namespace EonaCat.FirstLight.SaveTransfer.VdfGenerator.KeyValue;

/// <summary>
/// Provides functionality for tokenizing a key-value formatted string input, allowing sequential reading and validation of symbols and quoted strings.
/// </summary>
/// <param name="text">The input string to tokenize. Must not be null.</param>
public class KeyValueTokenizer(string text)
{
    private int _pos;

    /// <summary>
    /// Determines whether the next non-whitespace character in the input matches the specified character without advancing the current position.
    /// </summary>
    /// <param name="c">The character to compare with the next non-whitespace character in the input.</param>
    /// <returns><see langword="true"/> if the next non-whitespace character matches the specified character; otherwise, <see langword="false"/>.</returns>
    public bool PeekSymbol(char c)
    {
        SkipWhitespace();
        return _pos < text.Length && text[_pos] == c;
    }

    /// <summary>
    /// Reads the next non-whitespace character from the input and verifies that it matches the specified symbol.
    /// </summary>
    /// <param name="c">The character to match at the current position in the input.</param>
    /// <exception cref="Exception">Thrown if the next non-whitespace character does not match the specified symbol.</exception>
    public void ReadSymbol(char c)
    {
        SkipWhitespace();
        if (_pos >= text.Length || text[_pos] != c)
            throw new Exception($"Expected '{c}' at position {_pos}");

        _pos++;
    }

    /// <summary>
    /// Reads a string enclosed in quotation marks from the current position in the input text.
    /// </summary>
    /// <returns> The string located between the opening and closing quotation marks or an empty string if there are no characters between the quotes. </returns>
    /// <exception cref="Exception"> Thrown when the current position does not contain an opening quotation mark or when a closing quotation mark cannot be found. </exception>
    public string ReadString()
    {
        SkipWhitespace();

        if (_pos >= text.Length || text[_pos] != '"')
            throw new Exception($"Expected '\"' at position {_pos}, but found '{text[_pos]}'!");

        _pos++; // skip opening quote
        var start = _pos;

        while (_pos < text.Length && text[_pos] != '"')
            _pos++;

        if (_pos >= text.Length)
            throw new Exception("Unterminated string literal!");

        var result = text.Substring(start, _pos - start);
        _pos++; // skip closing quote

        return result;
    }

    /// <summary>
    /// Advances the current position past any consecutive whitespace characters in the input text.
    /// </summary>
    private void SkipWhitespace()
    {
        while (_pos < text.Length)
        {
            var c = text[_pos];

            if (c is ' ' or '\t' or '\n' or '\r')
            {
                _pos++;
                continue;
            }

            break;
        }
    }
}