using System.Collections;

namespace Numbers.NumberString;

/// <summary>
///     Represents a string consisting of digits and letters intermixed
/// </summary>
/// <inheritdoc cref="IEnumerable{T}" />
/// <inheritdoc cref="IComparable{T}" />
public sealed class NumberString : IEnumerable<int>, IComparable<NumberString>
{
    private readonly string _text;
    private int? _numericValue;

    /// <summary>
    ///     Creates a new instance of <see cref="NumberString" /> based on the given text.
    ///     As a simplification, the text is assumed to be a valid number string.
    /// </summary>
    /// <param name="text">A string containing digits and letters</param>
    public NumberString(string text)
    {
        _text = text;
        string digitsOnly = new string(text.Where(char.IsDigit).ToArray());
        _numericValue = digitsOnly.Length > 0 ? int.Parse(digitsOnly) : 0;
    }

    /// <summary>
    ///     Gets the numeric value contained in the number string
    /// </summary>
    public int NumericValue => _numericValue ?? 0;

    /// <summary>
    ///     Gets the digit at the given index.
    ///     If the index is out of range, -1 is returned.
    /// </summary>
    /// <param name="index">Digit index</param>
    public int this[int index]
    {
        get
        {
            string numberString = _numericValue.HasValue ? _numericValue.Value.ToString() : string.Empty;
            if (index < 0 || index >= numberString.Length || numberString == string.Empty)
            {
                return -1;
            }
            return GetDigit(numberString[index]);
            static int GetDigit(char c) => c - '0';
        }
    }

    public int CompareTo(NumberString? other)
    {
        if (ReferenceEquals(this, other))
        {
            return 1;
        }
        
        return NumericValue.CompareTo(other?.NumericValue);
    }

    public IEnumerator<int> GetEnumerator() => new DigitEnumerator(_text);

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

    public override bool Equals(object? obj)
    {
        if (obj is not NumberString numberString)
        {
            return false;
        }

        return numberString._numericValue == _numericValue;
    }

    public override int GetHashCode() => _numericValue?.GetHashCode() ?? 0;

    public static bool operator >(NumberString a, NumberString b) => a.CompareTo(b) > 0;

    public static bool operator <(NumberString a, NumberString b) => a.CompareTo(b) < 0;

    public static bool operator ==(NumberString a, NumberString b) => a?.Equals(b) ?? ReferenceEquals(b, null);

    public static bool operator !=(NumberString a, NumberString b) => !(a == b);
}