Ebiten example - piano

package main

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

    "github.com/hajimehoshi/ebiten"
    "github.com/hajimehoshi/ebiten/ebitenutil"
    "github.com/hajimehoshi/ebiten/example/common"
    "github.com/hajimehoshi/ebiten/exp/audio"
)

const (
    screenWidth  = 320
    screenHeight = 240
)

var pcm = make([]float64, 4*audio.SampleRate())

const baseFreq = 220

func init() {
    s := float64(audio.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) {
    f := int(freq)
    if n, ok := noteCache[f]; ok {
        audio.Play(-1, n)
        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
    audio.Play(-1, n)
}

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 pianoImage *ebiten.Image

func init() {
    var err error
    pianoImage, err = ebiten.NewImage(screenWidth, screenHeight, ebiten.FilterNearest)
    if err != nil {
        panic(err)
    }
    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
        pianoImage.DrawFilledRect(x, y, width-1, height, color.White)
        common.ArcadeFont.DrawText(pianoImage, 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
        pianoImage.DrawFilledRect(x, y, width-1, height, color.Black)
        common.ArcadeFont.DrawText(pianoImage, 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)
    }

    screen.Fill(color.RGBA{0x80, 0x80, 0xc0, 0xff})
    screen.DrawImage(pianoImage, 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)
    }
}