monty_hall

monty_hall.go

package main

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

const doorCount = 3
const (
	sheep = iota
	car
)

var doors = [doorCount]int{}

type input struct {
	retry bool
	win   bool
}

var outputTemplate = `
   retry | Y    | N
win     \|
Y        | %d| %d
N        | %d| %d`

func main() {
	Init()
	result := map[input]int{}

	for i := 0; i < 10000; i++ {
		retry := i&1 == 1
		win := oneLoop(retry)
		result[input{
			retry: retry,
			win:   win,
		}]++
	}

	fmt.Printf(outputTemplate, result[input{retry: true, win: true}], result[input{win: true}], result[input{retry: true}], result[input{}])
}

func Init() {
	rand.Seed(time.Now().Unix())
}

func resetDoors() {
	carIndex := rand.Intn(doorCount)
	for i := range doors {
		doors[i] = sheep
	}
	doors[carIndex] = car
}

func firstSelect() int {
	return rand.Intn(doorCount)
}

func openADoor(excludeIndex int) int {
	if doors[excludeIndex] == car {
		n := rand.Intn(doorCount - 1)
		if n >= excludeIndex {
			return n + 1
		}
		return n
	}

	for i := range doors {
		if i != excludeIndex && doors[i] == sheep {
			return i
		}
	}

	panic("should not happen")
}

func reSelect(a, b int) int {
	for i := range doors {
		if i != a && i != b {
			return i
		}
	}
	panic("should not happen")
}

func oneLoop(retry bool) (win bool) {
	resetDoors()
	selectI := firstSelect()
	openI := openADoor(selectI)
	if retry {
		selectI = reSelect(selectI, openI)
	}

	return doors[selectI] == car
}