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Make perm(::Vector{Int}) constructor and application of permutations to integers faster and allocate less #5476

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Make perm(::Vector{Int}) constructor and application of permutations to integers faster and allocate less #5476

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58 changes: 50 additions & 8 deletions src/Groups/perm.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -323,24 +323,51 @@ true
```
"""
function perm(g::PermGroup, L::AbstractVector{<:IntegerUnion})
x = GAPWrap.PermList(GapObj(L;recursive=true))
x = _make_gap_perm(degree(g), L)
@req x !== GAP.Globals.fail "the list does not describe a permutation"
@req (length(L) <= degree(g) && x in GapObj(g)) "the element does not embed in the group"
@req x in GapObj(g) "the element does not embed in the group"
return PermGroupElem(g, x)
end

perm(g::PermGroup, L::AbstractVector{<:ZZRingElem}) = perm(g, [Int(y) for y in L])

function (g::PermGroup)(L::AbstractVector{<:IntegerUnion})
x = GAPWrap.PermList(GapObj(L;recursive=true))
@req (length(L) <= degree(g) && x in GapObj(g)) "the element does not embed in the group"
x = _make_gap_perm(degree(g), L)
@req x in GapObj(g) "the element does not embed in the group"
return PermGroupElem(g, x)
end

function _make_gap_perm(deg::Int, L::AbstractVector{<:IntegerUnion})
@req 0 <= deg "degree is $deg, must be non-negative"
@req deg < 2^32 "degree is $deg, must be less than 2^32"
@req length(L) <= deg "length(L) = $(length(L)) exceeds degree bound $deg"
if deg < 2^16
x = _make_gap_perm(UInt16, deg, L)
else
x = _make_gap_perm(UInt32, deg, L)
end
return x
end

function _make_gap_perm(::Type{T}, deg::Int, L::AbstractVector{<:IntegerUnion}) where {T <: Union{UInt16, UInt32}}
if T === UInt16
perm = @ccall GAP.libgap.NEW_PERM2(deg::Cint)::GapObj
else
perm = @ccall GAP.libgap.NEW_PERM4(deg::Cint)::GapObj
end
addr = Ptr{T}(GAP.ADDR_OBJ(perm)+8) # skip 8 byte header

for i in 1:length(L)
unsafe_store!(addr, UInt64(L[i]) - 1, i)
end
for i in 1+length(L):deg
unsafe_store!(addr, i - 1, i)
end
return perm
Comment on lines +360 to +366
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@fingolfin fingolfin Nov 5, 2025

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Still fails because I only did the "unsafe" version which assumes the input is valid.

But the input could contain duplicates or "out of bounds" entries. I'll keep this one as unsafe_perm or so, and add one that performs those checks, just like the one in the GAP kernel (let's see how much time difference then remains)

end

(g::PermGroup)(L::AbstractVector{<:ZZRingElem}) = g([Int(y) for y in L])

# cperm stands for "cycle permutation", but we can change name if we want
# takes as input a list of vectors (not necessarily disjoint)
@doc raw"""
cperm(L::AbstractVector{<:T}...) where T <: IntegerUnion
cperm(L::AbstractVector{<:AbstractVector{T}}) where T <: IntegerUnion
Expand Down Expand Up @@ -501,9 +528,24 @@ Base.Vector(x::PermGroupElem, n::Int = x.parent.deg) = Vector{Int}(x,n)
#evaluation function
(x::PermGroupElem)(n::IntegerUnion) = n^x

^(n::T, x::PermGroupElem) where T <: IntegerUnion = T(GAP.Obj(n)^GapObj(x))
function ^(n::T, g::PermGroupElem) where T <: IntegerUnion
return fits(Int, n) ? T(Int(n)^g) : n
end

function ^(n::Int, g::PermGroupElem)
@req n > 0 "permutations only act on positive integers"
large = (GAP.TNUM_OBJ(GapObj(g)) == GAP.T_PERM4)
deg = GAP.SIZE_OBJ(GapObj(g)) - 8 # subtract 9 byte header
deg >>= large ? 2 : 1
n > deg && return n

^(n::Int, x::PermGroupElem) = (n^GapObj(x))::Int
addr = GAP.ADDR_OBJ(GapObj(g))+8 # skip 8 byte header
if large
return Int(unsafe_load(Ptr{UInt32}(addr), n))+1
else
return Int(unsafe_load(Ptr{UInt16}(addr), n))+1
end
end


@doc raw"""
Expand Down
2 changes: 1 addition & 1 deletion test/Groups/gsets.jl
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Expand Up @@ -135,7 +135,7 @@
@test unwrap(x) == omega
@test unwrap(omega) == omega
@test x^g == Omega(permuted(omega, g)) # action via `^` is defined for `x`
@test_throws ErrorException omega^g # ... but not for the unwrapped object
@test_throws ArgumentError omega^g # ... but not for the unwrapped object
orb = orbit(x)
@test length(orb) == 6
@test isa(orb, GSet)
Expand Down
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