New puzzle: `Slant', picked from the Japanese-language section of

nikoli.co.jp (which has quite a few puzzles that they don't seem to
have bothered to translate into English).

Minor structural change: the disjoint set forest code used in the
Net solver has come in handy again, so I've moved it out into its
own module dsf.c.

[originally from svn r6155]

dsf.c

@@ -0,0 +1,28 @@
+/*
+ * dsf.c: two small functions to handle a disjoint set forest,
+ * which is a data structure useful in any solver which has to
+ * worry about avoiding closed loops.
+ */
+
+int dsf_canonify(int *dsf, int val)
+{
+    int v2 = val;
+
+    while (dsf[val] != val)
+	val = dsf[val];
+
+    while (v2 != val) {
+	int tmp = dsf[v2];
+	dsf[v2] = val;
+	v2 = tmp;
+    }
+
+    return val;
+}
+
+void dsf_merge(int *dsf, int v1, int v2)
+{
+    v1 = dsf_canonify(dsf, v1);
+    v2 = dsf_canonify(dsf, v2);
+    dsf[v2] = v1;
+}