32 lines
1.2 KiB
JavaScript
32 lines
1.2 KiB
JavaScript
// m04 random: a tiny "state monad" for randomness. A Random<A> is a function seed -> [A, nextSeed],
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// so every draw is a pure function and the seed is passed along explicitly instead of hidden.
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/** mulberry32: a small, fast generator. `imul` is 32-bit integer multiplication (plain `*` would lose bits). */
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export const nextUint32 = (seed) => {
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const s = (seed + 0x6D2B79F5) >>> 0 // `>>> 0` keeps numbers unsigned 32-bit
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const t1 = Math.imul(s ^ (s >>> 15), s | 1)
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const t2 = t1 ^ (t1 + Math.imul(t1 ^ (t1 >>> 7), t1 | 61))
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return [(t2 ^ (t2 >>> 14)) >>> 0, s]
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}
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/** A Random that always gives `a` and leaves the seed untouched. */
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export const pure = (a) => (seed) => [a, seed]
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/** Transform the value a Random produces; the seed threading is unchanged. */
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export const map = (r, f) => (seed) => {
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const [a, next] = r(seed)
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return [f(a), next]
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}
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/** Run `r`, let `f` pick the next Random from its value, and run that with the seed `r` left behind. */
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export const flatMap = (r, f) => (seed) => {
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const [a, next] = r(seed)
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return f(a)(next)
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}
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/** A Random giving a number in `[0, n)`. */
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export const intBelow = (n) => map(nextUint32, (v) => v % n)
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/** Run a Random from a seed: `[value, nextSeed]`. */
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export const run = (r, seed) => r(seed)
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