Initial commit

2020/day13/day13.c

@@ -0,0 +1,89 @@
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+long egcd(long *x, long *y, long a, long b) {
+  if (b == 0) {
+    *x = 1;
+    *y = 0;
+    return a;
+  }
+
+  long x1, y1;
+  long gcd = egcd(&x1, &y1, b, a % b);
+  *x = y1;
+  *y = x1 - (a / b) * y1;
+
+  return gcd;
+}
+
+long modinv(long a, long m) {
+  long x, y;
+  long gcd = egcd(&x, &y, a, m);
+  if (gcd != 1) {
+    return -1;
+  }
+  if (x < 0) {
+    return m + x;
+  }
+  return x;
+}
+
+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 t;
+  fscanf(fp, "%d\n", &t);
+
+  int buses[100] = {0};
+  int n = 0;
+  while (fscanf(fp, "%d,", &buses[n++]) == 1 || fscanf(fp, "x,") == 0)
+    ;
+  n--;
+  fclose(fp);
+
+  // Part 1
+  int min = INT_MAX;
+  int tmin = 0;
+  for (int i = 0; i < n; ++i) {
+    if (buses[i] == 0) {
+      continue;
+    }
+    int x = buses[i] - t % buses[i];
+    if (x < min) {
+      min = x;
+      tmin = i;
+    }
+  }
+  printf("%d\n", min * buses[tmin]);
+
+  // Part 2
+  unsigned long N = 1;
+  for (int i = 0; i < n; ++i) {
+    if (buses[i] == 0) {
+      continue;
+    }
+    N *= buses[i];
+  }
+
+  long x = 0;
+  for (int i = 0; i < n; ++i) {
+    if (buses[i] == 0) {
+      continue;
+    }
+    long Ni = N / buses[i];
+    x += (buses[i] - i) * modinv(Ni, buses[i]) * Ni;
+  }
+  printf("%ld\n", x % N);
+
+  return 0;
+}