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;
+}

2020/day13/day13.dyalog

@@ -0,0 +1,18 @@
+ ⎕IO←0 ⋄ (⎕FR ⎕PP)←1287 34
+ p←⊃⎕NGET'input'1
+ t←⍎⊃p
+ b←⍎¨((1<',x'∘⍳)⊆⊢)1⊃p
+ b{×/((⊢⍳⌊/)⍵)⌷¨(⊂⍺),⊂⍵}b|-t  ⍝ Part 1
+ ⍝ https://www.jsoftware.com/papers/50/50_44.htm
+ xea←{(⍺,1 0){0=⊃⍵:⍺ ⋄ ⍵ ∇ ⍺-⍵×⌊(⊃⍺)÷⊃⍵}(⍵,0 1)}
+ ∇r←larg cr rarg
+  m r←larg
+  n s←rarg
+  gcd a b←m xea n
+  lcm←m×n÷gcd
+  c←lcm|gcd÷⍨(r×b×n)+(s×a×m)
+  r←lcm,c
+ ∇
+ c←','(≠⊆⊢)1⊃p
+ b←↓⍉⍎¨⍕¨↑(⊂~('x'≡⊃)¨c)/¨(⊂c),⊂-⍳⍴c
+ 1⊃⊃cr/{x y←⍵ ⋄ 0=y:x,y ⋄ 1:x(x+y)}¨b  ⍝ Part 2

2020/day13/day13.rkt

@@ -0,0 +1,42 @@
+#lang racket
+
+(require math/number-theory)
+
+(module+ test
+  (require rackunit))
+
+(module+ main
+  (displayln "Day 13"))
+
+(define (parse-buses str)
+  (map string->number (string-split str ",")))
+
+(define (read-input filename)
+  (define in (file->lines filename))
+  (values (string->number (car in))
+          (parse-buses (cadr in))))
+
+(define (part1 filename)
+  (define-values (t buses) (read-input filename))
+  (define waits (for/list ([b (in-list buses)]
+                           #:when b)
+                  (list b (- b (modulo t b)))))
+  (apply * (argmin cadr waits)))
+
+(module+ test
+  (check-equal? (part1 "test") 295))
+
+(module+ main
+  (displayln (part1 "input")))
+
+(define (part2 filename)
+  (define-values (t buses) (read-input filename))
+  (define active-buses (for/list ([b (in-list buses)] #:when b) b))
+  (define times (for/list ([b (in-list buses)] [i (in-naturals)] #:when b) (- i)))
+  (solve-chinese times active-buses))
+
+(module+ test
+  (check-equal? (part2 "test") 1068781))
+
+(module+ main
+  (displayln (part2 "input")))

2020/day13/input

@@ -0,0 +1,2 @@
+1000509
+17,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,739,x,29,x,x,x,x,x,x,x,x,x,x,13,x,x,x,x,x,x,x,x,x,23,x,x,x,x,x,x,x,971,x,x,x,x,x,x,x,x,x,41,x,x,x,x,x,x,x,x,19

2020/day13/test

@@ -0,0 +1,2 @@
+939
+7,13,x,x,59,x,31,19