Implementation of DigraphInsertEdge and DigraphReduceEdge

doc/oper.xml

@@ -2076,6 +2076,84 @@ gap> DigraphVertexLabels(D);
 </ManSection>
 <#/GAPDoc>
 
+<#GAPDoc Label="DigraphInsertEdge">
+<ManSection>
+    <Oper Name="DigraphInsertEdge" Arg="digraph, edge1, edge2"
+        Label="for a symmeric digraph and two lists of positive integers"/>
+  <Returns>A digraph.</Returns>
+  <Description>
+    If <A>edge1</A> and <A>edge2</A> are distinct pairs of vertices of <A>digraph</A>,
+    then then this operation inserts a new edge between <A>edge1</A> and
+    <A>edge2</A>.
+    <A>edge1</A> and <A>edge2</A> are split up into two new edges each, which are
+    all adjacent to the inserted edge. Hence, the vertices of the inserted edge both
+    have a degree of 3.
+
+    The opposite operation is <Ref Oper="DigraphReduceEdge"/>.
+
+    A new digraph constructed from <A>digraph</A> is returned,
+    unless <A>digraph</A> belongs to <Ref Filt="IsMutableDigraph"/>;
+    in this case changes are made directly to <A>digraph</A>, which is then returned. 
+    The <A>digraph</A> must not belong to <Ref Filt="IsMultiDigraph"/>. <P/>
+
+    <Example><![CDATA[
+gap> D := DigraphByEdges([[1, 2], [2, 1], [3, 4], [4, 3]]);
+<immutable digraph with 4 vertices, 4 edges>
+gap> D2 := DigraphInsertEdge(D, [1, 2], [3, 4]);
+<immutable digraph with 6 vertices, 10 edges>
+gap> DigraphEdges(D2);
+[ [ 1, 5 ], [ 2, 5 ], [ 3, 6 ], [ 4, 6 ], [ 5, 1 ], [ 5, 2 ], [ 5, 6 ], 
+  [ 6, 3 ], [ 6, 4 ], [ 6, 5 ] ]
+gap> D := DigraphByEdges([[1, 2], [2, 1], [2, 3], [3, 2]]);
+<immutable digraph with 3 vertices, 4 edges>
+gap> D2 := DigraphInsertEdge(D, [1, 2], [2, 3]);
+<immutable digraph with 5 vertices, 10 edges>
+gap> DigraphEdges(D2);
+[ [ 1, 4 ], [ 2, 4 ], [ 2, 5 ], [ 3, 5 ], [ 4, 1 ], [ 4, 2 ], [ 4, 5 ], 
+  [ 5, 2 ], [ 5, 3 ], [ 5, 4 ] ]
+]]></Example>
+  </Description>
+</ManSection>
+<#/GAPDoc>
+
+<#GAPDoc Label="DigraphReduceEdge">
+<ManSection>
+    <Oper Name="DigraphReduceEdge" Arg="digraph, edge"
+        Label="for a symmeric digraph and a list of positive integers"/>
+  <Returns>A digraph.</Returns>
+  <Description>
+    If <A>edge</A> is an edge of <A>digraph</A> and adjacent to four edges, then this
+    operation reduces this <A>edge</A>: <A>edge</A> will be removed, and for both
+    vertices of <A>edge</A> the two adjacent edges are combined to a single edge.
+    The vertices of <A>edge</A> must both have a degree of 3.
+
+    The opposite operation is <Ref Oper="DigraphInsertEdge"/>.
+
+    A new digraph constructed from <A>digraph</A> is returned,
+    unless <A>digraph</A> belongs to <Ref Filt="IsMutableDigraph"/>;
+    in this case changes are made directly to <A>digraph</A>, which is then returned. 
+    The <A>digraph</A> must not belong to <Ref Filt="IsMultiDigraph"/>. <P/>
+
+    <Example><![CDATA[
+gap> D := DigraphByEdges([[1, 5], [5, 1], [2, 5], [5, 2], [3, 6],
+  [6, 3], [4, 6], [6, 4], [5, 6], [6, 5]]);
+<immutable digraph with 6 vertices, 10 edges>
+gap> D2 := DigraphReduceEdge(D, [5, 6]);
+<immutable digraph with 4 vertices, 4 edges>
+gap> DigraphEdges(D2);
+[ [ 1, 2 ], [ 2, 1 ], [ 3, 4 ], [ 4, 3 ] ]
+gap> D := DigraphByEdges([[1, 4], [4, 1], [2, 4], [4, 2], [2, 5],
+  [5, 2], [3, 5], [5, 3], [4, 5], [5, 4]]);
+<immutable digraph with 5 vertices, 10 edges>
+gap> D2 := DigraphReduceEdge(D, [4, 5]);
+<immutable digraph with 3 vertices, 4 edges>
+gap> DigraphEdges(D2);
+[ [ 1, 2 ], [ 2, 1 ], [ 2, 3 ], [ 3, 2 ] ]
+]]></Example>
+  </Description>
+</ManSection>
+<#/GAPDoc>
+
 <#GAPDoc Label="IsMatching">
 <ManSection>
   <Oper Name="IsMatching" Arg="digraph, list"/>

