2016: d15: ex1: add solution

2016/d15/ex1/ex1.py

@@ -0,0 +1,61 @@
+#!/usr/bin/env python
+
+import math
+import sys
+from typing import NamedTuple
+
+
+class Disc(NamedTuple):
+    num: int
+    positions: int
+    start_offset: int
+
+    @classmethod
+    def from_str(cls, input: str) -> "Disc":
+        input = input.removeprefix("Disc #").removesuffix(".")
+        split_input = input.split()
+        return Disc(
+            int(split_input[0]),
+            int(split_input[2]),
+            int(split_input[-1]),
+        )
+
+
+def solve(input: str) -> int:
+    def parse(input: str) -> list[Disc]:
+        return [Disc.from_str(line) for line in input.splitlines()]
+
+    # https://rosettacode.org/wiki/Chinese_remainder_theorem#Python_3.6
+    def mul_inv(a: int, b: int) -> int:
+        b0 = b
+        x0, x1 = 0, 1
+        if b == 1:
+            return 1
+        while a > 1:
+            q = a // b
+            a, b = b, a % b
+            x0, x1 = x1 - q * x0, x0
+        if x1 < 0:
+            x1 += b0
+        return x1
+
+    def chinese_remainder(residue_mapping: dict[int, int]) -> int:
+        res = 0
+        prod = math.prod(residue_mapping)
+        for n_i, a_i in residue_mapping.items():
+            p = prod // n_i
+            res += a_i * mul_inv(p, n_i) * p
+        return res % prod
+
+    discs = parse(input)
+    residues = {disc.positions: -(disc.start_offset + disc.num) for disc in discs}
+    return chinese_remainder(residues)
+
+
+def main() -> None:
+    input = sys.stdin.read()
+    print(solve(input))
+
+
+if __name__ == "__main__":
+    main()