find cycle

src/day14/solve.c

@@ -137,8 +137,6 @@ void solve(const char *buf, size_t buf_size, Solution *result) {
 
     i64 part1, part2 = 0;
 
-    int total_cycles = 1000000000;
-
     XXH128_hash_t hashes[MAX_CYCLE_LEN];
     int hashes_count = 0;
     _cleanup_(ust_XXH128_hash_t_free) ust_XXH128_hash_t seen = ust_XXH128_hash_t_init(hash, equal);
@@ -148,9 +146,9 @@ void solve(const char *buf, size_t buf_size, Solution *result) {
     int duplicates_count = 0;
 
     // find cycle
-    for (int cycle = 1; cycle <= MAX_CYCLE_LEN; cycle++) {
+    for (int cycle = 0; cycle < MAX_CYCLE_LEN; cycle++) {
         tilt_north(rows, cols);
-        if (cycle == 1) { part1 = compute_total_load(rows, cols); }
+        if (cycle == 0) { part1 = compute_total_load(rows, cols); }
 
         tilt_west(rows, cols);
         tilt_south(rows, cols);
@@ -161,17 +159,30 @@ void solve(const char *buf, size_t buf_size, Solution *result) {
 
         ust_XXH128_hash_t_node *node = ust_XXH128_hash_t_find(&seen, value);
         if (node != NULL) {
-            log_debug("cycle %d seen before", cycle);
             duplicates[duplicates_count++] = value;
         } else {
             ust_XXH128_hash_t_insert(&seen, value);
         }
     }
 
-    for (int i = 0; i < duplicates_count; i++) {
-        // printf("duplicate %d: %lu%lu\n", i, duplicates[i].low64, duplicates[i].high64);
+    int cycle_start = 0;
+    while (!equal(&hashes[cycle_start], &duplicates[0])) { cycle_start++; }
+    int cycle_len = 1;
+    while (!equal(&hashes[cycle_start + cycle_len], &duplicates[0])) { cycle_len++; }
+    log_debug("cycle starts at %d and has length %d", cycle_start, cycle_len);
+
+    // sanity check
+    for (int i = 0; i < cycle_len; i++) {
+        assert(equal(&hashes[cycle_start + i], &hashes[cycle_start + cycle_len + i]));
     }
 
+    // from here on we know we have to compute:
+    // 1) Compute the first `cycle_start` cycles
+    // 2) Compute rem = total_cycles % cycle_len.
+    // 3) Compute the next `rem` cycles
+
+    int total_cycles = 1000000000;
+
     // print_platform(rows, cols);
 
     snprintf(result->part1, sizeof(result->part1), "%ld", part1);