// m04 random: a tiny "state monad" for randomness. A Random<A> is a function seed -> [A, nextSeed],
// so every draw is a pure function and the seed is passed along explicitly instead of hidden.

/** mulberry32: a small, fast generator. `imul` is 32-bit integer multiplication (plain `*` would lose bits). */
export const nextUint32 = (seed) => {
  const s = (seed + 0x6D2B79F5) >>> 0 // `>>> 0` keeps numbers unsigned 32-bit
  const t1 = Math.imul(s ^ (s >>> 15), s | 1)
  const t2 = t1 ^ (t1 + Math.imul(t1 ^ (t1 >>> 7), t1 | 61))
  return [(t2 ^ (t2 >>> 14)) >>> 0, s]
}

/** A Random that always gives `a` and leaves the seed untouched. */
export const pure = (a) => (seed) => [a, seed]

/** Transform the value a Random produces; the seed threading is unchanged. */
export const map = (r, f) => (seed) => {
  const [a, next] = r(seed)
  return [f(a), next]
}

/** Run `r`, let `f` pick the next Random from its value, and run that with the seed `r` left behind. */
export const flatMap = (r, f) => (seed) => {
  const [a, next] = r(seed)
  return f(a)(next)
}

/** A Random giving a number in `[0, n)`. */
export const intBelow = (n) => map(nextUint32, (v) => v % n)

/** Run a Random from a seed: `[value, nextSeed]`. */
export const run = (r, seed) => r(seed)