Mike Pinna has done some major reworking of the Loopy solver, giving

rise to a new Hard difficulty level.

[originally from svn r6880]

dsf.c

@@ -1,38 +1,156 @@
 /*
- * dsf.c: two small functions to handle a disjoint set forest,
+ * dsf.c: some functions to handle a disjoint set forest,
  * which is a data structure useful in any solver which has to
  * worry about avoiding closed loops.
  */
 
+#include <assert.h>
+#include <string.h>
+
 #include "puzzles.h"
 
-int dsf_canonify(int *dsf, int val)
+void print_dsf(int *dsf, int size)
 {
-    int v2 = val;
+    int *printed_elements = snewn(size, int);
+    int *equal_elements = snewn(size, int);
+    int *inverse_elements = snewn(size, int);
+    int printed_count = 0, equal_count, inverse_count;
+    int i, n, inverse;
 
-    while (dsf[val] != val)
+    memset(printed_elements, -1, sizeof(int) * size);
-	val = dsf[val];
 
-    while (v2 != val) {
+    while (1) {
-	int tmp = dsf[v2];
+        equal_count = 0;
-	dsf[v2] = val;
+        inverse_count = 0;
-	v2 = tmp;
+        for (i = 0; i < size; ++i) {
+            if (!memchr(printed_elements, i, sizeof(int) * size)) 
+                break;
+        }
+        if (i == size)
+            goto done;
+
+        i = dsf_canonify(dsf, i);
+
+        for (n = 0; n < size; ++n) {
+            if (edsf_canonify(dsf, n, &inverse) == i) {
+               if (inverse)
+                   inverse_elements[inverse_count++] = n;
+               else
+                   equal_elements[equal_count++] = n;
+            }
+        }
+        
+        for (n = 0; n < equal_count; ++n) {
+            fprintf(stderr, "%d ", equal_elements[n]);
+            printed_elements[printed_count++] = equal_elements[n];
+        }
+        if (inverse_count) {
+            fprintf(stderr, "!= ");
+            for (n = 0; n < inverse_count; ++n) {
+                fprintf(stderr, "%d ", inverse_elements[n]);
+                printed_elements[printed_count++] = inverse_elements[n];
+            }
+        }
+        fprintf(stderr, "\n");
+    }
+done:
+
+    sfree(printed_elements);
+    sfree(equal_elements);
+    sfree(inverse_elements);
+}
+
+int *snew_dsf(int size)
+{
+    int i;
+    int *ret;
+    
+    ret = snewn(size, int);
+    for (i = 0; i < size; i++) {
+        /* Bottom bit of each element of this array stores whether that element
+         * is opposite to its parent, which starts off as false */
+        ret[i] = i << 1; 
     }
 
-    return val;
+    /*print_dsf(ret, size); */
+
+    return ret;
+}
+
+int dsf_canonify(int *dsf, int index)
+{
+    return edsf_canonify(dsf, index, NULL);
 }
 
 void dsf_merge(int *dsf, int v1, int v2)
 {
-    v1 = dsf_canonify(dsf, v1);
+    edsf_merge(dsf, v1, v2, FALSE);
-    v2 = dsf_canonify(dsf, v2);
-    dsf[v2] = v1;
 }
 
-void dsf_init(int *dsf, int len)
+int edsf_canonify(int *dsf, int index, int *inverse_return)
 {
-    int i;
+    int start_index = index, canonical_index;
+    int inverse = 0;
 
-    for (i = 0; i < len; i++)
+/*    fprintf(stderr, "dsf = %p\n", dsf); */
-	dsf[i] = i;
+/*    fprintf(stderr, "Canonify %2d\n", index); */
+
+    assert(index >= 0);
+
+    /* Find the index of the canonical element of the 'equivalence class' of
+     * which start_index is a member, and figure out whether start_index is the
+     * same as or inverse to that. */
+    while ((dsf[index] >> 1) != index) {
+        inverse ^= (dsf[index] & 1);
+	index = dsf[index] >> 1;
+/*        fprintf(stderr, "index = %2d, ", index); */
+/*        fprintf(stderr, "inverse = %d\n", inverse); */
+    }
+    canonical_index = index;
+    
+    if (inverse_return)
+        *inverse_return = inverse;
+    
+    /* Update every member of this 'equivalence class' to point directly at the
+     * canonical member. */
+    index = start_index;
+    while (index != canonical_index) {
+	int nextindex = dsf[index] >> 1;
+        int nextinverse = inverse ^ (dsf[index] & 1);
+	dsf[index] = (canonical_index << 1) | inverse;
+        inverse = nextinverse;
+	index = nextindex;
+    }
+
+    assert(inverse == 0);
+
+/*    fprintf(stderr, "Return %2d\n", index); */
+    
+    return index;
+}
+
+void edsf_merge(int *dsf, int v1, int v2, int inverse)
+{
+    int i1, i2;
+    
+/*    fprintf(stderr, "dsf = %p\n", dsf); */
+/*    fprintf(stderr, "Merge [%2d,%2d], %d\n", v1, v2, inverse); */
+    
+    v1 = edsf_canonify(dsf, v1, &i1);
+    inverse ^= i1;
+    v2 = edsf_canonify(dsf, v2, &i2);
+    inverse ^= i2;
+
+/*    fprintf(stderr, "Doing [%2d,%2d], %d\n", v1, v2, inverse); */
+
+    if (v1 == v2)
+        assert(!inverse);
+    else
+        dsf[v2] = (v1 << 1) | !!inverse;
+    
+    v2 = edsf_canonify(dsf, v2, &i2);
+    assert(v2 == v1);
+    assert(i2 == inverse);
+
+/*    fprintf(stderr, "dsf[%2d] = %2d\n", v2, dsf[v2]); */
 }

puzzles.h

@@ -278,7 +278,22 @@ void draw_rect_outline(drawing *dr, int x, int y, int w, int h,
 /*
  * dsf.c
  */
+int *snew_dsf(int size);
+
+void print_dsf(int *dsf, int size);
+
+/* Return the canonical element of the equivalence class containing element
+ * val.  If 'inverse' is non-NULL, this function will put into it a flag
+ * indicating whether the canonical element is inverse to val. */
+int edsf_canonify(int *dsf, int val, int *inverse);
 int dsf_canonify(int *dsf, int val);
+
+/* Allow the caller to specify that two elements should be in the same
+ * equivalence class.  If 'inverse' is TRUE, the elements are actually opposite
+ * to one another in some sense.  This function will fail an assertion if the
+ * caller gives it self-contradictory data, ie if two elements are claimed to
+ * be both opposite and non-opposite. */
+void edsf_merge(int *dsf, int v1, int v2, int inverse);
 void dsf_merge(int *dsf, int v1, int v2);
 void dsf_init(int *dsf, int len);