global using System;
global using System.Collections.Generic;
global using System.IO;
global using System.Text;
using Microsoft.Extensions.ObjectPool;

namespace ParadoxSaveParser.Lib;

/// <summary>
///     Sequential parser that doesn't cache anything.
/// </summary>
public class SaveParserEU4
{
    protected readonly Stream _saveFile;
    private readonly BufferedEnumerator<Token> _tokens;
    private readonly ObjectPool<StringBuilder> _stringBuilderPool;
    private ISearchExpression? _searchExprCurrent;

    /// <param name="savefile">
    ///     Uncompressed stream of <c>gamestate</c> file which can be extracted from save archive
    /// </param>
    /// <param name="query">
    ///     Parsing whole save takes 10 seconds on mid pc and takes 1GB of RAM,
    ///     so you should specify what exactly you want to get from save file
    /// </param>
    public SaveParserEU4(Stream savefile, ISearchExpression? query)
    {
        _saveFile = savefile;
        _searchExprCurrent = query;
        const int tokenBufSize = 5;
        _tokens = new BufferedEnumerator<Token>(LexTextSave(), tokenBufSize);
        _stringBuilderPool = new DefaultObjectPool<StringBuilder>(
            new StringBuilderPooledObjectPolicy
            {
                InitialCapacity = tokenBufSize * 13,
                MaximumRetainedCapacity = tokenBufSize * 13,
            });
    }

    protected IEnumerator<Token> LexTextSave()
    {
        string expectedHeader = "EU4txt";
        byte[] headBytes = new byte[expectedHeader.Length];
        _saveFile.ReadExactly(headBytes);
        string headStr = Encoding.UTF8.GetString(headBytes);
        if (headStr != expectedHeader)
            throw new Exception($"Invalid gamestate header. Expected '{expectedHeader}', got '{headStr}'.");

        StringBuilder strb = _stringBuilderPool.Get();
        int line = 2;
        int column = 0;
        bool isQuoteOpen = false;
        bool isStrInQuotes = false;
        Token strToken = new()
        {
            type = TokenType.Invalid,
            column = -1,
            line = -1,
            value = null,
        };

        bool TryCompleteStringToken()
        {
            if (isQuoteOpen)
                return false;

            // strings in quotes may be empty
            if (!isStrInQuotes && (strb.Length <= 0 || strb[0] == '#'))
                return false;

            strToken = new Token
            {
                type = TokenType.StringOrNumber,
                column = (short)(column - strb.Length),
                line = line,
                value = strb,
            };
            strb = _stringBuilderPool.Get();
            isStrInQuotes = false;
            return true;
        }

        while (_saveFile.CanRead)
        {
            int c = _saveFile.ReadByte();
            column++;
            switch (c)
            {
                case -1:
                    if (TryCompleteStringToken())
                        yield return strToken;
                    _stringBuilderPool.Return(strb);
                    yield break;
                case '\"':
                    isQuoteOpen = !isQuoteOpen;
                    isStrInQuotes = true;
                    break;
                case ' ':
                case '\t':
                case '\r':
                    if (TryCompleteStringToken())
                        yield return strToken;
                    break;
                case '\n':
                    if (TryCompleteStringToken())
                        yield return strToken;
                    line++;
                    column = 0;
                    break;
                case '=':
                    if (TryCompleteStringToken())
                        yield return strToken;
                    yield return new Token
                    {
                        type = TokenType.Equals,
                        line = line, column = (short)column
                    };
                    break;
                case '{':
                    if (TryCompleteStringToken())
                        yield return strToken;
                    yield return new Token
                    {
                        type = TokenType.BracketOpen,
                        line = line, column = (short)column
                    };
                    break;
                case '}':
                    if (TryCompleteStringToken())
                        yield return strToken;
                    yield return new Token
                    {
                        type = TokenType.BracketClose,
                        line = line, column = (short)column
                    };
                    break;
                default:
                    // Skip control characters, which are invisible and causing frontend bugs.
                    // I dont know why there are so many of them in strings.
                    if (c >= 0x20)
                        strb.Append((char)c);
                    break;
            }
        }

        _stringBuilderPool.Return(strb);
    }


    // doesn't move next
    private object? ParseValue()
    {
        var tok = _tokens.Current.Value;
        switch (tok.type)
        {
            case TokenType.StringOrNumber:
                // string values can be empty
                if(tok.value!.Length == 0)
                    return string.Empty;
                
                string tokStr = tok.value.ToString();
                _stringBuilderPool.Return(tok.value);
                if (tokStr[0] != '-' && !char.IsDigit(tokStr[0]))
                    return tokStr;
                if (tokStr.Contains('.') && double.TryParse(tokStr, out double d))
                    return d;
                if (long.TryParse(tokStr, out long l))
                    return l;
                return tokStr;
            case TokenType.BracketOpen:
                object obj = ParseListOrDict();
                return obj;
            case TokenType.BracketClose:
                return null;
            default:
                throw new UnexpectedTokenException(tok);
        }
    }


    // skips next value
    /// <returns>true if skipped value, false if current token is closing bracket</returns>
    private bool SkipValue()
    {
        var tok = _tokens.Current.Value;
        switch (tok.type)
        {
            case TokenType.BracketOpen:
                SkipObject();
                return true;
            case TokenType.StringOrNumber:
                _stringBuilderPool.Return(tok.value!);
                return true;
            case TokenType.BracketClose:
                return false;
            default:
                throw new UnexpectedTokenException(tok);
        }
    }

