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Digraph decode/encode #912

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b596022
Fix: faulty encoding without faithfulness
anujthak Mar 25, 2026
1a1fc00
Fix: faulty encoding without faithfulness
anujthak Mar 25, 2026
69fe761
Fix: faulty encoding without faithfulness
anujthak Mar 25, 2026
b4feb62
Adding test coverage for new error catching
anujthak Mar 31, 2026
2c5aa59
linting edits
anujthak Mar 31, 2026
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31 changes: 26 additions & 5 deletions gap/io.gi
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Original file line number Diff line number Diff line change
Expand Up @@ -2222,10 +2222,15 @@ InstallMethod(Graph6String, "for a digraph by out-neighbours",
[IsDigraphByOutNeighboursRep],
function(D)
local list, adj, n, lenlist, tablen, blist, i, j, pos, block;
if (IsMultiDigraph(D) or not IsSymmetricDigraph(D)
or DigraphHasLoops(D)) then
ErrorNoReturn("the argument <D> must be a symmetric digraph ",
"with no loops or multiple edges,");
if IsMultiDigraph(D) then
ErrorNoReturn("the argument <D> must not have multiple edges; ",
"consider encoding in Disparse6 or Digraph6");
elif not IsSymmetricDigraph(D) then
ErrorNoReturn("the argument <D> must be a symmetric digraph; ",
"consider encoding in Sparse6 or Disparse6");
elif DigraphHasLoops(D) then
ErrorNoReturn("the argument <D> must not have loops; ",
"consider encoding in Sparse6 or Disparse6");
Comment on lines +2227 to +2233
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@mtorpey mtorpey Apr 1, 2026

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I think all three of these "consider" lines are wrong. Revise this with Wilf's summary from his issue:

Graph6: can encode any symmetric digraph without loops or multiple edges
Sparse6: can encode any symmetric digraph
Digraph6: can encode any digraph without multiple edges
Disparse6: can encode any digraph

fi;

list := [];
Expand Down Expand Up @@ -2285,6 +2290,15 @@ function(D)
adj := OutNeighbours(D);
n := Length(DigraphVertices(D));

if IsMultiDigraph(D) then
if IsSymmetricDigraph(D) then
ErrorNoReturn("the argument <D> must not have multiple edges ",
"consider encoding in Sparse6 or Disparse6, ");
fi;
ErrorNoReturn("the argument <D> must not have multiple edges ",
"consider encoding in Disparse6, ");
fi;

# First write the special character '&'
Add(list, -25);

Expand Down Expand Up @@ -2337,8 +2351,15 @@ InstallMethod(Sparse6String, "for a digraph by out-neighbours",
function(D)
local list, n, lenlist, adj, nredges, k, blist, v, nextbit, i, j,
bitstopad, pos, block;

if not IsSymmetricDigraph(D) then
ErrorNoReturn("the argument <D> must be a symmetric digraph,");
if IsMultiDigraph(D) then
ErrorNoReturn("the argument <D> must be a symmetric digraph; ",
"consider encoding in Disparse6");
else
ErrorNoReturn("the argument <D> must be a symmetric digraph; ",
"consider encoding in Digraph6 or Disparse6");
fi;
fi;

list := [];
Expand Down
33 changes: 26 additions & 7 deletions tst/standard/io.tst
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Expand Up @@ -283,8 +283,8 @@ true
gap> gr[3] := Digraph([[1, 2], [1, 2]]);
<immutable digraph with 2 vertices, 4 edges>
gap> WriteDigraphs(filename, Digraph([[2], []]), Graph6String);
Error, the argument <D> must be a symmetric digraph with no loops or multiple \
edges,
Error, the argument <D> must be a symmetric digraph; consider encoding in Spar\
se6 or Disparse6
gap> OnBreak := oldOnBreak;;
Comment on lines -286 to 288
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@mtorpey mtorpey Apr 1, 2026

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The fact these break across lines is probably an argument for putting \n after the semicolon. Could you do this in all cases?

gap> IO_Close(IO.OpenFiles[Length(IO.OpenFiles)]);;
gap> filename := Concatenation(DIGRAPHS_Dir(), "/tst/out/test.s6.bz2");;
Expand Down Expand Up @@ -320,8 +320,8 @@ gap> ReadDigraphs(f);
gap> IO_Close(f);;
gap> f := DigraphFile(filename, "a");;
gap> WriteDigraphs(f, CycleDigraph(5));
Error, the argument <D> must be a symmetric digraph with no loops or multiple \
edges,
Error, the argument <D> must be a symmetric digraph; consider encoding in Spar\
se6 or Disparse6
gap> WriteDigraphs(f, JohnsonDigraph(6, 3));
IO_OK
gap> IO_Close(f);;
Expand Down Expand Up @@ -546,7 +546,8 @@ Error, the 2nd argument <s> is not a valid disparse6 string,

# Special format characters
gap> Sparse6String(ChainDigraph(3));
Error, the argument <D> must be a symmetric digraph,
Error, the argument <D> must be a symmetric digraph; consider encoding in Digr\
aph6 or Disparse6
gap> Sparse6String(CompleteDigraph(1));
":@"
gap> gr := Digraph([[1], []]);;
Expand All @@ -559,6 +560,24 @@ gap> DigraphFromSparse6String(":TdBkJ`Kq?x");
gap> Sparse6String(last);
":TdBkJ`Kq?"

# Multiple Edges Digraph6
gap> gr := Digraph([[1, 1], [2]]);;
gap> Digraph6String(gr);
Error, the argument <D> must not have multiple edges consider encoding in Spar\
se6 or Disparse6,

# Non-symmetric Digraph in Sparse6
gap> gr := Digraph([[2], []]);;
gap> Sparse6String(gr);
Error, the argument <D> must be a symmetric digraph; consider encoding in Digr\
aph6 or Disparse6

# Digraph with loops in Sparse6
gap> gr := Digraph([[1], [2]]);;
gap> Graph6String(gr);
Error, the argument <D> must not have loops; consider encoding in Sparse6 or D\
isparse6

# DigraphPlainTextLineDecoder: bad input
gap> DigraphPlainTextLineDecoder(" ", " ", 1, ".");
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Expand Down Expand Up @@ -659,8 +678,8 @@ Error, cannot open the file given as the 1st argument <name>,

# DigraphPlainTextLineDecoder: bad input
gap> Graph6String(ChainDigraph(4));
Error, the argument <D> must be a symmetric digraph with no loops or multiple \
edges,
Error, the argument <D> must be a symmetric digraph; consider encoding in Spar\
se6 or Disparse6
gap> DIGRAPHS_Graph6Length(-1);
fail
gap> DIGRAPHS_Graph6Length(68719476737);
Expand Down
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