AdventOfCode23/Day9/Pyramid.cs

namespace Day9;

public class Pyramid
{
    public Sequence Bottom { get; }
    private readonly IList<Sequence> _layers = new List<Sequence>();

    public Pyramid(Sequence bottom)
    {
        Bottom = bottom;
        Build();
    }

    public long Predict()
    {
        var next = 0L;
        foreach (var sequence in _layers.Reverse().Skip(1))
        {
            next = sequence.Next(next);
        }

        return next;
    }
    
    public long Retrodict()
    {
        var next = 0L;
        foreach (var sequence in _layers.Reverse().Skip(1))
        {
            next = sequence.Prev(next);
        }

        return next;
    }

    private void Build()
    {
        _layers.Add(Bottom);
        while (!_layers.Last().IsZeros())
        {
            _layers.Add(_layers.Last().Difference());
        }
    }
}