Initial commit

2020/day15/day15.c

@@ -0,0 +1,34 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#define SIZE 30000000
+
+size_t a[SIZE];
+
+int main(int argc, char *argv[]) {
+  if (argc < 2) {
+    printf("Usage: %s <input list>\n", argv[0]);
+    return 0;
+  }
+
+  for (size_t i = 1; i < argc - 1; ++i) {
+    a[atoi(argv[i])] = i;
+  }
+
+  size_t n = atoi(argv[argc - 1]);
+  for (size_t i = argc - 1; i < SIZE; ++i) {
+    if (i == 2020) { // Part 1
+      printf("%zu\n", n);
+    }
+    size_t prev_n = n;
+    if (a[n] == 0) {
+      n = 0;
+    } else {
+      n = i - a[n];
+    }
+    a[prev_n] = i;
+  }
+  printf("%zu\n", n); // Part 2
+
+  return 0;
+}