using System.Globalization;
using System.Text;

using Lua;

namespace AIStudio.Tools.PluginSystem;

/// <summary>Reads one exported configuration assignment as data. No Lua state is created.</summary>
public sealed class ConfigurationSnippetParser
{
    private readonly string source;
    private int position;

    private ConfigurationSnippetParser(string source) => this.source = source;

    public static bool TryParse(string source, string expectedSection, out LuaTable table, out string issue)
    {
        table = new LuaTable();
        issue = string.Empty;
        if (string.IsNullOrWhiteSpace(source) || source.Length > 1_000_000)
        {
            issue = "Paste one exported configuration snippet (up to 1 MB).";
            return false;
        }

        try
        {
            var parser = new ConfigurationSnippetParser(source);
            parser.ExpectWord("CONFIG");
            var section = parser.ReadBracketedString();
            if (section != expectedSection)
                throw new FormatException($"This is a {section} snippet. Paste a {expectedSection} snippet here.");

            parser.Expect('[');
            parser.Expect('#');
            parser.ExpectWord("CONFIG");
            if (parser.ReadBracketedString() != expectedSection)
                throw new FormatException("The configuration section names do not match.");
            parser.Expect('+');
            parser.Expect('1');
            parser.Expect(']');
            parser.Expect('=');
            table = parser.ReadTable(0);
            parser.SkipTrivia();
            if (parser.position != source.Length)
                throw new FormatException("The snippet must contain exactly one table assignment and no executable code.");
            return true;
        }
        catch (FormatException exception)
        {
            issue = exception.Message;
            return false;
        }
    }

    private LuaTable ReadTable(int depth)
    {
        if (depth > 32)
            throw new FormatException("The snippet contains too many nested tables.");
        this.Expect('{');
        var table = new LuaTable();
        var arrayIndex = 1;
        var names = new HashSet<string>(StringComparer.Ordinal);
        while (true)
        {
            this.SkipTrivia();
            if (this.Take('}'))
                return table;

            if (this.Take('['))
            {
                var key = this.ReadString();
                this.Expect(']');
                this.Expect('=');
                if (!names.Add(key))
                    throw new FormatException($"The field '{key}' occurs more than once.");
                table[key] = this.ReadValue(depth + 1);
            }
            else
                table[arrayIndex++] = this.ReadValue(depth + 1);

            this.SkipTrivia();
            if (this.Take('}'))
                return table;
            if (!this.Take(',') && !this.Take(';'))
                throw new FormatException($"Expected a comma or closing brace at character {this.position + 1}.");
        }
    }

    private LuaValue ReadValue(int depth)
    {
        this.SkipTrivia();
        if (this.Peek() == '{')
            return this.ReadTable(depth);
        if (this.Peek() is '"' or '\'' || this.Peek() == '[')
            return this.ReadString();
        if (this.TakeWord("true"))
            return true;
        if (this.TakeWord("false"))
            return false;
        if (this.TakeWord("nil"))
            return LuaValue.Nil;

        var start = this.position;
        if (this.Peek() == '-')
            this.position++;
        while (char.IsAsciiDigit(this.Peek()))
            this.position++;
        if (this.Peek() == '.')
        {
            this.position++;
            while (char.IsAsciiDigit(this.Peek()))
                this.position++;
        }
        if (this.position > start && double.TryParse(this.source[start..this.position], NumberStyles.Float, CultureInfo.InvariantCulture, out var number) && double.IsFinite(number))
            return number;
        throw new FormatException($"Only literal values are allowed at character {start + 1}; executable Lua is not accepted.");
    }

    private string ReadBracketedString()
    {
        this.Expect('[');
        var result = this.ReadString();
        this.Expect(']');
        return result;
    }

    private string ReadString()
    {
        this.SkipTrivia();
        var quote = this.Peek();
        if (quote == '[')
        {
            this.position++;
            var equalsStart = this.position;
            while (this.Peek() == '=')
                this.position++;
            var equals = this.source[equalsStart..this.position];
            this.Expect('[');
            if (this.Peek() is '\r' or '\n')
            {
                if (this.Take('\r'))
                    this.Take('\n');
                else
                    this.position++;
            }
            var end = this.source.IndexOf("]" + equals + "]", this.position, StringComparison.Ordinal);
            if (end < 0)
                throw new FormatException("Unterminated long string.");
            var value = this.source[this.position..end];
            this.position = end + equals.Length + 2;
            return value;
        }
        if (quote is not ('"' or '\''))
            throw new FormatException($"Expected a quoted string at character {this.position + 1}.");
        this.position++;
        var builder = new StringBuilder();
        while (this.position < this.source.Length)
        {
            var c = this.source[this.position++];
            if (c == quote)
                return builder.ToString();
            if (c is '\r' or '\n')
                throw new FormatException("A quoted string contains an unescaped newline.");
            if (c != '\\')
            {
                builder.Append(c);
                continue;
            }
            if (this.position == this.source.Length)
                break;
            c = this.source[this.position++];
            builder.Append(c switch
            {
                'n' => '\n', 'r' => '\r', 't' => '\t', 'a' => '\a', 'b' => '\b', 'f' => '\f', 'v' => '\v',
                '\\' => '\\', '"' => '"', '\'' => '\'',
                _ => throw new FormatException($"Unsupported string escape \\{c}."),
            });
        }
        throw new FormatException("Unterminated quoted string.");
    }

    private void SkipTrivia()
    {
        while (this.position < this.source.Length)
        {
            if (char.IsWhiteSpace(this.source[this.position]))
            {
                this.position++;
                continue;
            }
            if (this.source.AsSpan(this.position).StartsWith("--"))
            {
                this.position += 2;
                while (this.position < this.source.Length && this.source[this.position] != '\n')
                    this.position++;
                continue;
            }
            break;
        }
    }

    private char Peek() => this.position < this.source.Length ? this.source[this.position] : '\0';

    private bool Take(char c)
    {
        this.SkipTrivia();
        if (this.Peek() != c)
            return false;
        this.position++;
        return true;
    }

    private void Expect(char c)
    {
        if (!this.Take(c))
            throw new FormatException($"Expected '{c}' at character {this.position + 1}.");
    }

    private bool TakeWord(string word)
    {
        this.SkipTrivia();
        if (!this.source.AsSpan(this.position).StartsWith(word) ||
            (this.position + word.Length < this.source.Length && (char.IsLetterOrDigit(this.source[this.position + word.Length]) || this.source[this.position + word.Length] == '_')))
            return false;
        this.position += word.Length;
        return true;
    }

    private void ExpectWord(string word)
    {
        if (!this.TakeWord(word))
            throw new FormatException($"Expected '{word}' at character {this.position + 1}.");
    }
}