// math/random — SplitMix64 PRNG.
//
// Mirrors Hare's math::random surface. The state is a single u64;
// Hare types it as `random = u64`. Callers thread a pointer through
// next/u32n/u64n so each call advances the state in place.
// Deterministic — same seed reproduces the same sequence.

package random;

export type random = u64;

// init — initialize a generator with `seed`. Same seed reproduces the
// same sequence on every run. Mirrors Hare's random::init.
export fn init(seed: u64) random = { return seed: random; };

// next — return a pseudo-random 64-bit value and advance the state.
// SplitMix64, per Hare's random::next.
export fn next(r: *random) u64 = {
	let s: u64 = (*r): u64 + 0x9E3779B97F4A7C15u64;
	*r = s: random;
	let a: u64 = s;
	a = (a ^ (a >> 30u64)) * 0xBF58476D1CE4E5B9u64;
	a = (a ^ (a >> 27u64)) * 0x94D049BB133111EBu64;
	return a ^ (a >> 31u64);
};

// u32n — pseudo-random u32 in [0, n). n must be > 0. Uses Lemire's
// fast unbiased mapping (mulhi-then-leftover-reject). Mirrors Hare's
// random::u32n.
export fn u32n(r: *random, n: u32) u32 = {
	assert(n != 0u32);	// ref/hare/math/random/random.ha:26
	let x: u32 = next(r): u32;
	let prod: u64 = (x: u64) * (n: u64);
	let leftover: u32 = prod: u32;
	if (leftover < n) {
		// thresh = -n mod n  (two's-complement on u32).
		let neg: u32 = 0u32 - n;
		let thresh: u32 = neg % n;
		for (leftover < thresh) {
			x = next(r): u32;
			prod = (x: u64) * (n: u64);
			leftover = prod: u32;
		};
	};
	return (prod >> 32u64): u32;
};

// u64n — pseudo-random u64 in [0, n). n must be > 0. Power-of-2 fast
// path; otherwise rejection-sample to avoid modulo bias. Mirrors
// Hare's random::u64n.
export fn u64n(r: *random, n: u64) u64 = {
	assert(n != 0u64);	// ref/hare/math/random/random.ha:42
	if ((n & (n - 1u64)) == 0u64) { return next(r) & (n - 1u64); };
	// max = U64_MAX - (U64_MAX+1) % n = -1 - (-n % n)
	let neg: u64 = (0u64 - n);
	let max: u64 = 0xFFFFFFFFFFFFFFFFu64 - (neg % n);
	let out: u64 = next(r);
	for (out > max) { out = next(r); };
	return out % n;
};