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fix(fsharp): mulberry32 wraps through uint64 so the Fable build matches the fixtures

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Fable compiles uint32 * and + to plain JavaScript arithmetic (no wrap, inexact
above 2^53); 2 of 37 RNG fixture cases matched in the browser build. uint64
helpers are exact on .NET and in Fable (BigInt).

Co-Authored-By: Claude Opus 5.5 <[email protected]>
This commit is contained in:
Lucas Winther
2026-10-29 01:04:10 +01:00
co-authored by Claude Opus 5.5
parent ab8abaaa77
commit 527b2cd157
2 changed files with 16 additions and 8 deletions
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fsharp/solutions/Random.fs
+11 -5
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@@ -19,13 +19,19 @@ let bind (f: 'a -> Random<'b>) (r: Random<'a>) : Random<'b> =
let a, next = run seed r
run next (f a))
// mulberry32. uint32 arithmetic wraps around at 2^32 (F# operators are unchecked) and `>>>` on uint32 is an
// unsigned shift, so the formula can be written down as it is.
// 32-bit wrap-around `*` and `+`. On .NET plain uint32 `*` and `+` already wrap at 2^32, but Fable (the F# to
// JavaScript compiler behind the browser page) compiles them to plain JavaScript `*` and `+`, which neither wrap nor
// stay exact for big products. Going through uint64 is exact on both: the product of two uint32 values fits in a
// uint64, and converting back to uint32 keeps the low 32 bits.
let private mul (a: uint32) (b: uint32) : uint32 = uint32 (uint64 a * uint64 b)
let private add (a: uint32) (b: uint32) : uint32 = uint32 (uint64 a + uint64 b)
// mulberry32. `^^^`, `|||` and `>>>` (an unsigned shift on uint32) behave the same everywhere.
let nextUint32: Random<uint32> =
Random(fun seed ->
let s = seed + 0x6D2B79F5u
let t1 = (s ^^^ (s >>> 15)) * (s ||| 1u)
let t2 = t1 ^^^ (t1 + (t1 ^^^ (t1 >>> 7)) * (t1 ||| 61u))
let s = add seed 0x6D2B79F5u
let t1 = mul (s ^^^ (s >>> 15)) (s ||| 1u)
let t2 = t1 ^^^ add t1 (mul (t1 ^^^ (t1 >>> 7)) (t1 ||| 61u))
(t2 ^^^ (t2 >>> 14), s))
let intBelow (n: int) : Random<int> = map (fun v -> int (v % uint32 n)) nextUint32