bab-verify.js

const crypto = require("crypto")

function gameResult(seed, salt) {
  const nBits = 52 // number of most significant bits to use

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

  // 2. r = 52 most significant bits
  seed = seed.slice(0, nBits/4)
  const r = parseInt(seed, 16)

  // 3. X = r / 2^52
  let X = r / Math.pow(2, nBits) // 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)
  return Math.max(1, result / 100)
}