Initial commit

2020/day03/day03.c

@@ -0,0 +1,79 @@
+#include <stdbool.h>
+#include <stdio.h>
+
+int count_lines(FILE *fp) {
+  int count = 0;
+
+  for (char c = getc(fp); c != EOF; c = getc(fp)) {
+    if (c == '\n') {
+      count++;
+    }
+  }
+  return count;
+}
+
+int line_length(FILE *fp) {
+  int count = 0;
+
+  for (char c = getc(fp); c != '\n'; c = getc(fp)) {
+    count++;
+  }
+
+  return count;
+}
+
+int count_trees(int n, int m, int grid[n][m], int down_offset, int right_offset) {
+  int count = 0;
+
+  for (int i = 0; i < n / down_offset; ++i) {
+    count += grid[i * down_offset][i * right_offset % m];
+  }
+
+  return count;
+}
+
+int main(int argc, char *argv[])
+{
+  if (argc < 2) {
+    printf("Usage: %s <input file>\n", argv[0]);
+    return 0;
+  }
+
+  FILE *fp = fopen(argv[1], "r");
+  if (fp == NULL) {
+    printf("Could not open file %s\n", argv[1]);
+    return 1;
+  }
+
+  int n = count_lines(fp);
+  rewind(fp);
+  int m = line_length(fp);
+  rewind(fp);
+
+  printf("Grid size: %d×%d\n", n, m);
+
+  int grid[n][m];
+  char c;
+  for (int i = 0; i < n; ++i) {
+    for (int j = 0; j < m; ++j) {
+      c = getc(fp);
+      if (c == '\n' || c == EOF) {
+	c = getc(fp);
+      }
+      grid[i][j] = (c == '#');
+    }
+  }
+  fclose(fp);
+
+  printf("%d\n", count_trees(n, m, grid, 1, 3));
+
+  int total = 1;
+  int down_offsets[] = {1, 1, 1, 1, 2};
+  int right_offsets[] = {1, 3, 5, 7, 1};
+  for (int i = 0; i < 5; ++i) {
+    total *= count_trees(n, m, grid, down_offsets[i], right_offsets[i]);
+  }
+  printf("%d\n", total);
+
+  return 0;
+}