Add DigraphRandomWalk with doc and tests

doc/oper.xml

@@ -1535,6 +1535,36 @@ gap> DigraphShortestPath(D, 1, 1);
 </ManSection>
 <#/GAPDoc>
 
+<#GAPDoc Label="DigraphRandomWalk">
+<ManSection>
+  <Oper Name="DigraphRandomWalk" Arg="digraph, v, t"/>
+  <Returns>A pair of lists.</Returns>
+  <Description>
+    Returns a directed path corresponding to a <E>random walk</E> in the digraph
+    <A>digraph</A>, starting at vertex <A>v</A> and having length no more than
+    <A>t</A>.
+    <P/>
+
+    A random walk is defined as follows.  The path begins at <A>v</A>, and at
+    each step it follows a random edge leaving the current vertex.  It continues
+    through the digraph in this way until it has traversed <A>t</A> edges, or
+    until it reaches a vertex with no out-edges (a <E>sink</E>) and therefore
+    cannot continue.
+    <P/>
+
+    The output has the same form as that of <Ref Oper="DigraphPath"/>.
+    <P/>
+
+<Log><![CDATA[
+gap> D := Digraph([[1, 2], [3], [2, 4], [1], [2, 4]]);                
+<immutable digraph with 5 vertices, 8 edges>
+gap> DigraphRandomWalk(D, 1, 4);
+[ [ 1, 2, 3, 2, 3 ], [ 2, 1, 1, 1 ] ]
+]]></Log>
+  </Description>
+</ManSection>
+<#/GAPDoc>
+
 <#GAPDoc Label="IteratorOfPaths">
 <ManSection>
   <Oper Name="IteratorOfPaths" Arg="digraph, u, v"/>

doc/z-chap4.xml

@@ -68,6 +68,7 @@
     <#Include Label="VerticesReachableFrom">
     <#Include Label="DigraphPath">
     <#Include Label="DigraphShortestPath">
+    <#Include Label="DigraphRandomWalk">
     <#Include Label="Dominators">
     <#Include Label="DominatorTree">
     <#Include Label="IteratorOfPaths">

gap/oper.gd

@@ -125,6 +125,7 @@ DeclareOperation("IteratorOfPaths", [IsList, IsPosInt, IsPosInt]);
 DeclareOperation("IteratorOfPathsNC", [IsList, IsPosInt, IsPosInt]);
 DeclareOperation("IsReachable", [IsDigraph, IsPosInt, IsPosInt]);
 DeclareOperation("DigraphLongestDistanceFromVertex", [IsDigraph, IsPosInt]);
+DeclareOperation("DigraphRandomWalk", [IsDigraph, IsPosInt, IsInt]);
 
 DeclareOperation("DigraphLayers", [IsDigraph, IsPosInt]);
 DeclareAttribute("DIGRAPHS_Layers", IsDigraph, "mutable");

gap/oper.gi

@@ -1727,6 +1727,41 @@ function(D, v)
   return dist;
 end);
 
+InstallMethod(DigraphRandomWalk,
+"for a digraph, a pos int and a non-negative int",
+[IsDigraph, IsPosInt, IsInt],
+function(D, v, t)
+  local vertices, edge_indices, i, neighbours, index;
+
+  # Check input
+  if v > DigraphNrVertices(D) then
+    ErrorNoReturn("the 2nd argument <v> must be ",
+                  "a vertex of the 1st argument <D>,");
+  elif t < 0 then
+    ErrorNoReturn("the 3rd argument <t> must be a non-negative int,");
+  fi;
+
+  # Prepare output lists
+  vertices     := [v];
+  edge_indices := [];
+
+  # Iterate to desired length
+  for i in [1 .. t] do
+    neighbours := OutNeighboursOfVertex(D, v);
+    if IsEmpty(neighbours) then
+      break;  # Sink: path ends here
+    fi;
+    # Follow a random edge
+    index := Random(1, Length(neighbours));
+    v     := neighbours[index];
+    vertices[i + 1] := v;
+    edge_indices[i] := index;
+  od;
+
+  # Format matches that of DigraphPath
+  return [vertices, edge_indices];
+end);
+
 InstallMethod(DigraphLayers, "for a digraph, and a positive integer",
 [IsDigraph, IsPosInt],
 function(D, v)

tst/standard/oper.tst

@@ -1443,6 +1443,36 @@ infinity
 gap> DigraphLongestDistanceFromVertex(gr, 16);
 Error, the 2nd argument <v> must be a vertex of the 1st argument <D>,
 
+#  DigraphRandomWalk
+gap> gr := CompleteDigraph(5);
+<immutable complete digraph with 5 vertices>
+gap> path := DigraphRandomWalk(gr, 1, 100);;
+gap> Length(path[1]);
+101
+gap> ForAll(path[1], i -> i in [1 .. 5]);
+true
+gap> Length(path[2]);
+100
+gap> ForAll(path[2], i -> i in [1 .. 4]);
+true
+gap> gr := ChainDigraph(5);
+<immutable chain digraph with 5 vertices>
+gap> DigraphRandomWalk(gr, 2, 100);
+[ [ 2, 3, 4, 5 ], [ 1, 1, 1 ] ]
+gap> DigraphRandomWalk(gr, 2, 2);  
+[ [ 2, 3, 4 ], [ 1, 1 ] ]
+gap> DigraphRandomWalk(gr, 5, 100);
+[ [ 5 ], [  ] ]
+gap> gr := CompleteBipartiteDigraph(10, 8);;
+gap> DigraphRandomWalk(gr, 3, 0);           
+[ [ 3 ], [  ] ]
+gap> DigraphRandomWalk(gr, 19, 5);
+Error, the 2nd argument <v> must be a vertex of the 1st argument <D>,
+gap> DigraphRandomWalk(gr, 123, 5);
+Error, the 2nd argument <v> must be a vertex of the 1st argument <D>,
+gap> DigraphRandomWalk(gr, 3, -1); 
+Error, the 3rd argument <t> must be a non-negative int,
+
 #  DigraphLayers
 gap> gr := CompleteDigraph(4);
 <immutable complete digraph with 4 vertices>