resamplingSourceLength(length int64) int64 {
	if length == 0 {
		return -1
	}
	return length
}

func validateResamplingSampleRates(from, to int) {
	if from <= 0 {
		panic(fmt.Sprintf("audio: original sample rate must be positive but was %d", from))
	}
	if to <= 0 {
		panic(fmt.Sprintf("audio: target sample rate must be positive but was %d", to))
	}
}