using System.Collections;

namespace Numbers.NumberFactory;

/// <summary>
///     Base class for <see cref="INumbers" /> implementations
/// </summary>
/// <inheritdoc cref="INumbers" />
public abstract class AbstractNumbers : INumbers
{
    private readonly List<long> _list;

    /// <summary>
    ///     Creates a new instance of <see cref="AbstractNumbers" /> with the given bounds.
    ///     Will generate the appropriate numbers based on the implementation of <see cref="PickNumber" />.
    /// </summary>
    /// <param name="lowerBound">Lower bound of the number range</param>
    /// <param name="upperBound">Upper bound of the number range</param>
    protected AbstractNumbers(long lowerBound, long upperBound)
    {
        LowerBound = lowerBound;
        UpperBound = upperBound;
        _list = [];
        GenerateNumbers();
    }

    public long this[int index]
    {
        get
        {
            if (index < 0 || index >= _list.Count)
            {
                return -1;
            }
            
            return _list[index];
        }
    }

    public long LowerBound { get; }

    public long UpperBound { get; }

    public int Length => _list.Count;
    public IEnumerator<long> GetEnumerator() => new NumbersEnumerator(_list);

    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();

    private void GenerateNumbers()
    {
        for (var i = LowerBound; i <= UpperBound; i++)
        {
            if (PickNumber(i))
            {
                _list.Add(i);
            }
        }
    }

    /// <summary>
    ///     Determines whether the given number should be picked based on the task of a
    ///     specific <see cref="INumbers" /> implementation
    /// </summary>
    /// <param name="number">Number to check</param>
    /// <returns>True if the number should be picked; false otherwise</returns>
    protected abstract bool PickNumber(long number);
}