using System.Collections;

namespace Numbers.NumberString;

/// <summary>
///     Allows to enumerate only the digits in a string
/// </summary>
/// <inheritdoc cref="IEnumerator{T}"/>
public sealed class DigitEnumerator : IEnumerator<int>
{
    private readonly string _text;
    private int _index;

    /// <summary>
    ///     Creates a new instance of <see cref="DigitEnumerator"/> based on the given text.
    ///     This will be called within <see cref="NumberString.GetEnumerator"/>.
    /// </summary>
    /// <param name="text">The text to iterate over (containing both letters and digits)</param>
    public DigitEnumerator(string text)
    {
        _text = text;
        _index = -1;
    }

    public bool MoveNext()
    {
        while (++_index < _text.Length)
        {
            if (char.IsDigit(_text[_index]))
            {
                return true;
            }
        }
        return false;
    }

    public void Reset()
    {
        _index = -1;
    }

    public int Current
    {
        get
        {
            if (_index < 0 || _index >= _text.Length || !char.IsDigit(_text[_index]))
            {
                return -1;
            }
            
            return GetDigit(_text[_index]);
            static int GetDigit(char c) => c - '0';
        }
    }

    object IEnumerator.Current => Current;

    public void Dispose() { }
}