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Speed up AlgebraicExtension and AlgebraicExtensionNC for larger degrees #4622

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Jul 30, 2021
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Speed up AlgebraicExtension and AlgebraicExtensionNC for larger degrees #4622

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36 changes: 17 additions & 19 deletions lib/algfld.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -65,7 +65,7 @@ end;
InstallMethod(AlgebraicElementsFamily,"generic",true,
[IsField,IsUnivariatePolynomial,IsBool],0,
function(f,p,check)
local fam,i,cof,red,rchar,impattr,deg,tmp;
local fam, i, impattr, rchar, deg, neg, red, z, new, c;
if check and not IsIrreducibleRingElement(PolynomialRing(f,
[IndeterminateNumberOfLaurentPolynomial(p)]),p) then
Error("<p> must be irreducible over f");
Expand Down Expand Up @@ -106,27 +106,25 @@ local fam,i,cof,red,rchar,impattr,deg,tmp;
fam!.indeterminateName:=MakeImmutable("a");

# reductions
#red:=IdentityMat(deg,fam!.oneCoefficient);
red:=[];
for i in [deg..2*deg-2] do
cof:=ListWithIdenticalEntries(i,fam!.zeroCoefficient);
Add(cof,fam!.oneCoefficient);
if rchar>0 then
if IsHPCGAP then
tmp := CopyToVectorRep(cof,rchar); # rchar is <= 256
if tmp <> fail then
cof := tmp;
fi;
else
ConvertToVectorRep(cof,rchar);
neg := -fam!.polCoeffs{[1..deg]};
if not IsOne(fam!.polCoeffs[deg+1]) then
neg := fam!.polCoeffs[deg+1]^-1 * neg;
fi;
red := [neg];
z := 0*neg;
for i in [1..deg-2] do
new := ShiftedCoeffs(red[i], 1);
if Length(new) > deg then
c := Remove(new);
if not IsZero(c) then
new := new + c*neg;
fi;
elif Length(new) < deg then
Append(new, z{[1..deg-Length(new)]});
fi;
ReduceCoeffs(cof,fam!.polCoeffs);
while Length(cof)<deg do
Add(cof,fam!.zeroCoefficient);
od;
Add(red,cof{[1..deg]});
Add(red, new);
od;

red:=ImmutableMatrix(fam!.baseField,red);
fam!.reductionMat:=red;
fam!.prodlen:=Length(red);
Expand Down