ViewString: special case for trees and forests

digraphs · Mar 17, 2021 · 33828b3 · 33828b3
1 parent 00ad997
commit 33828b3
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Showing 6 changed files with 17 additions and 5 deletions.
diff --git a/doc/attr.xml b/doc/attr.xml
@@ -2144,7 +2144,7 @@ gap> D := Digraph([[1, 2, 1, 3], [1], [4], [3, 4, 3]]);
 gap> UndirectedSpanningTree(D);
 fail
 gap> forest := UndirectedSpanningForest(D);
-<immutable symmetric digraph with 4 vertices, 4 edges>
+<immutable undirected forest digraph with 4 vertices, 4 edges>
 gap> OutNeighbours(forest);
 [ [ 2 ], [ 1 ], [ 4 ], [ 3 ] ]
 gap> IsUndirectedSpanningForest(D, forest);
@@ -2159,7 +2159,7 @@ true
 gap> D := CompleteDigraph(4);
 <immutable complete digraph with 4 vertices>
 gap> tree := UndirectedSpanningTree(D);
-<immutable symmetric digraph with 4 vertices, 6 edges>
+<immutable undirected tree digraph with 4 vertices>
 gap> IsUndirectedSpanningTree(D, tree);
 true
 gap> tree = UndirectedSpanningForest(D);

diff --git a/doc/oper.xml b/doc/oper.xml
@@ -1436,7 +1436,7 @@ false
 gap> DigraphShortestPathSpanningTree(D, 1);
 fail
 gap> tree := DigraphShortestPathSpanningTree(D, 5);
-<immutable digraph with 5 vertices, 4 edges>
+<immutable directed tree digraph with 5 vertices>
 gap> OutNeighbours(tree);
 [ [  ], [ 3 ], [  ], [ 1 ], [ 2, 4 ] ]
 gap> ForAll(DigraphVertices(D), v ->

diff --git a/gap/attr.gi b/gap/attr.gi
@@ -2093,6 +2093,7 @@ InstallMethod(UndirectedSpanningTree, "for an immutable digraph",
 InstallMethod(UndirectedSpanningTreeAttr, "for an immutable digraph",
 [IsImmutableDigraph],
 function(D)
+  local out;
   if DigraphNrVertices(D) = 0
       or not IsStronglyConnectedDigraph(D)
       or (HasMaximalSymmetricSubdigraphAttr(D)
@@ -2101,7 +2102,9 @@ function(D)
           <> 2 * (DigraphNrVertices(D) - 1)) then
     return fail;
   fi;
-  return UndirectedSpanningForest(D);
+  out := UndirectedSpanningForest(D);
+  SetIsUndirectedTree(out, true);
+  return out;
 end);
 
 InstallMethod(DigraphMycielskian, "for a digraph",

diff --git a/gap/digraph.gi b/gap/digraph.gi
@@ -479,6 +479,14 @@ function(D)
       Append(str, "join semilattice ");
     elif HasIsMeetSemilatticeDigraph(D) and IsMeetSemilatticeDigraph(D) then
       Append(str, "meet semilattice ");
+    elif HasIsUndirectedTree(D) and IsUndirectedTree(D) then
+      Append(str, "undirected tree ");
+      display_nredges := false;
+    elif HasIsUndirectedForest(D) and IsUndirectedForest(D) then
+      Append(str, "undirected forest ");
+    elif HasIsDirectedTree(D) and IsDirectedTree(D) then
+      Append(str, "directed tree ");
+      display_nredges := false;
     else
       if HasIsEulerianDigraph(D) and IsEulerianDigraph(D) then
         Append(str, "Eulerian ");

diff --git a/gap/prop.gd b/gap/prop.gd
@@ -70,6 +70,7 @@ InstallTrueMethod(IsAcyclicDigraph, IsTournament and IsTransitiveDigraph);
 InstallTrueMethod(IsAntisymmetricDigraph, IsAcyclicDigraph);
 InstallTrueMethod(IsAntisymmetricDigraph, IsTournament);
 InstallTrueMethod(IsBipartiteDigraph, IsCompleteBipartiteDigraph);
+InstallTrueMethod(IsBipartiteDigraph, IsUndirectedForest);
 InstallTrueMethod(IsCompleteMultipartiteDigraph, IsCompleteBipartiteDigraph);
 InstallTrueMethod(IsConnectedDigraph, IsBiconnectedDigraph);
 InstallTrueMethod(IsConnectedDigraph, IsStronglyConnectedDigraph);

diff --git a/tst/standard/attr.tst b/tst/standard/attr.tst
@@ -1631,7 +1631,7 @@ true
 gap> D := DigraphFromDigraph6String("&I~~~~^Znn~|~~x^|v{");
 <immutable digraph with 10 vertices, 89 edges>
 gap> tree := UndirectedSpanningTree(D);
-<immutable symmetric digraph with 10 vertices, 18 edges>
+<immutable undirected tree digraph with 10 vertices>
 gap> IsUndirectedSpanningTree(D, tree);
 true
 gap> tree := UndirectedSpanningTree(DigraphMutableCopy(D));