Day 14, Part 2: WIP

src/day14/solve.c

@@ -9,11 +9,23 @@
 #include "solve.h"
 #include "aoc/all.h"
 
+#include <inttypes.h>
+#include <xxhash.h>
+
 #define MAX_DIM 100
 #define ROLLING 'O'
 #define FIXED_POINT '#'
 #define EMPTY '.'
 
+typedef struct {
+    XXH64_hash_t hash;
+    int cycle;
+} Seen;
+
+#define P
+#define T Seen
+#include <ust.h>
+
 static char platform[MAX_DIM][MAX_DIM];
 
 static void tilt_north(int rows, int cols) {
@@ -22,16 +34,64 @@ static void tilt_north(int rows, int cols) {
             char c = platform[y][x];
             if (c != ROLLING) continue;
             int new_y = y;
-            while (new_y >= 1 && platform[new_y - 1][x] == EMPTY) { new_y--; }
+            while (new_y - 1 >= 0 && platform[new_y - 1][x] == EMPTY) { new_y--; }
+            if (new_y != y) {
+                // log_debug("row %d, col %d moves to row %d", y, x, new_y);
+                platform[new_y][x] = c;
+                platform[y][x] = EMPTY;
+            }
+        }
+    }
+}
+
+static void tilt_south(int rows, int cols) {
+    for (int y = rows - 2; y >= 0; y--) {
+        for (int x = 0; x < cols; x++) {
+            char c = platform[y][x];
+            if (c != ROLLING) continue;
+            int new_y = y;
+            while (new_y + 1 < rows && platform[new_y + 1][x] == EMPTY) { new_y++; }
             if (new_y != y) {
-                log_debug("row %d, col %d moves to row %d", y, x, new_y);
+                // log_debug("row %d, col %d moves to row %d", y, x, new_y);
                 platform[new_y][x] = c;
                 platform[y][x] = EMPTY;
             }
         }
     }
 }
 
+static void tilt_east(int rows, int cols) {
+    for (int x = cols - 2; x >= 0; x--) {
+        for (int y = 0; y < rows; y++) {
+            char c = platform[y][x];
+            if (c != ROLLING) continue;
+            int new_x = x;
+            while (new_x + 1 < cols && platform[y][new_x + 1] == EMPTY) { new_x++; }
+            if (new_x != x) {
+                // log_debug("row %d, col %d moves to col %d", y, x, new_x);
+                platform[y][new_x] = c;
+                platform[y][x] = EMPTY;
+            }
+        }
+    }
+}
+
+static void tilt_west(int rows, int cols) {
+    for (int x = 1; x < cols; x++) {
+        for (int y = 0; y < rows; y++) {
+            char c = platform[y][x];
+            if (c != ROLLING) continue;
+            int new_x = x;
+            while (new_x - 1 >= 0 && platform[y][new_x - 1] == EMPTY) { new_x--; }
+            if (new_x != x) {
+                // log_debug("row %d, col %d moves to col %d", y, x, new_x);
+                platform[y][new_x] = c;
+                platform[y][x] = EMPTY;
+            }
+        }
+    }
+}
+
 static i64 compute_total_load(int rows, int cols) {
     i64 result = 0;
     for (int y = 0; y < rows; y++) {
@@ -50,6 +110,12 @@ static void print_platform(int rows, int cols) {
     }
 }
 
+static inline XXH64_hash_t hash_platform(int rows, int cols) { return XXH3_64bits(&platform[0][0], rows * cols); }
+
+static size_t hash_helper(Seen *s) { return s->hash; }
+
+static int equal_helper(Seen *lhs, Seen *rhs) { return lhs->hash == rhs->hash; }
+
 void solve(const char *buf, size_t buf_size, Solution *result) {
     size_t pos = 0;
     int rows, cols;
@@ -68,10 +134,33 @@ void solve(const char *buf, size_t buf_size, Solution *result) {
         rows = y;
     }
 
-    tilt_north(rows, cols);
-
     i64 part1, part2 = 0;
-    part1 = compute_total_load(rows, cols);
+
+    int total_cycles = 1000000000;
+    // TODO: find repeating pattern
+
+    _cleanup_(ust_Seen_free) ust_Seen seen = ust_Seen_init(hash_helper, equal_helper);
+
+    for (int cycle = 1; cycle <= 1000000; cycle++) {
+        tilt_north(rows, cols);
+        if (cycle == 1) { part1 = compute_total_load(rows, cols); }
+
+        tilt_west(rows, cols);
+        tilt_south(rows, cols);
+        tilt_east(rows, cols);
+
+        if (cycle == 175) print_platform(rows, cols);
+
+        Seen s = {.hash = hash_platform(rows, cols), .cycle = cycle};
+        ust_Seen_node *node = ust_Seen_find(&seen, s);
+        if (node != NULL) {
+            printf("cycle %d collides with entry from cycle %d\n", cycle, node->key.cycle);
+            break;
+        }
+        ust_Seen_insert(&seen, s);
+    }
+
+    print_platform(rows, cols);
 
     snprintf(result->part1, sizeof(result->part1), "%ld", part1);
     snprintf(result->part2, sizeof(result->part2), "%ld", part2);

src/day14/solve_test.c

@@ -25,7 +25,7 @@ O.#..O.#.#\n\
     Solution solution;
     solve(buf, strlen(buf), &solution);
     ASSERT_STR("136", solution.part1);
-    // ASSERT_STR("0", solution.part2);
+    ASSERT_STR("64", solution.part2);
 }
 
 #ifdef HAVE_INPUTS