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grahom.gi test coverage #928

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7e1baef
Tidy up some items in grahom.gi
mtorpey May 1, 2026
26ae4e7
Add some tests for complete codecov of grahom.gi.
mtorpey May 1, 2026
cf3c0d0
Fix broken error message
mtorpey May 6, 2026
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14 changes: 4 additions & 10 deletions gap/grahom.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -521,8 +521,8 @@ InstallMethod(DigraphsRespectsColouring,
[IsDigraph, IsDigraph, IsTransformation, IsList, IsList],
function(src, ran, x, cols1, cols2)
if Maximum(OnTuples(DigraphVertices(src), x)) > DigraphNrVertices(ran) then
ErrorNoReturn("the third argument <x> must map the vertices of the first ",
"argument <src> into the vertices of the second argument ",
ErrorNoReturn("the 3rd argument <x> must map the vertices of the 1st ",
"argument <src> into the vertices of the 2nd argument ",
"<ran>,");
fi;
DIGRAPHS_ValidateVertexColouring(DigraphNrVertices(src), cols1);
Expand Down Expand Up @@ -738,7 +738,7 @@ end);
InstallMethod(MaximalCommonSubdigraph, "for a pair of digraphs",
[IsDigraph, IsDigraph],
function(A, B)
local D1, D2, MPG, nonloops, Clqus, M, l, n, m, embedding1, embedding2, iso;
local D1, D2, MPG, nonloops, Clqus, M, n, m, embedding1, embedding2, iso;

D1 := DigraphImmutableCopy(A);
D2 := DigraphImmutableCopy(B);
Expand All @@ -763,13 +763,7 @@ function(A, B)
nonloops := Filtered([1 .. n * m], x -> not x in OutNeighbours(MPG)[x]);
# We find a big clique
Clqus := DigraphMaximalCliquesReps(MPG, [], nonloops);
M := 1;
for l in [1 .. Size(Clqus)] do
if Size(Clqus[l]) > Size(Clqus[M]) then
M := l;
fi;
od;

M := PositionMaximum(Clqus, Size);
embedding1 := List(Clqus[M], x -> QuoInt(x - 1, m) + 1);
embedding2 := List(Clqus[M], x -> RemInt(x - 1, m) + 1);
return [InducedSubdigraph(D1, embedding1),
Expand Down
29 changes: 25 additions & 4 deletions tst/standard/grahom.tst
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Original file line number Diff line number Diff line change
Expand Up @@ -8,9 +8,9 @@
#############################################################################
##

#@local D, D1, D2, DD, G, H, N5, edges, epis, f, found, gens, gr, gr1, gr2
#@local homos, hook, mat, mono, monos, order_func, p, parts, ran, s, src, t, tt
#@local x
#@local D, D1, D2, DD, G, H, N5, c1, c2, edges, epis, f, found, gens, gr, gr1
#@local gr2, homos, hook, mat, mono, monos, order_func, p, parts, ran, s, src
#@local t, tt, x
gap> START_TEST("Digraphs package: standard/grahom.tst");
gap> LoadPackage("digraphs", false);;

Expand Down Expand Up @@ -168,6 +168,9 @@ gap> gr := DigraphTransitiveClosure(CompleteDigraph(2));
gap> DigraphHasLoops(gr);
true
gap> GeneratorsOfEndomorphismMonoid(gr);
[ Transformation( [ 2, 1 ] ), IdentityTransformation,
Transformation( [ 1, 1 ] ) ]
gap> GeneratorsOfEndomorphismMonoid(gr); # recall attribute
[ Transformation( [ 2, 1 ] ), IdentityTransformation,
Transformation( [ 1, 1 ] ) ]
gap> gr := EmptyDigraph(2);
Expand Down Expand Up @@ -2300,6 +2303,8 @@ gap> IsDigraphHomomorphism(gr1, gr2, Transformation([1, 2]), [1, 2], [1, 1]);
false
gap> IsDigraphHomomorphism(gr1, gr2, Transformation([1, 2]), [1, 1], [1, 1]);
true
gap> IsDigraphHomomorphism(gr1, gr2, (1, 2), [1, 1], [1, 1]);
true
gap> IsDigraphHomomorphism(gr1, gr2, Transformation([1, 2]), [1, 1], [1, 2]);
false
gap> gr1 := Digraph([[], []]);
Expand Down Expand Up @@ -2478,7 +2483,21 @@ false
gap> IsDigraphEmbedding(ran, src, (), [2, 1], [1, 1, 2]);
false

# MaximalCommSubdigraph and MinimalCommonSuperDigraph
# DigraphsRespectsColouring
gap> D1 := CompleteDigraph(4);;
gap> D2 := ChainDigraph(5);;
gap> c1 := [1, 2, 3, 4];;
gap> c2 := [2, 3, 3, 4, 1];;
gap> DigraphsRespectsColouring(D1, D2, Transformation([5, 1, 3, 4, 5]), c1, c2);
true
gap> DigraphsRespectsColouring(D1, D2, Transformation([4, 1, 3, 4, 5]), c1, c2);
false
gap> DigraphsRespectsColouring(D1, D2, Transformation([6, 1, 3, 4, 5, 6]),
> c1, c2);
Error, the 3rd argument <x> must map the vertices of the 1st argument <src> in\
to the vertices of the 2nd argument <ran>,

# MaximalCommonSubdigraph and MinimalCommonSuperDigraph
gap> MaximalCommonSubdigraph(NullDigraph(0), CompleteDigraph(10));
[ <immutable empty digraph with 0 vertices>, IdentityTransformation,
IdentityTransformation ]
Expand Down Expand Up @@ -2513,6 +2532,8 @@ gap> MinimalCommonSuperdigraph(PetersenGraph(),
> DigraphSymmetricClosure(CycleDigraph(5)));
[ <immutable digraph with 10 vertices, 30 edges>, IdentityTransformation,
IdentityTransformation ]
gap> MaximalCommonSubdigraph(Digraph("banner"), Digraph("diamond"))[1];
<immutable digraph with 3 vertices, 4 edges>
gap> MaximalCommonSubdigraph(Digraph([[1, 1]]), Digraph([[1]]));
Error, the 1st argument (a digraph) must not satisfy IsMultiDigraph
gap> MinimalCommonSuperdigraph(Digraph([[1, 1]]), Digraph([[1]]));
Expand Down
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