MersenneTwister: simplify seeding logic #60204
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Since at least v0.3, `MersenneTwister` has stored the user-provided
seed. More recently, this allowed `show` to print a round-trippable
representation of the RNG. However, storing an arbitrary deep-copied
user seed is not ideal, and the initialization logic has become more
complicated than necessary.
In particular, `MersenneTwister()` currently performs three steps:
1. seed = rand(RandomDevice(), UInt128) # generate a seed
2. initstate = rand(SeedHasher(seed), UInt32, 8) # expand it
3. initialize dSFMT with `initstate`
This commit merges the first two steps. The new initialization is:
1. initstate = rand(RandomDevice(), NTuple{2, UInt128})
2. convert `initstate` to the dSFMT format and initialize the state
We now store only `initstate` for use in `show`, preserving
round-trippability. For user-provided seeds, `initstate` is
derived from the seed via `SeedHasher`.
A small bonus is that `MersenneTwister()` is no longer restricted to
"only" 2^128 distinct initializations.
The cosmetic drawback is that the printed representation is less pretty:
`MersenneTwister(1)` now shows as something like
MersenneTwister(0xc53bc12f4c3f0b4efff..., 0x50a4aa153d208dd856d...)
oscardssmith
approved these changes
Nov 22, 2025
rfourquet
added
randomness
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Since at least v0.3,
MersenneTwisterhas stored the user-provided seed. More recently, this allowedshowto print a round-trippable representation of the RNG. However, storing an arbitrary deep-copied user seed is not ideal, and the initialization logic has become more complicated than necessary.In particular,
MersenneTwister()currently performs three steps:seed = rand(RandomDevice(), UInt128)# generate a seedinitstate = rand(SeedHasher(seed), UInt32, 8)# expand itinitstateThis commit merges the first two steps. The new initialization is:
initstate = rand(RandomDevice(), NTuple{2, UInt128})initstateto the dSFMT format and initialize the stateWe now store only
initstatefor use inshow, preserving round-trippability. For user-provided seeds,initstateis derived from the seed viaSeedHasher.A small bonus is that
MersenneTwister()is no longer restricted to "only" 2^128 distinct initializations.The cosmetic drawback is that the printed representation is less pretty:
MersenneTwister(1)now shows asThis is based off #60185 to avoid merge conflicts.