MersenneTwister: clean up constructors (JuliaLang#60185)

We had two sets of constructors:
1) user-facing ones, which mimic `show`
(e.g. `MersenneTwister(seed)`, or `MersenneTwister(1, (0, 1002, 0,
1))`),
2) internal ones:
   - `MersenneTwister(seed, state, vals, ...)`
   - `MersenneTwister(seed, state)`

Internal ones were not practical to use, so they are replaced by a
single `_MersenneTwister(undef)` constructor which prepares an
uninitialized instance, which can then be initialized by `seed!`,
`copy!`, etc.

This commit also makes `const` some internal fields, and replaces
`Vector` with `Memory` for one of them.

Author: rfourquet

stdlib/Random/src/MersenneTwister.jl

@@ -9,9 +9,9 @@ const MT_CACHE_I = 501 << 4 # number of bytes in the UInt128 cache
 
 mutable struct MersenneTwister <: AbstractRNG
     seed::Any
-    state::DSFMT_state
-    vals::Vector{Float64}
-    ints::Vector{UInt128}
+    const state::DSFMT_state
+    const vals::Memory{Float64}
+    const ints::Vector{UInt128} # it's temporarily resized internally
     idxF::Int
     idxI::Int
 
@@ -21,25 +21,13 @@ mutable struct MersenneTwister <: AbstractRNG
     adv_vals::Int64     # state of advance when vals is filled-up
     adv_ints::Int64     # state of advance when ints is filled-up
 
-    function MersenneTwister(seed, state, vals, ints, idxF, idxI,
-                             adv, adv_jump, adv_vals, adv_ints)
-        length(vals) == MT_CACHE_F && 0 <= idxF <= MT_CACHE_F ||
-            throw(DomainError((length(vals), idxF),
-                      "`length(vals)` and `idxF` must be consistent with $MT_CACHE_F"))
-        length(ints) == MT_CACHE_I >> 4 && 0 <= idxI <= MT_CACHE_I ||
-            throw(DomainError((length(ints), idxI),
-                      "`length(ints)` and `idxI` must be consistent with $MT_CACHE_I"))
-        new(seed, state, vals, ints, idxF, idxI,
-            adv, adv_jump, adv_vals, adv_ints)
-    end
+    global _MersenneTwister(::UndefInitializer) =
+        new(nothing, DSFMT_state(),
+            Memory{Float64}(undef, MT_CACHE_F),
+            Vector{UInt128}(undef, MT_CACHE_I >> 4),
+            MT_CACHE_F, 0, 0, Base.GMP.ZERO, -1, -1)
 end
 
-MersenneTwister(seed, state::DSFMT_state) =
-    MersenneTwister(seed, state,
-                    Vector{Float64}(undef, MT_CACHE_F),
-                    Vector{UInt128}(undef, MT_CACHE_I >> 4),
-                    MT_CACHE_F, 0, 0, 0, -1, -1)
-
 """
     MersenneTwister(seed)
     MersenneTwister()
@@ -72,8 +60,7 @@ julia> x1 == x2
 true
 ```
 """
-MersenneTwister(seed=nothing) =
-    seed!(MersenneTwister(Vector{UInt32}(), DSFMT_state()), seed)
+MersenneTwister(seed=nothing) = seed!(_MersenneTwister(undef), seed)
 
 
 function copy!(dst::MersenneTwister, src::MersenneTwister)
@@ -90,10 +77,7 @@ function copy!(dst::MersenneTwister, src::MersenneTwister)
     dst
 end
 
-copy(src::MersenneTwister) =
-    MersenneTwister(src.seed, copy(src.state), copy(src.vals), copy(src.ints),
-                    src.idxF, src.idxI, src.adv, src.adv_jump, src.adv_vals, src.adv_ints)
-
+copy(src::MersenneTwister) = copy!(_MersenneTwister(undef), src)
 
 ==(r1::MersenneTwister, r2::MersenneTwister) =
     r1.seed == r2.seed && r1.state == r2.state &&
@@ -250,7 +234,7 @@ function initstate!(r::MersenneTwister, data::StridedVector, seed)
     dsfmt_init_by_array(r.state, reinterpret(UInt32, data))
     reset_caches!(r)
     r.adv = 0
-    r.adv_jump = 0
+    r.adv_jump = Base.GMP.ZERO
     return r
 end
 
@@ -561,7 +545,9 @@ end
 function _randjump(r::MersenneTwister, jumppoly::DSFMT.GF2X)
     adv = r.adv
     adv_jump = r.adv_jump
-    s = MersenneTwister(r.seed, DSFMT.dsfmt_jump(r.state, jumppoly))
+    s = _MersenneTwister(undef)
+    s.seed = r.seed
+    copy!(s.state, DSFMT.dsfmt_jump(r.state, jumppoly))
     reset_caches!(s)
     s.adv = adv
     s.adv_jump = adv_jump

stdlib/Random/test/runtests.jl

@@ -646,18 +646,6 @@ end
 # MersenneTwister initialization with invalid values
 @test_throws DomainError DSFMT.DSFMT_state(zeros(Int32, rand(0:DSFMT.JN32-1)))
 
-@test_throws DomainError MersenneTwister(zeros(UInt32, 1), DSFMT.DSFMT_state(),
-                                         zeros(Float64, 10), zeros(UInt128, MT_CACHE_I>>4), 0, 0, 0, 0, -1, -1)
-
-@test_throws DomainError MersenneTwister(zeros(UInt32, 1), DSFMT.DSFMT_state(),
-                                         zeros(Float64, MT_CACHE_F), zeros(UInt128, MT_CACHE_I>>4), -1, 0, 0, 0, -1, -1)
-
-@test_throws DomainError MersenneTwister(zeros(UInt32, 1), DSFMT.DSFMT_state(),
-                                         zeros(Float64, MT_CACHE_F), zeros(UInt128, MT_CACHE_I>>3), 0, 0, 0, 0, -1, -1)
-
-@test_throws DomainError MersenneTwister(zeros(UInt32, 1), DSFMT.DSFMT_state(),
-                                         zeros(Float64, MT_CACHE_F), zeros(UInt128, MT_CACHE_I>>4), 0, -1, 0, 0, -1, -1)
-
 # seed is private to MersenneTwister
 let seed = rand(UInt32, 10)
     r = MersenneTwister(seed)