commentary: clarify and fix input vs output confusion

[matchsticks-search.git] / main.c

diff --git a/main.c b/main.c
index b248631b7b819329ffb8b186cad2030e086f1189..93dc7ebe5325269db8fda05f5fe954a081489ed3 100644 (file)
--- a/main.c
+++ b/main.c
@@ -58,7 +58,8 @@
  *
  * We search all possible adjacency matrices, and for each one we run
  * GLPK's simplex solver.  We represent the adjacency matrix as an
- * array of bitmaps.
+ * array of bitmaps: one word per input stick, with one bit per output
+ * stick.
  *
  * However, there are a couple of wrinkles:
  *
@@ -76,7 +77,7 @@
  * nondecreasing in array order.
  *
  * Once we have a solution, we also avoid considering any candidate
- * which involves dividing one of the output sticks into so many
+ * which involves dividing one of the input sticks into so many
  * fragment that the smallest fragment would necessarily be no bigger
  * than our best solution.  That is, we reject candidates where any of
  * the hamming weights of the adjacency bitmap words are too large.
@@ -101,7 +102,7 @@ static double best;
 static glp_prob *best_prob;
 static AdjWord *best_adjmatrix;
 
-static int n_over_best;
+static int n_max_frags, m_max_frags;
 static int *weight;
 
 static unsigned printcounter;
@@ -119,7 +120,18 @@ static void progress_eol(void) {
 
 static void set_best(double new_best) {
   best = new_best;
-  n_over_best = floor(n / best);
+  /*
+   * When computing n_max_frags, we want to set a value that will skip
+   * anything that won't provide strictly better solutions.  So we
+   * want
+   *             frags <      n / best
+   *                        _          _
+   *   <=>       frags <   |  n / best  |
+   *                        _          _
+   *   <=>       frags <=  |  n / best  | - 1
+   */
+  n_max_frags = ceil(n / best) - 1;
+  m_max_frags = ceil(m / best) - 1;
 }
 
 /*----- multicore support -----*/
@@ -355,7 +367,8 @@ static void prep(void) {
   glp_term_out(GLP_OFF);
   setlinebuf(stderr);
   weight = calloc(sizeof(*weight), m);  assert(weight);
-  n_over_best = INT_MAX;
+  n_max_frags = INT_MAX;
+  m_max_frags = INT_MAX;
 }
 
 #if 0
@@ -377,8 +390,7 @@ static int count_set_adj_bits(AdjWord w) {
 static int totalfrags;
 
 static bool maxhamweight_ok(void) {
-  double maxminsize = (double)m / maxhamweight;
-  return maxminsize > best;
+  return maxhamweight <= m_max_frags;
 }
 
 static bool preconsider_ok(int nwords, bool doprint) {
@@ -390,8 +402,7 @@ static bool preconsider_ok(int nwords, bool doprint) {
   for (i=0, totalfrags=0; i<nwords; i++) {
     int frags = count_set_adj_bits(adjmatrix[i]);
     PRINTF("%"PRADJ" ", adjmatrix[i]);
-    double maxminsize = (double)m / frags;
-    if (maxminsize <= best) {
+    if (frags > m_max_frags) {
       PRINTF(" too fine");
       goto out;
     }
@@ -502,7 +513,7 @@ static void optimise(bool doprint) {
   glp_set_obj_coef(prob, X_minimum, 1);
 
   for (i=0; i<n; i++) {
-    for (j=0, jbit=1; j<m; j++, jbit<<=1) {
+    FOR_BITS(j,m) {
       if (!(adjmatrix[i] & jbit))
        continue;
       /* x_total_i += x_minimum */
@@ -628,7 +639,7 @@ static void iterate_recurse(int i, AdjWord min) {
       if (adjmatrix[i] & jbit)
         weight[j]++;
     for (int j = 0; j < m; j++)
-      if (weight[j] >= n_over_best)
+      if (weight[j] >= n_max_frags)
         goto takeout;
 
     iterate_recurse(i+1, adjmatrix[i]);