day 2, part 2 working!

day_8/2.c

@@ -0,0 +1,123 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#define GRID_SIZE 50
+
+typedef struct point {
+  int x, y;
+} point_t;
+
+typedef struct lots_of_points {
+  point_t points[4];
+  unsigned char index;
+} lots_of_points_t;
+
+int main(void) {
+  FILE *fp = fopen("input", "r");
+
+  if (fp == NULL) {
+    perror("input");
+    return 1;
+  }
+
+  char antinodes[GRID_SIZE][GRID_SIZE] = {0};
+
+  lots_of_points_t data['z' + 1] = {0};
+
+  for (unsigned int i = 0; i < GRID_SIZE; i++) {
+    unsigned char buffer[GRID_SIZE] = {0};
+    size_t read = fread(&buffer, sizeof(char), GRID_SIZE, fp);
+
+    if (read == 0) {
+      fprintf(stderr, "oopsy daisy\n");
+      fclose(fp);
+      return 1;
+    }
+
+    // move past the newline
+    fseek(fp, 1, SEEK_CUR);
+
+    for (unsigned int j = 0; j < GRID_SIZE; j++) {
+      const unsigned char freq = buffer[j];
+      if (freq != '.') {
+        const point_t point = {(int)j, (int)i};
+        data[freq].points[data[freq].index] = point;
+        data[freq].index++;
+      }
+    }
+  }
+
+  unsigned int sum = 0;
+  for (unsigned int i = 0; i < 'z' + 1; i++) {
+    if (data[i].index == 0) {
+      continue;
+    }
+
+    for (unsigned int j = 0; j < data[i].index; j++) {
+      const point_t point = data[i].points[j];
+      for (unsigned int k = 0; k < data[i].index; k++) {
+        if (j == k) {
+          continue;
+        }
+
+        const point_t other_point = data[i].points[k];
+
+        for (int l = 0;; l++) {
+          const int dist_x = point.x - other_point.x;
+          const int dist_y = point.y - other_point.y;
+
+          const point_t antinode = {point.x + dist_x * l, point.y + dist_y * l};
+
+          if (antinode.x >= GRID_SIZE || antinode.x < 0 ||
+              antinode.y >= GRID_SIZE || antinode.y < 0) {
+            break;
+          }
+
+          if (antinodes[antinode.y][antinode.x] == 1) {
+            continue;
+          }
+
+          antinodes[antinode.y][antinode.x] = 1;
+
+          sum++;
+        }
+      }
+    }
+  }
+
+#ifdef DEBUG
+  for (unsigned int i = 0; i < GRID_SIZE; i++) {
+    for (unsigned int j = 0; j < GRID_SIZE; j++) {
+      unsigned char found_freq = 0;
+      for (unsigned int k = 0; k < 'z' + 1; k++) {
+        for (unsigned int l = 0; l < data[k].index; l++) {
+          if (data[k].points[l].x == j && data[k].points[l].y == i) {
+            printf("%c", k);
+            found_freq = 1;
+            break;
+          }
+        }
+        if (found_freq) {
+          break;
+        }
+      }
+
+      if (!found_freq) {
+        if (antinodes[i][j] == 1) {
+          printf("#");
+          continue;
+        }
+        printf(".");
+      }
+    }
+
+    printf("\n");
+  }
+#endif
+
+  printf("%u\n", sum);
+
+  fclose(fp);
+
+  return 0;
+}