Add hash function for digraphs

doc/attr.xml

@@ -2548,3 +2548,16 @@ fail
   </Description>
 </ManSection>
 <#/GAPDoc>
+
+<#GAPDoc Label="DigraphHash">
+<ManSection>
+  <Attr Name="DigraphHash" Arg="digraph"/>
+  <Returns>The hash of a digraph.</Returns>
+  <Description>
+    This function returns a hash of <A>digraph</A>. <P/>
+
+    Note that the underlying hashing function is system dependent, and so the
+    value of this attribute is not guaranteed to be the same on different systems.
+  </Description>
+</ManSection>
+<#/GAPDoc>

doc/z-chap4.xml

@@ -103,4 +103,8 @@
     <#Include Label="SubdigraphHomeomorphicToK">
   </Section>
 
+  <Section><Heading>Hashing</Heading>
+    <#Include Label="DigraphHash">
+  </Section>
+
 </Chapter>

gap/attr.gd

@@ -15,6 +15,7 @@ DeclareAttribute("DigraphNrVertices", IsDigraph);
 DeclareAttribute("DigraphEdges", IsDigraph);
 DeclareAttribute("DigraphNrEdges", IsDigraph);
 DeclareAttribute("DigraphNrLoops", IsDigraph);
+DeclareAttribute("DigraphHash", IsDigraph);
 
 DeclareAttribute("DigraphRange", IsDigraph);
 DeclareAttribute("DigraphSource", IsDigraph);

gap/attr.gi

@@ -750,6 +750,13 @@ function(D)
   return out;
 end);
 
+InstallMethod(DigraphHash, "for a digraph", [IsDigraph], DIGRAPH_HASH);
+
+InstallMethod(SparseIntKey, "for an object and digraph",
+[IsObject, IsDigraph],
+{coll, D} -> DigraphHash
+);
+
 # attributes for digraphs . . .
 
 InstallMethod(AsGraph, "for a digraph", [IsDigraph], Graph);

src/digraphs.c

@@ -1133,7 +1133,7 @@ static Obj FuncIS_MULTI_DIGRAPH(Obj self, Obj digraph) {
  *      - If true, for each vertex i, dist[i][i] is initially set to 0
  *      - If false, this step is skipped
  */
-static Obj FLOYD_WARSHALL(Obj  digraph,
+static Obj FLOYD_WARSHALL(Obj digraph,
                           void (*func)(Int** dist, Int i, Int j, Int k, Int n),
                           Int  val1,
                           Int  val2,
@@ -1384,6 +1384,56 @@ static bool EqJumbledPlists(Obj l, Obj r, Int nr, Int* buf) {
   return true;
 }
 
+//-----------------------------------------------------------------------------
+// MurmurHash3 was written by Austin Appleby, and is placed in the public
+// domain. The author hereby disclaims copyright to this source code.
+
+/* Minor modifications to get it to compile in C rather than C++ and
+integrate with GAP  SL*/
+
+/* Digraphs takes parts of this source from GAP. */
+
+#define BIG_CONSTANT(x) (x##LLU)
+
+static inline UInt fmix(UInt h) {
+#ifndef SYS_IS_64_BIT
+  h ^= h >> 16;
+  h *= 0x85ebca6b;
+  h ^= h >> 13;
+  h *= 0xc2b2ae35;
+  h ^= h >> 16;
+#else
+  h ^= h >> 33;
+  h *= BIG_CONSTANT(0xff51afd7ed558ccd);
+  h ^= h >> 33;
+  h *= BIG_CONSTANT(0xc4ceb9fe1a85ec53);
+  h ^= h >> 33;
+#endif
+
+  return h;
+}
+
+static Obj FuncDIGRAPH_HASH(Obj self, Obj digraph) {
+  UInt n, i, h;
+  Obj  out, a;
+  Int  nr, j;
+
+  h   = 0;
+  n   = DigraphNrVertices(digraph);
+  out = FuncOutNeighbours(self, digraph);
+
+  for (i = 1; i <= n; i++) {
+    a = ELM_PLIST(out, i);
+    PLAIN_LIST(a);
+    nr = LEN_PLIST(a);
+    for (j = 1; j <= nr; j++)
+      h += fmix(INT_INTOBJ(ELM_PLIST(a, j)));
+    h = fmix(h + 0x9e3779b9);
+  }
+
+  return INTOBJ_INT(h);
+}
+
 static Obj FuncDIGRAPH_EQUALS(Obj self, Obj digraph1, Obj digraph2) {
   UInt i, n1, n2, m1, m2;
   Obj  out1, out2, a, b;
@@ -2199,6 +2249,12 @@ static StructGVarFunc GVarFuncs[] = {
      FuncRANDOM_MULTI_DIGRAPH,
      "src/digraphs.c:FuncRANDOM_MULTI_DIGRAPH"},
 
+    {"DIGRAPH_HASH",
+     1,
+     "digraph",
+     FuncDIGRAPH_HASH,
+     "src/digraphs.c:FuncDIGRAPH_HASH"},
+
     {"DIGRAPH_EQUALS",
      2,
      "digraph1, digraph2",

tst/standard/attr.tst

@@ -2973,6 +2973,22 @@ gap> DigraphHomomorphism(NullDigraph(1), NullDigraph(65555));
 Error, the 2nd argument <digraph2> must have at most 65534 vertices, found 655\
 55,
 
+# Test Digraph hashing
+# This has a small chance to randomly fail. Sorry if it does!
+gap> D1 := RandomMultiDigraph(100);;
+gap> D2 := Digraph(List(OutNeighbours(D1), x -> Shuffle(ShallowCopy(x))));;
+gap> D1 = D2;
+true
+gap> OutNeighbours(D1) = OutNeighbours(D2);
+false
+gap> DigraphHash(D1) = DigraphHash(D2);
+true
+gap> while D1 = D2 do
+> D2 := RandomMultiDigraph(100);
+> od;;
+gap> DigraphHash(D1) = DigraphHash(D2);
+false
+
 # Unbind local variables, auto-generated by etc/tst-unbind-local-vars.py
 gap> Unbind(A);
 gap> Unbind(B);