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WIP: Fixes for vector spaces with DimensionOfVectors(V) = 0 (DO NOT MERGE) #2125

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WIP: Fixes for vector spaces with DimensionOfVectors(V) = 0 (DO NOT MERGE) #2125

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11 changes: 11 additions & 0 deletions hpcgap/lib/basis.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -1403,6 +1403,17 @@ InstallMethod( Coefficients,
fi;
end );

InstallOtherMethod( Coefficients,
"for empty basis and empty list",
[ IsBasis and IsEmpty, IsEmpty and IsList ], SUM_FLAGS,
function( B, v )
if v = Zero( UnderlyingLeftModule( B ) ) then
return [];
else
return fail;
fi;
end );

InstallMethod( LinearCombination,
"for empty basis and empty list",
[ IsBasis and IsEmpty, IsList and IsEmpty ], SUM_FLAGS,
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11 changes: 11 additions & 0 deletions lib/basis.gi
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Expand Up @@ -1398,6 +1398,17 @@ InstallMethod( Coefficients,
fi;
end );

InstallOtherMethod( Coefficients,
"for empty basis and empty list",
[ IsBasis and IsEmpty, IsEmpty and IsList ], SUM_FLAGS,
function( B, v )
if v = Zero( UnderlyingLeftModule( B ) ) then
return [];
else
return fail;
fi;
end );

InstallMethod( LinearCombination,
"for empty basis and empty list",
[ IsBasis and IsEmpty, IsList and IsEmpty ], SUM_FLAGS,
Expand Down
12 changes: 11 additions & 1 deletion lib/modulrow.gi
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Expand Up @@ -209,6 +209,15 @@ InstallMethod( \in,
and IsSubset( LeftActingDomain( M ), v );
end );

# TODO: HACK: properly document this
InstallMethod( \in,
"for empty list and vector space with length 0 vector",
[ IsEmpty and IsList, IsFreeLeftModule and IsFullRowModule ],
SUM_FLAGS, # can't do better
function( v, M )
return DimensionOfVectors(M) = 0;
end );


#############################################################################
##
Expand Down Expand Up @@ -535,7 +544,8 @@ InstallMethod( EnumeratorByBasis,
zerovector := Zero( V ),
dimension := Dimension( V ) ) );

if IsField( F ) and Size( F ) < 256 and IsInternalRep( One( F ) ) then
if IsField( F ) and Size( F ) < 256 and Dimension( V ) > 0 and
IsInternalRep( One( F ) ) then
# Use a more efficient method for `Position'.
enum!.NumberElement:= PosVecEnumFF;
SetFilterObj( enum, IsQuickPositionList );
Expand Down
17 changes: 17 additions & 0 deletions tst/testbugfix/2018-01-29-empty-list-in-vecspace.tst
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@@ -0,0 +1,17 @@
#
# See <https://github.com/gap-system/gap/issues/2117>
#
gap> V := GF(5)^0;;
gap> e := Enumerator(V);;
gap> Position(e, Zero(V));
1
gap> Coefficients(Basis(V), Zero(V));
[ ]

#
gap> V := GF(257)^0;;
gap> e := Enumerator(V);;
gap> Position(e, Zero(V));
1
gap> Coefficients(Basis(V), Zero(V));
[ ]