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Fix uint32 multiplication in PCG next and peel functions #28

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Fix uint32 multiplication in PCG next and peel functions #28

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24 changes: 21 additions & 3 deletions src/Random.elm
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Original file line number Diff line number Diff line change
Expand Up @@ -678,21 +678,39 @@ type Seed
next : Seed -> Seed
next (Seed state0 incr) =
-- The magic constant is from Numerical Recipes and is inlined for perf.
Seed (Bitwise.shiftRightZfBy 0 ((state0 * 1664525) + incr)) incr
Seed (Bitwise.shiftRightZfBy 0 ((imul32 state0 1664525) + incr)) incr


-- obtain a psuedorandom 32-bit integer from a seed
-- obtain a pseudorandom 32-bit integer from a seed
peel : Seed -> Int
peel (Seed state _) =
-- This is the RXS-M-SH version of PCG, see section 6.3.4 of the paper
-- and line 184 of pcg_variants.h in the 0.94 (non-minimal) C implementation,
-- the latter of which is the source of the magic constant.
let
word =
(Bitwise.xor state (Bitwise.shiftRightZfBy ((Bitwise.shiftRightZfBy 28 state) + 4) state)) * 277803737
imul32
(Bitwise.xor state (Bitwise.shiftRightZfBy ((Bitwise.shiftRightZfBy 28 state) + 4) state))
277803737
in
Bitwise.shiftRightZfBy 0 (Bitwise.xor (Bitwise.shiftRightZfBy 22 word) word)

{-| Needed for u32 * u32 multiplications. Emulates JavaScript's Math.imul(a,b) but returns the result as an _unsigned_ 32bit number.
This would ideally live in the Bitwise module.
-}
imul32 : Int -> Int -> Int
imul32 a b =
let
aHi = Bitwise.shiftRightZfBy 16 a
aLo = Bitwise.and 0xFFFF a
bHi = Bitwise.shiftRightZfBy 16 b
bLo = Bitwise.and 0xFFFF b
low = aLo * bLo
high = Bitwise.and 0xFFFF (aHi * bLo + aLo * bHi)
in
(low + high * 0x00010000)
|> Bitwise.shiftRightZfBy 0


{-| A `Generator` is a **recipe** for generating random values. For example,
here is a generator for numbers between 1 and 10 inclusive:
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