using System.Collections.Generic;
using System.Linq;
using System.Numerics;

namespace Algorithms.Sequences;

/// <summary>
///     <para>
///         Tribonacci numbers: a(n) = a(n-1) + a(n-2) + a(n-3) with a(0)=a(1)=a(2)=1.
///     </para>
///     <para>
///         OEIS: https://oeis.org/A000213.
///     </para>
/// </summary>
public class TribonacciNumbersSequence : ISequence
{
    public IEnumerable<BigInteger> Sequence
    {
        get
        {
            var buffer = Enumerable.Repeat(BigInteger.One, 4).ToArray();
            while (true)
            {
                yield return buffer[0];
                var next = buffer[0] + buffer[1] + buffer[2];
                buffer[0] = buffer[1];
                buffer[1] = buffer[2];
                buffer[2] = next;
            }
        }
    }
}