gap/oper.gd

@@ -38,6 +38,10 @@ DeclareOperation("DigraphClosure", [IsDigraph, IsPosInt]);
 DeclareOperation("DigraphContractEdge", [IsDigraph, IsPosInt, IsPosInt]);
 DeclareOperation("DigraphContractEdge", [IsDigraph, IsDenseList]);
 
+DeclareOperation("DigraphInsertEdge", [IsDigraph, IsDenseList, IsDenseList]);
+
+DeclareOperation("DigraphReduceEdge", [IsDigraph, IsDenseList]);
+
 # 3. Ways of combining digraphs . . .
 DeclareGlobalFunction("DigraphDisjointUnion");
 DeclareGlobalFunction("DigraphJoin");

gap/oper.gi

@@ -603,6 +603,183 @@ function(D, edge)
   return DigraphContractEdge(D, edge[1], edge[2]);
 end);
 
+DIGRAPHS_CheckInsertEdgeDigraph := function(D, edge1, edge2)
+  if IsEmptyDigraph(D) then
+    ErrorNoReturn("the 1st argument <D> must not be an empty digraph");
+  fi;
+  if not IsSymmetricDigraph(D) then
+    ErrorNoReturn("the 1st argument <D> must be a symmetric digraph");
+  fi;
+
+  if Length(edge1) <> 2 then
+    ErrorNoReturn("the 2nd argument <edge1> must be a list of length 2");
+  fi;
+  if not IsDigraphEdge(D, edge1) then
+    ErrorNoReturn("the 2nd argument <edge1> must be an edge of the ",
+                  "digraph <D>");
+  fi;
+
+  if Length(edge2) <> 2 then
+    ErrorNoReturn("the 3rd argument <edge2> must be a list of length 2");
+  fi;
+  if not IsDigraphEdge(D, edge2) then
+    ErrorNoReturn("the 3rd argument <edge2> must be an edge of the ",
+                  "digraph <D>");
+  fi;
+
+  if Length(Union(edge1, edge2)) < 3 then
+    ErrorNoReturn("the 2nd and 3rd argument <edge1> and <edge2> must ",
+                  "be two different edges");
+  fi;
+
+end;
+
+InstallMethod(DigraphInsertEdge,
+"for a symmetric digraph and two dense lists",
+[IsMutableDigraph, IsDenseList, IsDenseList],
+function(D, edge1, edge2)
+  local numVertices;
+
+  DIGRAPHS_CheckInsertEdgeDigraph(D, edge1, edge2);
+
+  numVertices := Maximum(DigraphVertices(D));
+
+  D := DigraphRemoveEdge(D, edge1);
+  D := DigraphRemoveEdge(D, Reversed(edge1));
+  D := DigraphRemoveEdge(D, edge2);
+  D := DigraphRemoveEdge(D, Reversed(edge2));
+
+  DigraphAddVertex(D, numVertices + 1);  # vertex A
+  DigraphAddVertex(D, numVertices + 2);  # vertex B
+
+  # Add two connected edges between each former edge
+  #
+  # Add edges intersecting former edge1 with vertex A
+  DigraphAddEdge(D, edge1[1], numVertices + 1);
+  DigraphAddEdge(D, numVertices + 1, edge1[1]);
+  #
+  DigraphAddEdge(D, edge1[2], numVertices + 1);
+  DigraphAddEdge(D, numVertices + 1, edge1[2]);
+  #
+  # Add edges intersecting former edge2 with vertex B
+  DigraphAddEdge(D, edge2[1], numVertices + 2);
+  DigraphAddEdge(D, numVertices + 2, edge2[1]);
+  #
+  DigraphAddEdge(D, edge2[2], numVertices + 2);
+  DigraphAddEdge(D, numVertices + 2, edge2[2]);
+  #
+  # Add edges connecting vertex A and vertex B
+  DigraphAddEdge(D, numVertices + 1, numVertices + 2);
+  DigraphAddEdge(D, numVertices + 2, numVertices + 1);
+
+  # TODO: Call SetIsSymmetricDigraph and/or
+  # SetDigraphSymmetricClosureAttr here?
+
+  return D;
+end);
+
+InstallMethod(DigraphInsertEdge,
+"for a symmetric digraph and two dense lists",
+[IsImmutableDigraph, IsDenseList, IsDenseList],
