a monty-hall solution

monty-hall.js

// Generate a random integer r with equal chance in  min <= r < max.
function randomInt(min, max) {
    var range = max - min;
    if (range <= 0) {
        throw new Exception('max must be larger than min');
    }
    var requestBytes = Math.ceil(Math.log2(range) / 8);
    if (!requestBytes) { // No randomness required
        return min;
    }
    var maxNum = Math.pow(256, requestBytes);
    var ar = new Uint8Array(requestBytes);

    while (true) {
        window.crypto.getRandomValues(ar);

        var val = 0;
        for (var i = 0;i < requestBytes;i++) {
            val = (val << 8) + ar[i];
        }

        if (val < maxNum - maxNum % range) {
            return min + (val % range);
        }
    }
}


function makeGames (length) {
  return new Array(length).fill(0).map(() => {
    const carDoor = randomInt(0, 3)
    const playerChoice = randomInt(0, 3)
    const hostChoices = [0,1,2].filter(door => {
      if (door === carDoor) return false
      if (door === playerChoice) return false
      return true
    })
    const hostChoiceIndex = randomInt(0, hostChoices.length)
    const hostChoice = hostChoices[hostChoiceIndex]

    const noSwapWin = carDoor === playerChoice
    const remainingDoors = [0,1,2].filter(door => door !== hostChoice)
    const swapDoor = remainingDoors.filter(door => door !== playerChoice)[0]
    const swapWin = carDoor === swapDoor
    
    const randomSwap = randomInt(0, 2)
    const randomSwapWin = randomSwap ? noSwapWin : swapWin
    const game =  {
      carDoor,
      playerChoice,
      hostChoice,
      noSwapWin,
      swapWin,
      randomSwapWin
    }

    return game
  })
}


const totalGames = 50000
const games = makeGames(totalGames)
// console.log(games)
const stats = games.reduce((previous, current) => {
  if (current.swapWin) previous.swapWins++
  if (current.noSwapWin) previous.noSwapWins++
  if (current.randomSwapWin) previous.randomSwapWins++

  return previous
}, {
  swapWins: 0,
  noSwapWins: 0,
  randomSwapWins: 0
})
stats.totalGames = totalGames
stats.swapWinsPercentage = `${(stats.swapWins / totalGames) * 100}%`
stats.noSwapWinsPercentage = `${(stats.noSwapWins / totalGames) * 100}%`
stats.randomSwapWinsPercentage = `${(stats.randomSwapWins / totalGames) * 100}%`
console.log(stats)