AdventOfCode/AdventOfCode.Solutions/2023/Day 09/Day09.cs

using AdventOfCode.Core.Shared.IO;

namespace AdventOfCode.Solutions._2023
{
    public class Day09 : IChallange
    {
        public int Year => 2023;
        public int Day => 9;

        private readonly IInputReader _inputReader;

        public Day09(IInputReader inputReader)
        {
            _inputReader = inputReader;
            _inputReader.SetInput(this);
        }

        public async Task<string> GetSolutionPart1()
        {
            long sum = 0;
            await foreach (long[] list in _inputReader.ReadLineAsLongArray(" "))
            {
                sum += GetNextValue(list);
            }

            return sum.ToString();
        }

        public async Task<string> GetSolutionPart2()
        {
            long sum = 0;
            await foreach (long[] list in _inputReader.ReadLineAsLongArray(" "))
            {
                sum += GetNextValue(list.Reverse().ToArray());
            }

            return sum.ToString();
        }

        private long GetNextValue(long[] list) => list[^1] + GetNextValueExtrapolated(list);

        private long GetNextValueExtrapolated(long[] list)
        {
            long[] deltas = list.Skip(1).Select((v, i) => v - list[i]).ToArray();
            if (deltas.All(d => d == 0))
            {
                return deltas[0];
            }

            return deltas[^1] + GetNextValueExtrapolated(deltas);
        }
    }
}