Added a helper function and updated the existing methods to preserve edge weights when removing vertices, edges or copying digraphs (issue #683)

gap/digraph.gd

@@ -152,3 +152,5 @@ DeclareOperation("RandomLattice", [IsFunction, IsPosInt]);
 # in the not-too-distant future!
 DeclareOperation("RandomMultiDigraph", [IsPosInt]);
 DeclareOperation("RandomMultiDigraph", [IsPosInt, IsPosInt]);
+
+DeclareGlobalFunction("CopyEdgeWeightsForSubdigraph");

gap/digraph.gi

@@ -172,6 +172,11 @@ function(D)
   if HaveEdgeLabelsBeenAssigned(D) then
     SetDigraphEdgeLabelsNC(copy, StructuralCopy(DigraphEdgeLabelsNC(D)));
   fi;
+
+  if HasEdgeWeights(D) then
+      SetEdgeWeights(copy, StructuralCopy(EdgeWeights(D)));
+  fi;
+
   return copy;
 end);
 
@@ -1837,3 +1842,28 @@ n -> RandomLatticeCons(IsImmutableDigraph, n));
 
 InstallMethod(RandomLattice, "for a func and a pos int", [IsFunction, IsPosInt],
 RandomLatticeCons);
+
+InstallGlobalFunction(CopyEdgeWeightsForSubdigraph, 
+function(oldDigraph, newDigraph, removedVertices)
+    local oldWeights, newWeights, i, j, weight, shiftVertices;
+
+    if not HasEdgeWeights(oldDigraph) then
+        return;
+    fi;
+
+    oldWeights := EdgeWeights(oldDigraph);
+    newWeights := [];
+    for i in [1 .. DigraphNrVertices(newDigraph)] do
+        newWeights[i] := [];
+
+        for j in OutNeighbours(newDigraph, i) do
+            shiftVertices := function(x)
+                return x + Length(Filtered(removedVertices, v -> v <= x));
+            end;
+            weight := oldWeights[shiftVertices(i)][shiftVertices(j)];
+            Add(newWeights[i], weight);
+        od;
+    od;
+
+    SetEdgeWeights(newDigraph, newWeights);
+end);

gap/oper.gi

@@ -107,7 +107,8 @@ InstallMethod(DigraphRemoveVertex,
 "for a mutable digraph by out-neighbours and positive integer",
 [IsMutableDigraph and IsDigraphByOutNeighboursRep, IsPosInt],
 function(D, u)
-  local pos, w, v;
+  local pos, w, v, oldD;
+  oldD := StructuralCopy(D);
   if u > DigraphNrVertices(D) then
     return D;
   fi;
@@ -131,17 +132,30 @@ function(D, u)
       fi;
     od;
   od;
+
+  if HasEdgeWeights(D) then
+      CopyEdgeWeightsForSubdigraph(oldD, D, [u]);
+  fi;
+
   return D;
 end);
 
 InstallMethod(DigraphRemoveVertex,
 "for an immutable digraph and positive integer",
 [IsImmutableDigraph, IsPosInt],
 function(D, u)
+  local newD;
   if u > DigraphNrVertices(D) then
     return D;
   fi;
-  return MakeImmutable(DigraphRemoveVertex(DigraphMutableCopy(D), u));
+
+  newD := DigraphRemoveVertex(DigraphMutableCopy(D), u);
+  if HasEdgeWeights(D) then
+      CopyEdgeWeightsForSubdigraph(D, newD, [u]);
+  fi;
+  MakeImmutable(newD);
+
+  return newD;
 end);
 
 InstallMethod(DigraphRemoveVertices, "for a mutable digraph and a list",
@@ -227,7 +241,7 @@ InstallMethod(DigraphRemoveEdge,
 "for a mutable digraph by out-neighbours and two positive integers",
 [IsMutableDigraph and IsDigraphByOutNeighboursRep, IsPosInt, IsPosInt],
 function(D, src, ran)
-  local pos;
+  local pos, weights;
   if IsMultiDigraph(D) then
     ErrorNoReturn("the 1st argument <D> must be a digraph with no multiple ",
                   "edges,");
@@ -243,6 +257,15 @@ function(D, src, ran)
     Remove(D!.OutNeighbours[src], pos);
     RemoveDigraphEdgeLabel(D, src, pos);
   fi;
+
+  if HasEdgeWeights(D) then
+      weights := StructuralCopy(EdgeWeights(D));
+      if IsBound(weights[src]) and IsBound(weights[src][ran]) then
+          weights[src][ran] := fail;
+      fi;
+      SetEdgeWeights(D, weights);
+  fi;
+
   return D;
 end);