Initial commit

2020/day10/day10.c

@@ -0,0 +1,63 @@
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <limits.h>
+
+int compare(const void* a, const void* b) {
+  unsigned int x = *(const unsigned int*)a;
+  unsigned int y = *(const unsigned int*)b;
+
+  if (x < y) return -1;
+  if (x > y) return 1;
+  return 0;
+}
+
+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;
+  }
+
+  unsigned int input[128] = {0};
+  unsigned int n = 1;
+  unsigned int d = 0;
+  unsigned int max = 0;
+  while (fscanf(fp, "%d\n", &d) == 1) {
+    input[n] = d;
+    n++;
+    max = d > max ? d : max;
+  }
+  max += 3;
+  input[n] = max;
+  n++;
+  fclose(fp);
+
+  qsort(input, n, sizeof(unsigned int), compare);
+
+  // Part 1
+  int ones = 0;
+  int threes = 0;
+  for (int i = 1; i < n; ++i) {
+    int diff = input[i] - input[i-1];
+    ones += (diff == 1);
+    threes += (diff == 3);
+  }
+  printf("%d\n", ones * threes);
+
+  // Part 2
+  unsigned long counts[1024] = {0};
+  counts[2] = 1;
+  for (int i = 1; i < n; ++i) {
+    unsigned int x = input[i];
+    counts[x+2] = counts[x-1] + counts[x] + counts[x+1];
+  }
+  printf("%lu\n", counts[max+2]);
+
+  return 0;
+}