    // skips all tokens inside curly braces block
    private void SkipObject(int bracketBalance = 1)
    {
        while (bracketBalance != 0 && _tokens.MoveNext())
        {
            var tok = _tokens.Current.Value;
            if (tok.type == TokenType.BracketOpen)
                bracketBalance++;
            else if (tok.type == TokenType.BracketClose)
                bracketBalance--;
            else if (tok.type == TokenType.StringOrNumber)
            {
                _stringBuilderPool.Return(tok.value!);
            }
        }
    }

    private static bool IsEmptyCollection(object value)
        => value is Dictionary<string, object> { Count: 0 } or List<object> { Count: 0 };

    // doesn't move next
    private object ParseListOrDict()
    {
        var first = _tokens.Current.Next;
        var second = _tokens.Current.Next?.Next;
        if (first?.Value.type == TokenType.StringOrNumber && second?.Value.type == TokenType.Equals)
            return ParseDict();

        return ParseList();
    }

    // moves next
    private List<object> ParseList()
    {
        List<object> list = new();
        for (int i = 0; ; i++)
        {
            if (!_tokens.MoveNext())
                throw new Exception("Unexpected end of file");

            ISearchExpression? searchExprNext = null;
            if (_searchExprCurrent != null 
                && !_searchExprCurrent.DoesMatch(new SearchArgs(i, string.Empty), out searchExprNext))
            {
                if(!SkipValue())
                    break;
                continue;
            }
            var searchExprPrev = _searchExprCurrent;
            _searchExprCurrent = searchExprNext;
            object? value = ParseValue();
            _searchExprCurrent = searchExprPrev;
            if (value is null)
                break;

            // do dot add empty collections into list
            if (IsEmptyCollection(value))
                continue;
            
            list.Add(value);
        }

        return list;
    }

    // moves next
    private Dictionary<string, object> ParseDict()
    {
        Dictionary<string, object> dict = new();

        // root is a dict without closing bracket, so this method must check _tokenIndex < _tokens.Count
        for (int localIndex = 0; _tokens.MoveNext(); localIndex++)
        {
            var tok = _tokens.Current.Value;
            // end of dictionary 
            if (tok.type == TokenType.BracketClose)
                break;

            // Saves may contain some blocks without key.
            // Such blocks are skipped because idk where to put them.
            // Example: `technology_group=tech_cannorian{ }
            //          { } { } { }`
            if (tok.type == TokenType.BracketOpen)
            {
                SkipObject();
                continue;
            }

            if (tok.type != TokenType.StringOrNumber)
                throw new UnexpectedTokenException(tok);

            var keySB = tok.value!;

            // next token should be `=` or `{`
            if (!_tokens.MoveNext())
                throw new UnexpectedTokenException(tok);
            tok = _tokens.Current.Value;
            if (tok.type == TokenType.Equals)
            {
                // skip `=`
                if (!_tokens.MoveNext())
                    throw new UnexpectedTokenException(tok);
            }
            // Saves may contain object definition without `=`.
            // Example: `map_area_data {` instead of `map_area_data = {`
            else if (tok.type != TokenType.BracketOpen)
            {
                throw new UnexpectedTokenException(tok);
            }

            ISearchExpression? searchExprNext = null;
            if (_searchExprCurrent != null
                && !_searchExprCurrent.DoesMatch(new SearchArgs(localIndex, keySB), out searchExprNext))
            {
                if(!SkipValue())
                    throw new UnexpectedTokenException(_tokens.Current.Value);
                _stringBuilderPool.Return(keySB);
                continue;
            }

            var searExpressionPrevious = _searchExprCurrent;
            _searchExprCurrent = searchExprNext;
            object? value = ParseValue();
            if (value is null)
                throw new UnexpectedTokenException(_tokens.Current.Value);
            _searchExprCurrent = searExpressionPrevious;

            string keyStr = keySB.ToString();
            _stringBuilderPool.Return(keySB);
            
            // Paradox save format has another way of defining list:
            // a = 1
            // a = 2
            // It means `a = { 1 2 }`
            if (dict.TryGetValue(keyStr, out var firstValue))
            {
                // Do dot add empty collections into list.
                // `key:{}` is okay, but i don't want to see `key:[{},{},{},{},{},{}]`
                if (IsEmptyCollection(value))
                    continue;
                
                if (firstValue is List<object> existingList)
                    existingList.Add(value);
                else dict[keyStr] = new List<object> { firstValue, value };
            }
            else
            {
                dict.Add(keyStr, value);
            }
        }

        return dict;
    }

    public Dictionary<string, object> Parse()
    {
        var root = ParseDict();
        return root;
    }

    protected enum TokenType : byte
    {
        Invalid,
        StringOrNumber,
        Equals,
        BracketOpen,
        BracketClose
    }

    protected struct Token
    {
        public required TokenType type;
        public required short column;
        public required int line;
        public StringBuilder? value;

        public override string ToString()
        {
            string s;
            switch (type)
            {
                case TokenType.Invalid:
                    s = "INVALID_TOKEN";
                    break;
                case TokenType.StringOrNumber:
                    if (value == null || value.Length == 0)
                        s = "NULL";
                    else s = value.ToString();
                    break;
                case TokenType.Equals:
                    s = "=";
                    break;
                case TokenType.BracketOpen:
                    s = "{";
                    break;
                case TokenType.BracketClose:
                    s = "}";
                    break;
                default:
                    throw new ArgumentOutOfRangeException(type.ToString());
            }

            return $"{line}:{column} '{s}'";
        }
    }

    protected class UnexpectedTokenException : Exception
    {
        public UnexpectedTokenException(Token token) :
            base($"Unexpected token: {token}")
        {
        }
    }
}