day 13

r0f1 · r0f1 · commit 93b56095ede6 · 2020-12-13T12:07:53.000+01:00
diff --git a/README.md b/README.md
@@ -14,7 +14,7 @@
 |[10](https://adventofcode.com/2020/day/10)|Adapter Array|[py](/day10/main.py), [alt](/day10/alt.py)|
 |[11](https://adventofcode.com/2020/day/11)|Seating System|[py](/day11/main.py)|
 |[12](https://adventofcode.com/2020/day/12)|Rain Risk|[py](/day12/main.py)|
-|[13](https://adventofcode.com/2020/day/13)|-|-|
+|[13](https://adventofcode.com/2020/day/13)|Shuttle Search|[py](/day13/main.py)|
 |[14](https://adventofcode.com/2020/day/14)|-|-|
 |[15](https://adventofcode.com/2020/day/15)|-|-|
 |[16](https://adventofcode.com/2020/day/16)|-|-|
diff --git a/day13/main.py b/day13/main.py
@@ -0,0 +1,20 @@
+import numpy as np
+from math import prod
+
+with open("input.txt") as f:
+    lines = [x.strip() for x in f]
+
+arrival = int(lines[0])
+buses = [(i, int(e)) for i, e in enumerate(lines[1].split(",")) if e.isdigit()]
+
+offsets, times = zip(*buses)
+b = [e - (arrival % e) for e in times]
+print(np.min(b) * times[np.argmin(b)])
+
+def chinese_remainder(ns, rems):
+    p = prod(ns)
+    x = sum(r * (p // n) * pow(p // n, -1, n) for r, n in zip(rems, ns))
+    return x % p
+
+rems = [time-offset for offset, time in buses]
+print(chinese_remainder(times, rems))
