using System.Text;
using FluentResults;
using static System.Net.Mime.MediaTypeNames;

namespace pgLabII.PgUtils.ConnectionStrings;

public class PqConnectionStringTokenizer : IPqConnectionStringTokenizer
{
    private readonly string input;
    private int position = 0;

    public bool IsEof
    {
        get
        {
            ConsumeWhitespace();
            return position >= input.Length;
        }
    }

    public PqConnectionStringTokenizer(string input)
    {
        this.input = input;
        position = 0;
    }

    public Result<string> GetKeyword()
    {
        if (IsEof)
            return Result.Fail($"Unexpected end of file was expecting a keyword at position {position}");

        return GetString(forKeyword: true);
    }

    public Result ConsumeEquals()
    {
        ConsumeWhitespace();
        if (position < input.Length && input[position] == '=')
        {
            position++;
            return Result.Ok();
        }
        else
            return Result.Fail($"Was expecting '=' after keyword at position {position}");
    }

    public Result<string> GetValue()
    {
        if (IsEof)
            return Result.Fail($"Unexpected end of file was expecting a keyword at position {position}");

        return GetString(forKeyword: false);
    }

    private Result<string> GetString(bool forKeyword)
    {
        if (forKeyword && input[position] == '=')
            return Result.Fail($"Unexpected '=' was expecting keyword at position {position}");

        if (input[position] == '\'')
            return ParseQuotedText();

        return UnquotedString(forKeyword);
    }

    private void ConsumeWhitespace()
    {
        while (position < input.Length && char.IsWhiteSpace(input[position]))
            position++;
    }

    private string UnquotedString(bool forKeyword)
    {
        int start = position;
        while (++position < input.Length)
        {
            char c = input[position];
            // Libpq syntax does not use semicolons as pair separators; treat ';' as invalid here
            if (c == ';')
            {
                // Force tokenizer to stop and later cause a parse error by making GetValue/keyword incomplete
                break;
            }
            if (char.IsWhiteSpace(c)) break;
            if (forKeyword && c == '=') break;
        }
        return input.Substring(start, position - start);
    }

    private Result<string> ParseQuotedText()
    {
        bool escape = false;
        StringBuilder sb = new();
        int start = position;
        while (++position < input.Length)
        {
            char c = input[position];
            if (escape)
            {
                switch (c)
                {
                    case '\'':
                    case '\\':
                        sb.Append(c);
                        escape = false;
                        break;
                    default:
                        return Result.Fail($"Invalid escape sequence at position {position}");
                }
            }
            else
            {
                if (c == '\'')
                {
                    ++position;
                    return sb.ToString();
                }
                else if (c == '\\')
                {
                    escape = true;
                }
                else
                {
                    sb.Append(c);
                }
            }
        }
        return Result.Fail($"Missing end quote on value starting at {start}");
    }
}