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Add optimized is_trivial methods for zzModRing and ZZModRing #1949

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Nov 26, 2024
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Add optimized is_trivial methods for zzModRing and ZZModRing #1949

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4 changes: 4 additions & 0 deletions src/flint/FlintTypes.jl
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Expand Up @@ -463,6 +463,10 @@ For the modulus being an [`Int`](@ref) see [`zzModRing`](@ref).
ninv::fmpz_mod_ctx_struct

function ZZModRing(n::ZZRingElem, cached::Bool=true)
# Modulus of zero cannot be supported. E.g. Flint library could not be expected to
# do matrices over Z/0 using a Z/nZ type. The former is multiprecision, the latter not.
n <= 0 && throw(DomainError(n, "Modulus must be positive"))

return get_cached!(FmpzModRingID, n, cached) do
ninv = fmpz_mod_ctx_struct()
@ccall libflint.fmpz_mod_ctx_init(ninv::Ref{fmpz_mod_ctx_struct}, n::Ref{ZZRingElem})::Nothing
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9 changes: 4 additions & 5 deletions src/flint/fmpz_mod.jl
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Expand Up @@ -56,13 +56,15 @@

modulus(R::ZZModRing) = R.n

characteristic(R::ZZModRing) = modulus(R)

is_trivial(a::ZZModRing) = is_one(modulus(a)) # constructor ensures the modulus is > 0

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perhaps this makes it clearer?

Suggested change
is_trivial(a::ZZModRing) = is_one(modulus(a)) # constructor ensures the modulus is > 0
is_trivial(a::ZZModRing) = is_one(modulus(a)) # this is correct because the constructor ensures the modulus is > 0


function deepcopy_internal(a::ZZModRingElem, dict::IdDict)
R = parent(a)
return ZZModRingElem(deepcopy(a.data), R)
end

characteristic(R::ZZModRing) = modulus(R)

function _reduce(a::ZZRingElem, ctx::fmpz_mod_ctx_struct)
b = ZZRingElem()
@ccall libflint.fmpz_mod_set_fmpz(b::Ref{ZZRingElem}, a::Ref{ZZRingElem}, ctx::Ref{fmpz_mod_ctx_struct})::Nothing
Expand Down Expand Up @@ -449,9 +451,6 @@
###############################################################################

function residue_ring(R::ZZRing, n::ZZRingElem; cached::Bool=true)
# Modulus of zero cannot be supported. E.g. Flint library could not be expected to
# do matrices over Z/0 using a Z/nZ type. The former is multiprecision, the latter not.
n <= 0 && throw(DomainError(n, "Modulus must be positive"))
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S = ZZModRing(n, cached)
f = Generic.EuclideanRingResidueMap(R, S)
return S, f
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6 changes: 4 additions & 2 deletions src/flint/nmod.jl
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Expand Up @@ -56,13 +56,15 @@

modulus(R::zzModRing) = R.n

characteristic(R::zzModRing) = ZZRingElem(modulus(R))

is_trivial(a::zzModRing) = is_unit(modulus(a))

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Is there a reason you use is_unit here but is_one in the other file?

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is_unit is mathematically correct. here it gets optimized to just checking for one as the modulus is unsigned . for the ZZ case using isone is an optimization to save 1 or 2 machine instructions checking for -1

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ah ok. thanks for the explanation


function deepcopy_internal(a::zzModRingElem, dict::IdDict)
R = parent(a)
return zzModRingElem(deepcopy(a.data), R)
end

characteristic(R::zzModRing) = ZZRingElem(modulus(R))

###############################################################################
#
# Canonicalisation
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