flinch.io game result calculation

game.go

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"math"
	"math/big"
)

// GameResult calculates the corresponding crash point for a game hash and salt.
//
// Crash points have two decimal places, so a result of 1234 is interpreted as
// 12.34x. The largest possible result is 6966505673588736.
func GameResult(seed, salt []byte) uint64 {
	const nBits = 46 // number of most significant bits to use

	// 1. HMAC_SHA256(key=salt, message=seed)
	hmacHash := hmac.New(sha256.New, salt)
	hmacHash.Write(seed)
	seed = hmacHash.Sum(nil)

	// 2. r = 46 most significant bits
	seedInt := new(big.Int).SetBytes(seed)
	seedInt.Rsh(seedInt, sha256.Size*8-nBits)
	r := seedInt.Uint64()

	// 3. X = r / 2^46
	X := float64(r) / math.Pow(2, 46) // uniformly distributed in [0; 1)

	// 4. X = 99 / (1-X)
	X = 99 / (1 - X)

	// 5. return max(trunc(X), 100)
	result := uint64(X)
	if result == 99 {
		return 100
	}
	return result
}

game.js

const { createHmac } = require("crypto")


// gameResult calculates the corresponding crash point for a game hash and salt.
//
// Crash points have two decimal places, so a result of 1234 is interpreted as
// 12.34x. The largest possible result is 6966505673588736.
function gameResult(seed, salt) {
  const nBits = 46 // number of most significant bits to use

  // 1. HMAC_SHA256(key=salt, message=seed)
  const hmac = createHmac("sha256", salt)
  hmac.update(seed)
  seed = hmac.digest()

  // 2. r = 46 most significant bits
  seed = seed.readUIntBE(0, 6).toString(2)
  while (seed.length < 48) {
    seed = '0' + seed
  }
  seed = seed.substr(0, 46)
  const r = parseInt(seed, 2)

  // 3. X = r / 2^46
  let X = r / Math.pow(2, 46) // uniformly distributed in [0; 1)

  // 4. X = 99 / (1-X)
  X = 99 / (1 - X)

  // 5. return max(trunc(X), 100)
  const result = Math.floor(X)
  if (result === 99) {
    return 100
  }
  return result
}