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using System;
using System.Collections.Generic;
using System.Linq;
using aoc2019.lib;
namespace aoc2019
{
internal sealed class Day16 : Day
{
private static readonly int[] BasePattern = {0, 1, 0, -1};
private readonly int[] initialList;
public Day16() : base(16, "Flawed Frequency Transmission")
{
initialList = Input.First().Select(c => int.Parse($"{c}")).ToArray();
}
protected override string Part1()
{
const int phaseCount = 100;
var signal0 = initialList.ToArray();
var signal1 = new int[signal0.Length];
for (var i = 0; i < phaseCount; i++)
CalculateSignal(i % 2 == 0 ? signal0 : signal1, i % 2 == 0 ? signal1 : signal0);
return new string(
signal0.Take(8).Select(c => (char) (c + '0'))
.ToArray());
}
protected override string Part2()
{
const int phaseCount = 100;
var messageOffset = initialList.Take(7).Aggregate((n, i) => n * 10 + i);
var signal = initialList.Repeat(10_000).Skip(messageOffset).ToArray();
for (var p = 0; p < phaseCount; p++)
{
signal[^1] %= 10;
for (var i = signal.Length - 2; i >= 0; i--)
signal[i] = (signal[i + 1] + signal[i]) % 10;
}
return new string(signal.Take(8).Select(c => (char) (c + '0')).ToArray());
}
private static void CalculateSignal(IReadOnlyList<int> input, IList<int> output)
{
for (var outputIndex = 0; outputIndex < output.Count; outputIndex++)
output[outputIndex] =
Math.Abs(PatternValues(outputIndex, input.Count).Select((pv, i) => pv * input[i] % 10).Sum()) % 10;
}
private static IEnumerable<int> PatternValues(int index, int count)
{
return BasePattern
.SelectMany(v => Enumerable.Repeat(v, index + 1))
.Repeat(int.MaxValue)
.Skip(1)
.Take(count);
}
}
}
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