2023/day3: add part2 C solution

.gitignore

@@ -3,3 +3,4 @@
 *.bin
 *.d
 *.map
+*.out

2023/day3/_solution.c

@@ -66,15 +66,9 @@ uint32_t ix_from_xy(uint8_t const * const start,
   return (stride * y + x);
 }
 
-static const uint8_t symbols[] = {'@', '&', '=', '/', '-', '+', '%', '*', '$', '#'};
+bool is_symbol(uint8_t const c)
-
-bool is_symbol(uint8_t const * const start,
-               uint32_t const stride,
-               uint32_t const x,
-               uint32_t const y)
 {
-  uint32_t const ix = ix_from_xy(start, stride, x, y);
+  static uint8_t const symbols[] = {'@', '&', '=', '/', '-', '+', '%', '*', '$', '#'};
-  uint8_t const c = start[ix];
 
   for (uint32_t i = 0; i < (sizeof (symbols)) / (sizeof (*symbols)); i++) {
     if (c == symbols[i]) {
@@ -84,38 +78,90 @@ bool is_symbol(uint8_t const * const start,
   return false;
 }
 
+bool is_gear(uint8_t const c)
+{
+  return c == '*';
+}
+
+struct adjacency {
+  uint32_t ix;
+  int32_t count;
+  int32_t neighbors[4];
+};
+
+int32_t last_gear = 0;
+struct adjacency gears[512] = {0};
+
+void update_gear_adjacencies(uint32_t gear_ix, uint32_t num)
+{
+  for (int i = 0; i < (sizeof (gears)) / (sizeof (*gears)); i++) {
+    if (i >= last_gear) {
+      gears[last_gear++].ix = gear_ix;
+    }
+
+    if (gears[i].ix == gear_ix) {
+      if (gears[i].count < 4) {
+	gears[i].neighbors[gears[i].count++] = num;
+      }
+
+      break;
+    }
+  }
+}
+
+void adjacency_check(uint8_t const * const start,
+		     uint32_t const stride,
+		     int32_t const max_x,
+		     int32_t const max_y,
+		     int32_t const x,
+		     int32_t const y,
+		     uint32_t const num,
+		     bool * neighbor)
+{
+  if (xy_in_bounds(max_x, max_y, x, y)) {
+    uint32_t const ix = ix_from_xy(start, stride, x, y);
+    uint8_t const c = start[ix];
+    if (is_symbol(c))
+      *neighbor = true;
+    if (is_gear(c))
+      update_gear_adjacencies(ix, num);
+  }
+}
+
 bool has_adjacent_symbol(uint8_t const * const start,
                          uint32_t const stride,
                          int32_t const max_x,
                          int32_t const max_y,
                          int32_t const x,
                          int32_t const y,
-                         int32_t const digits)
+			 uint32_t const num,
+                         int32_t const digits
+			 )
 {
+  bool neighbor = false;
+
   /* check top/bottom/diagonals */
   for (int32_t xi = x - 1; xi <= x + digits; xi++) {
-    if (xy_in_bounds(max_x, max_y, xi, y - 1)) {
+    adjacency_check(start, stride, max_x, max_y, xi, y + 1, num, &neighbor);
-      if (is_symbol(start, stride, xi, y - 1))
+    adjacency_check(start, stride, max_x, max_y, xi, y - 1, num, &neighbor);
-        return true;
-    }
-    if (xy_in_bounds(max_x, max_y, xi, y + 1)) {
-      if (is_symbol(start, stride, xi, y + 1))
-        return true;
-    }
   }
 
   /* check left/right side */
-  if (xy_in_bounds(max_x, max_y, x + digits, y)) {
+  adjacency_check(start, stride, max_x, max_y, x + digits, y, num, &neighbor);
-    if (is_symbol(start, stride, x + digits, y))
+  adjacency_check(start, stride, max_x, max_y, x - 1     , y, num, &neighbor);
-      return true;
+
+  return neighbor;
 }
 
-  if (xy_in_bounds(max_x, max_y, x - 1, y)) {
+uint64_t part2_mac(void)
-    if (is_symbol(start, stride, x - 1, y))
+{
-      return true;
+  uint64_t mac = 0;
+  for (int i = 0; i < last_gear; i++) {
+    if (gears[i].count == 2) {
+      mac += (uint64_t)gears[i].neighbors[0] * (uint64_t)gears[i].neighbors[1];
     }
-
+  }
-  return false;
+  return mac;
 }
 
 void solve(uint8_t const * const start,
@@ -146,12 +192,18 @@ void solve(uint8_t const * const start,
                                          max_y,
                                          x,
                                          y,
-                                         (int32_t)digits);
+					 num,
+                                         (int32_t)digits
+					 );
     if (has)
       sum += num;
   }
 
   printf("part1 sum: %d\n", sum);
+
+  printf("part2:\n");
+  printf("  gears : %d\n", last_gear);
+  printf("  answer: %ld\n", part2_mac());
 }
 
 int main(void)