using System.Text;

namespace pgLabII.PgUtils;

public class Acl(string grantee, string rights, string grantor)
{
    public string Grantee { get; } = grantee;
    public string Rights { get; } = rights;
    public string Grantor { get; } = grantor;
}
/// <summary>
/// Parser for acl's. Acl's have a fairly simple structure grantee=right/grantor see also https://www.postgresql.org/docs/current/ddl-priv.html
/// When the grantee is empty it is "PUBLIC" which you can see as a wildcard that matches everybody that can authenticate.
/// Because the grantee and grantor name can contain special characters there is an escape mechinism in place.
/// When the name contains a conflicting character it is quoted. If the name contains a quote it is doubles. 
/// </summary>
public class AclParser
{
    private int _position = 0;
    private string _input = null!;
    
    public Acl Parse(string acl)
    {
        _input = acl;
        _position = 0;
        ConsumeWhitespace();
        string grantee = ConsumeRoleName();
        string rights = ConsumeRights();
        if (Current() != '/')
            throw new ArgumentException("Unexpected symbol in acl string");
        _position++;
        string grantor = ConsumeRoleName();
        return new Acl(grantee, rights, grantor);
    }

    /// <summary>
    /// There is a limited set of possible characters for the rights
    /// </summary>
    /// <returns></returns>
    /// <exception cref="NotImplementedException"></exception>
    private string ConsumeRights()
    {
        if (Current() != '=')
            throw new ArgumentException("Unexpected symbol in acl string");
        _position++;
        int start = _position;
        while (NotAtEnd() && Current() != '/')
            _position++;
        
        return _input.Substring(start, _position - start);
    }

    // Whitespace should have been consumed
    // There should be either a " in which case the name is quoted
    // a = in which case there is no name ie PUBLIC
    // all other cases it is an unquoted name
    private string ConsumeRoleName() =>
        _input[_position] switch
        {
            '=' => "",
            '"' => ParseQuotedName(),
            _ => ParseUnquotedName()
        };

    private string ParseQuotedName()
    {
        _position++; // consume opening quote
        int start = _position;
        StringBuilder sb = new();

        while (true)
        {
            while (NotAtEnd() && Current() != '"')
                _position++;

            char peek = _position + 1 < _input.Length ? _input[_position + 1] : '\0';  
            if (peek == '"')
            {
                _position++;
                sb.Append(_input.AsSpan(start, _position - start));
                _position++;
                start = _position;
            }
            else
            {
                sb.Append(_input.AsSpan(start, _position - start));
                _position++;
                break;
            }
        }

        return sb.ToString();
    }

    private string ParseUnquotedName()
    {
        int start = _position;
        while (NotAtEnd() && Current() != '=')
            _position++;
        
        return _input.Substring(start, _position - start);
    }

    private void ConsumeWhitespace()
    {
        while (NotAtEnd() && char.IsWhiteSpace(Current()))
            ++_position;
    }
    
    private bool NotAtEnd() => _position < _input.Length;
    
    private char Current() => _input[_position];
}