Ebiten example - piano

// +build example

package main

import (
        "fmt"
        "image/color"
        "log"
        "math"

        "github.com/hajimehoshi/ebiten"
        "github.com/hajimehoshi/ebiten/audio"
        "github.com/hajimehoshi/ebiten/ebitenutil"
        "github.com/hajimehoshi/ebiten/examples/common"
)

const (
        screenWidth  = 320
        screenHeight = 240
        sampleRate   = 44100
)

var audioContext *audio.Context

func init() {
        var err error
        audioContext, err = audio.NewContext(sampleRate)
        if err != nil {
                log.Fatal(err)
        }
}

var pcm = make([]float64, 4*sampleRate)

const baseFreq = 220

func init() {
        s := float64(sampleRate)
        amp := []float64{1.0, 0.8, 0.6, 0.4, 0.2}
        x := []float64{4.0, 2.0, 1.0, 0.5, 0.25}
        for i := 0; i < len(pcm); i++ {
                v := 0.0
                twoPiF := 2.0 * math.Pi * baseFreq
                for j := 0; j < len(amp); j++ {
                        a := amp[j] * math.Exp(-5*float64(i)/(x[j]*s))
                        v += a * math.Sin(float64(i)*twoPiF*float64(j+1)/s)
                }
                pcm[i] = v / 5.0
        }
}

var (
        noteCache = map[int][]byte{}
)

func toBytes(l, r []int16) []byte {
        if len(l) != len(r) {
                panic("len(l) must equal to len(r)")
        }
        b := make([]byte, len(l)*4)
        for i := range l {
                b[4*i] = byte(l[i])
                b[4*i+1] = byte(l[i] >> 8)
                b[4*i+2] = byte(r[i])
                b[4*i+3] = byte(r[i] >> 8)
        }
        return b
}

func addNote(freq float64, vol float64) {
        // TODO: Call Close method of *audio.Player.
        // However, this works without Close because Close is automatically called when GC
        // collects a *audio.Player object.
        f := int(freq)
        if n, ok := noteCache[f]; ok {
                p, _ := audio.NewPlayerFromBytes(audioContext, n)
                p.Play()
                return
        }
        length := len(pcm) * baseFreq / f
        l := make([]int16, length)
        r := make([]int16, length)
        j := 0
        jj := 0
        for i := 0; i < len(l); i++ {
                p := pcm[j]
                l[i] = int16(p * vol * math.MaxInt16)
                r[i] = l[i]
                jj += f
                j = jj / baseFreq
        }
        n := toBytes(l, r)
        noteCache[f] = n
        p, _ := audio.NewPlayerFromBytes(audioContext, n)
        p.Play()
        return
}

var keys = []ebiten.Key{
        ebiten.KeyQ,
        ebiten.KeyA,
        ebiten.KeyW,
        ebiten.KeyS,
        ebiten.KeyD,
        ebiten.KeyR,
        ebiten.KeyF,
        ebiten.KeyT,
        ebiten.KeyG,
        ebiten.KeyH,
        ebiten.KeyU,
        ebiten.KeyJ,
        ebiten.KeyI,
        ebiten.KeyK,
        ebiten.KeyO,
        ebiten.KeyL,
}

var keyStates = map[ebiten.Key]int{}

func init() {
        for _, key := range keys {
                keyStates[key] = 0
        }
}

func updateInput() {
        for _, key := range keys {
                if !ebiten.IsKeyPressed(key) {
                        keyStates[key] = 0
                        continue
                }
                keyStates[key]++
        }
}

var (
        imagePiano *ebiten.Image
)

func init() {
        imagePiano, _ = ebiten.NewImage(screenWidth, screenHeight, ebiten.FilterNearest)
        whiteKeys := []string{"A", "S", "D", "F", "G", "H", "J", "K", "L"}
        width := 24
        y := 48
        for i, k := range whiteKeys {
                x := i*width + 36
                height := 112
                ebitenutil.DrawRect(imagePiano, float64(x), float64(y), float64(width-1), float64(height), color.White)
                common.ArcadeFont.DrawText(imagePiano, k, x+8, y+height-16, 1, color.Black)
        }

        blackKeys := []string{"Q", "W", "", "R", "T", "", "U", "I", "O"}
        for i, k := range blackKeys {
                if k == "" {
                        continue
                }
                x := i*width + 24
                height := 64
                ebitenutil.DrawRect(imagePiano, float64(x), float64(y), float64(width-1), float64(height), color.Black)
                common.ArcadeFont.DrawText(imagePiano, k, x+8, y+height-16, 1, color.White)
        }
}

func update(screen *ebiten.Image) error {
        updateInput()
        for i, key := range keys {
                if keyStates[key] != 1 {
                        continue
                }
                addNote(220*math.Exp2(float64(i-1)/12.0), 1.0)
        }
        if err := audioContext.Update(); err != nil {
                return err
        }
        if ebiten.IsRunningSlowly() {
                return nil
        }
        screen.Fill(color.RGBA{0x80, 0x80, 0xc0, 0xff})
        screen.DrawImage(imagePiano, nil)

        ebitenutil.DebugPrint(screen, fmt.Sprintf("FPS: %0.2f", ebiten.CurrentFPS()))
        return nil
}

func main() {
        if err := ebiten.Run(update, screenWidth, screenHeight, 2, "Piano (Ebiten Demo)"); err != nil {
                log.Fatal(err)
        }
}