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Day13.cs
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Day13.cs
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using System;
using System.Collections.Generic;
using System.Numerics;
using AdventOfCode.CSharp.Common;
namespace AdventOfCode.CSharp.Y2020.Solvers;
public class Day13 : ISolver
{
public static void Solve(ReadOnlySpan<byte> input, Solution solution)
{
var reader = new SpanReader(input.TrimEnd((byte)'\n'));
int earliestTime = reader.ReadPosIntUntil('\n');
int part1Id = -1;
int part1Time = int.MaxValue;
BigInteger product = 1;
var buses = new List<(int n, int a)>();
for (int i = 0; !reader.Done; i++)
{
if (reader.Peek() == 'x')
{
reader.SkipLength("x,".Length);
continue;
}
int busId = reader.ReadPosIntUntil(',');
// Part 1
int waitTime = busId - (earliestTime % busId);
if (waitTime < part1Time)
{
part1Id = busId;
part1Time = waitTime;
}
// Part 2
product *= busId;
int remainder = (busId - i) % busId;
if (remainder < 0)
{
remainder += busId;
}
buses.Add((busId, remainder));
}
int part1 = part1Id * part1Time;
BigInteger part2 = 0;
foreach ((int n, int a) in buses)
{
BigInteger p = product / n;
part2 += a * BigInteger.ModPow(p, n - 2, n) * p;
}
part2 %= product;
solution.SubmitPart1(part1);
solution.SubmitPart2(part2);
}
}