Initial commit

2021/day03/day03.c

@@ -0,0 +1,35 @@
+#include <stdio.h>
+
+#define N 1000
+#define M 12
+
+int main() {
+  char c = 0;
+  int i = 0, j = 0;
+  char in[N][M] = {0};
+
+  while ((c = getchar()) != EOF) {
+    switch (c) {
+    case '\n': i++; j = 0; break;
+    case '1': in[i][j] = 1; j++; break;
+    case '0': j++; break;
+    default: fprintf(stderr, "unknown digit %c", c); return 1;
+    }
+  }
+
+  // Part 1
+  int counts[M] = {0};
+  for (i = 0; i < N; ++i) {
+    for (j = 0; j < M; ++j) {
+      counts[j] += in[i][j];
+    }
+  }
+  int gamma = 0, epsilon = 0;
+  for (j = 0; j < M; ++j) {
+    gamma = (gamma << 1) + (counts[j] > N/2);
+    epsilon = (epsilon << 1) + (counts[j] <= N/2);
+  }
+  printf("%d\n", gamma * epsilon);
+
+  return 0;
+}