package main

import (
    "context"
    "log"
    "math/rand"
    "runtime"
    "sync"
    "time"
)

var timeout = 3 * time.Second

var worker = runtime.GOMAXPROCS(0)

// pi = 4 * P(l <= r)
func main() {
    // trace.Start(os.Stdout)
    // defer trace.Stop()

    // f, err := os.Create("cpuprofile")
    // if err != nil {
    //  panic(err)
    // }
    // err = pprof.StartCPUProfile(f)
    // if err != nil {
    //  panic(err)
    // }
    // defer pprof.StopCPUProfile()
    wg := sync.WaitGroup{}
    ctx, _ := context.WithTimeout(context.Background(), timeout)
    inCh, totalCh := make(chan int, worker), make(chan int, worker)
    for i := 0; i < worker; i++ {
        wg.Add(1)
        r := rand.New(rand.NewSource(time.Now().UnixNano()))
        go func() {
            defer wg.Done()
            in, total := 0, 0
            for i := 0; ; i++ {
                if i%1000 == 0 {
                    select {
                    case <-ctx.Done():
                        inCh <- in
                        totalCh <- total
                        return
                    default:
                    }
                }
                total++
                x, y := r.Float64(), r.Float64()
                if isIn(x, y) {
                    in++
                }
            }
        }()
    }
    wg.Wait()
    in, total := 0, 0
    close(inCh)
    close(totalCh)
    for v := range inCh {
        in += v
    }
    for v := range totalCh {
        total += v
    }
    log.Printf("pi is %5f, sample size is %d", float64(in)/float64(total)*4, total)
}

func isIn(x, y float64) bool {
    return x*x+y*y <= 1
}