eureka-about-golang-channels

the_day_I_understood_channels.go

package main

import (
  "fmt"
  "math/rand"
  "time"
  "flag"
)

const (
  N = 5
  T_MIN = 1
  T_MAX = 5
)

var P int

type Res struct {
  id int
  timestamp time.Time
}

func worker(numCh chan int, standingCh chan Res, workerIdx int) {
  for {
    randN := rand.Intn(100)
    numCh <- randN
    if randN <= P {
      break
    }
    t := time.Duration(rand.Intn(T_MAX - T_MIN + 1) + T_MIN)
    time.Sleep(t * time.Second)
  }
  standingCh <- Res{workerIdx, time.Now()}
  close(numCh)
}

func printCh(chIdx, randN int) {
  var msg string
  if randN <= P {
    msg = fmt.Sprintf("Channel %d: OK ==> [%d] ----------", chIdx, randN)
  } else {
    msg = fmt.Sprintf("Channel %d: continue (%d)...", chIdx, randN)
  }
  fmt.Println(msg)
}

func main() {
  rand.Seed(time.Now().UnixNano())

  flag.IntVar(&P, "p", 20, "Probability of success")
  flag.Parse()

  standingCh := make(chan Res, N)
  numChs := []chan int{}
  for i := 0; i < N; i++ {
    numChs = append(numChs, make(chan int))
  }

  fmt.Printf("Spawning %d workers with p=%d...\n\n", N, P)
  for i := 0; i < N; i++ {
    go worker(numChs[i], standingCh, i)
  }

  var n0, n1, n2, n3, n4 int
  ok0, ok1, ok2, ok3, ok4 := true, true, true, true, true
  var standing []string

  for len(standing) != N || ok0 || ok1 || ok2 || ok3 || ok4 {
    select {
    case n0, ok0 = <-numChs[0]:
      if ok0 {
        printCh(0, n0)
      }
    case n1, ok1 = <-numChs[1]:
      if ok1 {
        printCh(1, n1)
      }
    case n2, ok2 = <-numChs[2]:
      if ok2 {
        printCh(2, n2)
      }
    case n3, ok3 = <-numChs[3]:
      if ok3 {
        printCh(3, n3)
      }
    case n4, ok4 = <-numChs[4]:
      if ok4 {
        printCh(4, n4)
      }
    case r := <-standingCh:
      standing = append(standing, fmt.Sprintf("%d -- %v", r.id, r.timestamp))
    }
  }
  allDone := !ok0 && !ok1 && !ok2 && !ok3 && !ok4 

  fmt.Printf("\nEnd of program!\n\n")
  if allDone {
    fmt.Println("SUCCESS!")
    for idx, result := range standing {
      fmt.Printf("%d) %s\n", idx+1, result)
    }
  // For DEBUG purpose
  // } else {
  //   fmt.Println("FAIL :(")
  //   fmt.Println("n0 =", n0, "n1 =", n1, "n2 =", n2, "n3 =", n3, "n4 =", n4)
  }
}