day 7 part 2 (C), day 8 init

2021/day07/README.txt

@@ -34,8 +34,6 @@ This costs a total of 37 fuel. This is the cheapest possible outcome; more expen
 Determine the horizontal position that the crabs can align to using the least fuel possible. How much fuel must they spend to align to that position?
 
 Your puzzle answer was 349769.
-
-The first half of this puzzle is complete! It provides one gold star: *
 --- Part Two ---
 
 The crabs don't seem interested in your proposed solution. Perhaps you misunderstand crab engineering?
@@ -58,3 +56,7 @@ As each crab moves, moving further becomes more expensive. This changes the best
 This costs a total of 168 fuel. This is the new cheapest possible outcome; the old alignment position (2) now costs 206 fuel instead.
 
 Determine the horizontal position that the crabs can align to using the least fuel possible so they can make you an escape route! How much fuel must they spend to align to that position?
+
+Your puzzle answer was 99540554.
+
+Both parts of this puzzle are complete! They provide two gold stars: **

2021/day07/aoc-c.c

@@ -28,7 +28,7 @@ typedef struct crab {
 
 crab_t *crabs = NULL;
 
-//#ifdef DEBUG
+#ifdef DEBUG
 static void print_crab()
 {
     int i = 0;
@@ -38,7 +38,7 @@ static void print_crab()
         printf("%s%d/%d", i? ", ": "", crabs[i].pos, crabs[i].ncrabs);
     printf("\n");
 }
-//#endif
+#endif
 
 static crab_t *grow_crabs()
 {
@@ -93,19 +93,30 @@ static u64 read_crab()
     return ncrabs;
 }
 
-static int doit(int part)
+static u64 doit(int part)
 {
-    int try, best_dist = 0, best_fuel = crab_max * crab_max;
+    int try;
+    u64 best_fuel = (u64) -1;
 
     if (part == 1) {
         for (try = 0; try < crab_max; ++try) {
-            int fuel = 0;
-            for (int crab = 0; crab < ncrabs; ++crab)
+            u64 fuel = 0;
+            for (int crab = 0; crab < ncrabs; ++crab) {
                 fuel += abs(crabs[crab].pos - try) * crabs[crab].ncrabs;
-            if (fuel < best_fuel) {
-                best_dist = try;
-                best_fuel = fuel;
             }
+            if (fuel < best_fuel)
+                best_fuel = fuel;
+        }
+    } else {
+        for (try = 0; try < crab_max; ++try) {
+            u64 fuel = 0;
+            for (int crab = 0; crab < ncrabs; ++crab) {
+                /* n * (n + 1) / 2 */
+                fuel += abs(crabs[crab].pos - try) *
+                    (abs(crabs[crab].pos - try) + 1) * crabs[crab].ncrabs / 2;
+            }
+            if (fuel < best_fuel)
+                best_fuel = fuel;
         }
     }
     return best_fuel;
@@ -121,7 +132,7 @@ int main(int ac, char **av)
 {
     int opt;
     u32 exercise = 1;
-    int res;
+    u64 res;
 
     while ((opt = getopt(ac, av, "d:p:")) != -1) {
         switch (opt) {
@@ -142,6 +153,6 @@ int main(int ac, char **av)
 
     read_crab();
     res = doit(exercise);
-    printf ("%s : res=%d\n", *av, res);
+    printf ("%s : res=%lu\n", *av, res);
     exit (0);
 }