Hojun-Cho
aadc6618f0
Every // ---- section banner dies (132 -> 0): names carry the WHAT. Narration deleted (filename restatements, run-with lines, what-the- next-line-does); every ref/hare cite, task cite, divergence, ABI/ layout contract, and ownership qualifier kept (borrowed-view lines restored where the sweep over-cut). Comment-only proven: all 442 walk-workdir .s and 32 import-probe .s byte-identical before/after; libbyteid 56-roster all-ID.
59 lines
1.8 KiB
Plaintext
59 lines
1.8 KiB
Plaintext
// math/random — SplitMix64 PRNG.
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//
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// Mirrors Hare's math::random surface. The state is a single u64;
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// Hare types it as `random = u64`. Callers thread a pointer through
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// next/u32n/u64n so each call advances the state in place.
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// Deterministic — same seed reproduces the same sequence.
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package random;
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export type random = u64;
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// Mirrors Hare's random::init.
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export fn init(seed: u64) random = { return seed: random; };
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// SplitMix64, per Hare's random::next.
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export fn next(r: *random) u64 = {
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let s: u64 = (*r): u64 + 0x9E3779B97F4A7C15u64;
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*r = s: random;
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let a: u64 = s;
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a = (a ^ (a >> 30u64)) * 0xBF58476D1CE4E5B9u64;
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a = (a ^ (a >> 27u64)) * 0x94D049BB133111EBu64;
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return a ^ (a >> 31u64);
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};
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// u32n — pseudo-random u32 in [0, n). n must be > 0. Uses Lemire's
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// fast unbiased mapping (mulhi-then-leftover-reject). Mirrors Hare's
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// random::u32n.
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export fn u32n(r: *random, n: u32) u32 = {
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assert(n != 0u32); // ref/hare/math/random/random.ha:26
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let x: u32 = next(r): u32;
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let prod: u64 = (x: u64) * (n: u64);
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let leftover: u32 = prod: u32;
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if (leftover < n) {
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// thresh = -n mod n (two's-complement on u32).
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let neg: u32 = 0u32 - n;
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let thresh: u32 = neg % n;
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for (leftover < thresh) {
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x = next(r): u32;
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prod = (x: u64) * (n: u64);
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leftover = prod: u32;
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};
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};
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return (prod >> 32u64): u32;
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};
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// u64n — pseudo-random u64 in [0, n). n must be > 0. Power-of-2 fast
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// path; otherwise rejection-sample to avoid modulo bias. Mirrors
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// Hare's random::u64n.
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export fn u64n(r: *random, n: u64) u64 = {
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assert(n != 0u64); // ref/hare/math/random/random.ha:42
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if ((n & (n - 1u64)) == 0u64) { return next(r) & (n - 1u64); };
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// max = U64_MAX - (U64_MAX+1) % n = -1 - (-n % n)
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let neg: u64 = (0u64 - n);
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let max: u64 = 0xFFFFFFFFFFFFFFFFu64 - (neg % n);
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let out: u64 = next(r);
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for (out > max) { out = next(r); };
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return out % n;
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};
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