Use visited map like a stack to walk full tree (including duplicate pointers)

deephash.go

@@ -33,6 +33,19 @@ func deepHash(src reflect.Value, visited map[uintptr]*visit, depth int) []byte {
 		}
 		// Remember, remember...
 		visited[h] = &visit{addr, typ, seen}
+		defer func() {
+			// If we get here, we've either added a new entry in visited or
+			// a new type to the end of a list in visited
+			if seen != nil {
+				// If we just added a type to the end, remove it when
+				// returning from this level of recursion
+				visited[h] = seen
+			} else {
+				// If this is the first time we've seen this memory address,
+				// pop it off when returning from this level of recursion
+				delete(visited, h)
+			}
+		}()
 	}
 
 	hash := fnv.New64a()

deephash_test.go

@@ -259,3 +259,30 @@ func TestBooleans(t *testing.T) {
 		t.Fatal("Expecting true to hash differently than false")
 	}
 }
+
+type parent struct {
+	c1, c2 *child
+}
+
+type child struct {
+	val string
+}
+
+func TestStackCycle(t *testing.T) {
+	c1 := child{val: "child"}
+	c2 := child{val: "child"}
+
+	ch1 := Hash(&c1)
+	ch2 := Hash(&c2)
+	if !bytes.Equal(ch1, ch2) {
+		t.Fatalf("got %d != %d, want equal", ch1, ch2)
+	}
+
+	p1 := parent{c1: &c1, c2: &c1}
+	p2 := parent{c1: &c1, c2: &c2}
+	ph1 := Hash(p1)
+	ph2 := Hash(p2)
+	if !bytes.Equal(ph1, ph2) {
+		t.Errorf("got %d != %d, want equal", ph1, ph2)
+	}
+}