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// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/

// Run with "web" command-line argument for web server.
// See page 13.
//!+main

// Lissajous generates GIF animations of random Lissajous figures.
package main

import (
    "image"
    "image/color"
    "image/gif"
    "io"
    "math"
    "math/rand"
    "os"
)

//!-main
// Packages not needed by version in book.
import (
    "log"
    "net/http"
    "time"
)

//!+main

var palette = []color.Color{color.White, color.Black}

const (
    whiteIndex = 0 // first color in palette
    blackIndex = 1 // next color in palette
)

func main() {
    //!-main
    // The sequence of images is deterministic unless we seed
    // the pseudo-random number generator using the current time.
    // Thanks to Randall McPherson for pointing out the omission.
    rand.Seed(time.Now().UTC().UnixNano())

    if len(os.Args) > 1 && os.Args[1] == "web" {
        //!+http
        handler := func(w http.ResponseWriter, r *http.Request) {
            lissajous(w)
        }
        http.HandleFunc("/", handler)
        //!-http
        log.Fatal(http.ListenAndServe("localhost:8000", nil))
        return
    }
    //!+main
    lissajous(os.Stdout)
}

func lissajous(out io.Writer) {
    const (
        cycles  = 5     // number of complete x oscillator revolutions
        res     = 0.001 // angular resolution
        size    = 100   // image canvas covers [-size..+size]
        nframes = 64    // number of animation frames
        delay   = 8     // delay between frames in 10ms units
    )
    freq := rand.Float64() * 3.0 // relative frequency of y oscillator
    anim := gif.GIF{LoopCount: nframes}
    phase := 0.0 // phase difference
    for i := 0; i < nframes; i++ {
        rect := image.Rect(0, 0, 2*size+1, 2*size+1)
        img := image.NewPaletted(rect, palette)
        for t := 0.0; t < cycles*2*math.Pi; t += res {
            x := math.Sin(t)
            y := math.Sin(t*freq + phase)
            img.SetColorIndex(size+int(x*size+0.5), size+int(y*size+0.5),
                blackIndex)
        }
        phase += 0.1
        anim.Delay = append(anim.Delay, delay)
        anim.Image = append(anim.Image, img)
    }
    gif.EncodeAll(out, &anim) // NOTE: ignoring encoding errors
}

//!-main