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Day25Solver.cs
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Day25Solver.cs
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using System;
using System.Linq;
using AdventOfCode.Abstractions;
namespace AdventOfCode.Year2020.Day25;
public class Day25Solver : DaySolver
{
private readonly int _cardPublicKey;
private readonly int _doorPublicKey;
public Day25Solver(string inputFilePath) : base(inputFilePath)
{
_cardPublicKey = int.Parse(InputLines.First());
_doorPublicKey = int.Parse(InputLines.Skip(1).First());
}
private int BruteForceLoopSizeFromPublicKey(int subjectNumber, int publicKey)
{
int value = 1;
for (int loopSize = 0; loopSize < 20201227; loopSize++)
{
if (value == publicKey) return loopSize;
value = (int)((long)value * subjectNumber) % 20201227;
}
throw new Exception();
}
private int Transform(int subjectNumber, int loopSize)
{
int value = 1;
for (int i = 0; i < loopSize; i++)
value = (int)((long)value * subjectNumber % 20201227);
return value;
}
public override string SolvePart1()
{
int cardLoopSize = BruteForceLoopSizeFromPublicKey(7, _cardPublicKey);
int doorLoopSize = BruteForceLoopSizeFromPublicKey(7, _doorPublicKey);
int encryptionKey = Transform(_cardPublicKey, doorLoopSize);
return encryptionKey.ToString();
}
public override string SolvePart2() => string.Empty;
}