+function(D, edge1, edge2)
+  D := DigraphInsertEdge(DigraphMutableCopy(D), edge1, edge2);
+
+  return MakeImmutable(D);
+end);
+
+DIGRAPHS_CheckReduceEdgeDigraph := function(D, edge)
+  local leftVertexOutNeighbours, rightVertexOutNeighbours,
+        allVertexOutNeighbours;
+
+  if IsEmptyDigraph(D) then
+    ErrorNoReturn("the 1st argument <D> must not be an empty digraph");
+  fi;
+  if not IsSymmetricDigraph(D) then
+    ErrorNoReturn("the 1st argument <D> must be a symmetric digraph");
+  fi;
+
+  if Length(edge) <> 2 then
+    ErrorNoReturn("the 2nd argument <edge> must be a list of length 2");
+  fi;
+  if not IsDigraphEdge(D, edge) then
+    ErrorNoReturn("the 2nd argument <edge> must be an edge of the digraph <D>");
+  fi;
+
+  leftVertexOutNeighbours := OutNeighboursOfVertex(D, edge[1]);
+  rightVertexOutNeighbours := OutNeighboursOfVertex(D, edge[2]);
+  allVertexOutNeighbours := Union(leftVertexOutNeighbours,
+                                  rightVertexOutNeighbours);
+
+  # Check if edge vertices have degree three
+  if Length(leftVertexOutNeighbours) <> 3 or
+      Length(rightVertexOutNeighbours) <> 3 then
+    ErrorNoReturn("the 2nd argument <edge> must be an edge where the ",
+                  "incident vertices have degree three");
+  fi;
+  if not Length(allVertexOutNeighbours) in [5, 6] then
+    ErrorNoReturn("the 2nd argument <edge> must be an edge where the ",
+                  "incident vertices have a maximum of one common ",
+                  "neighbour");
+  fi;
+
+end;
+
+InstallMethod(DigraphReduceEdge,
+"for a symmetric digraph and a dense list",
+[IsMutableDigraph, IsDenseList],
+function(D, edge)
+  local neighbours, neighbour, newEdge;
+
+  DIGRAPHS_CheckReduceEdgeDigraph(D, edge);
+
+  # Add new edges
+  #
+  # left side of edge
+  newEdge := [];
+  neighbours := OutNeighboursOfVertex(D, edge[1]);
+  for neighbour in neighbours do
+    if neighbour <> edge[2] then
+      Add(newEdge, neighbour);
+    fi;
+  od;
+  for neighbour in neighbours do
+    DigraphRemoveEdge(D, [edge[1], neighbour]);
+  od;
+  DigraphAddEdge(D, newEdge[1], newEdge[2]);
+  DigraphAddEdge(D, newEdge[2], newEdge[1]);
+  #
+  # right side of intersecting edge
+  newEdge := [];
+  neighbours := OutNeighboursOfVertex(D, edge[2]);
+  for neighbour in neighbours do
+    if neighbour <> edge[1] then
+      Add(newEdge, neighbour);
+    fi;
+  od;
+  for neighbour in neighbours do
+    DigraphRemoveEdge(D, [edge[2], neighbour]);
+  od;
+  DigraphAddEdge(D, newEdge[1], newEdge[2]);
+  DigraphAddEdge(D, newEdge[2], newEdge[1]);
+
+  # Remove old vertices
+  DigraphRemoveVertices(D, edge);
+
+  # TODO: Call SetIsSymmetricDigraph and/or
+  # SetDigraphSymmetricClosureAttr here?
+
+  return D;
+end);
+
+InstallMethod(DigraphReduceEdge,
+"for a symmetric digraph and a dense list",
+[IsImmutableDigraph, IsDenseList],
+function(D, edge)
+  D := DigraphReduceEdge(DigraphMutableCopy(D), edge);
+
+  return MakeImmutable(D);
+end);
+
 #############################################################################
 # 3. Ways of combining digraphs
 #############################################################################