using NubLang.Common;
using NubLang.Diagnostics;

namespace NubLang.Syntax.Tokenization;

public sealed class Tokenizer
{
    private static readonly Dictionary<string, Symbol> Keywords = new()
    {
        ["namespace"] = Symbol.Namespace,
        ["func"] = Symbol.Func,
        ["if"] = Symbol.If,
        ["else"] = Symbol.Else,
        ["while"] = Symbol.While,
        ["break"] = Symbol.Break,
        ["continue"] = Symbol.Continue,
        ["return"] = Symbol.Return,
        ["alloc"] = Symbol.Alloc,
        ["struct"] = Symbol.Struct,
        ["let"] = Symbol.Let,
        ["calls"] = Symbol.Calls,
        ["trait"] = Symbol.Trait,
        ["impl"] = Symbol.Impl,
        ["for"] = Symbol.For,
        ["extern"] = Symbol.Extern
    };

    private static readonly Dictionary<char[], Symbol> Chians = new()
    {
        [['=', '=']] = Symbol.Equal,
        [['!', '=']] = Symbol.NotEqual,
        [['<', '=']] = Symbol.LessThanOrEqual,
        [['>', '=']] = Symbol.GreaterThanOrEqual,
        [[':', ':']] = Symbol.DoubleColon,
    };

    private static readonly Dictionary<char, Symbol> Chars = new()
    {
        [':'] = Symbol.Colon,
        ['('] = Symbol.OpenParen,
        [')'] = Symbol.CloseParen,
        ['{'] = Symbol.OpenBrace,
        ['}'] = Symbol.CloseBrace,
        ['['] = Symbol.OpenBracket,
        [']'] = Symbol.CloseBracket,
        [','] = Symbol.Comma,
        ['.'] = Symbol.Period,
        ['='] = Symbol.Assign,
        ['<'] = Symbol.LessThan,
        ['>'] = Symbol.GreaterThan,
        ['+'] = Symbol.Plus,
        ['-'] = Symbol.Minus,
        ['*'] = Symbol.Star,
        ['/'] = Symbol.ForwardSlash,
        ['!'] = Symbol.Bang,
        ['^'] = Symbol.Caret,
        ['&'] = Symbol.Ampersand,
        [';'] = Symbol.Semi,
    };

    private readonly SourceText _sourceText;
    private int _index;

    public Tokenizer(SourceText sourceText)
    {
        _sourceText = sourceText;
    }

    public IEnumerable<Token> Tokenize(out IEnumerable<Diagnostic> diagnostics)
    {
        _index = 0;

        List<Token> tokens = [];
        while (ParseToken().TryGetValue(out var token))
        {
            tokens.Add(token);
        }

        // TODO: Implement diagnostics
        diagnostics = [];
        return tokens;
    }

    private Optional<Token> ParseToken()
    {
        var startIndex = _index;

        if (!Peek().TryGetValue(out var current))
        {
            return Optional<Token>.Empty();
        }

        if (Peek().TryGetValue(out var character) && char.IsWhiteSpace(character))
        {
            Next();

            return ParseToken();
        }

        if (current == '/' && Peek(1).TryGetValue(out var nextChar) && nextChar == '/')
        {
            Next();
            Next();

            while (Peek().TryGetValue(out var ch) && ch != '\n')
            {
                Next();
            }

            return ParseToken();
        }

        if (char.IsLetter(current) || current == '_')
        {
            var buffer = string.Empty;

            while (Peek().TryGetValue(out var next) && (char.IsLetterOrDigit(next) || next == '_'))
            {
                buffer += next;
                Next();
            }

            if (Keywords.TryGetValue(buffer, out var keywordSymbol))
            {
                return new SymbolToken(CreateSpan(startIndex), keywordSymbol);
            }

            if (buffer is "true" or "false")
            {
                return new LiteralToken(CreateSpan(startIndex), LiteralKind.Bool, buffer);
            }

            return new IdentifierToken(CreateSpan(startIndex), buffer);
        }

        if (char.IsDigit(current))
        {
            var isFloat = false;
            var buffer = string.Empty;

            while (Peek().TryGetValue(out var next))
            {
                if (next == '.')
                {
                    if (isFloat)
                    {
                        throw new Exception("More than one period found in float literal");
                    }

                    isFloat = true;
                    buffer += next;
                    Next();
                }
                else if (char.IsDigit(next))
                {
                    buffer += next;
                    Next();
                }
                else
                {
                    break;
                }
            }

            return new LiteralToken(CreateSpan(startIndex), isFloat ? LiteralKind.Float : LiteralKind.Integer, buffer);
        }

        // TODO: Revisit this
        foreach (var chain in Chians)
        {
            if (current != chain.Key[0]) continue;

            for (var i = 1; i < chain.Key.Length; i++)
            {
                var c = Peek(i);
                if (!c.HasValue || c.Value != chain.Key[i]) break;

                if (i == chain.Key.Length - 1)
                {
                    for (var j = 0; j <= i; j++)
                    {
                        Next();
                    }

                    return new SymbolToken(CreateSpan(startIndex), chain.Value);
                }
            }
        }

        if (Chars.TryGetValue(current, out var charSymbol))
        {
            Next();
            return new SymbolToken(CreateSpan(startIndex), charSymbol);
        }

        if (current == '"')
        {
            Next();
            var buffer = string.Empty;

            while (true)
            {
                if (!Peek().TryGetValue(out var next))
                {
                    throw new Exception("Unclosed string literal");
                }

                if (next == '"')
                {
                    Next();
                    break;
                }

                buffer += next;
                Next();
            }

            return new LiteralToken(CreateSpan(startIndex), LiteralKind.String, buffer);
        }

        throw new Exception($"Unknown character {current}");
    }

    private SourceLocation CreateLocation(int index)
    {
        var line = 1;
        var column = 1;
        for (var i = 0; i < Math.Min(index, _sourceText.Content.Length - 1); i++)
        {
            if (_sourceText.Content[i] == '\n')
            {
                column = 1;
                line += 1;
            }
            else
            {
                column += 1;
            }
        }

        return new SourceLocation(line, column);
    }

    private SourceSpan CreateSpan(int startIndex)
    {
        return new SourceSpan(_sourceText, CreateLocation(startIndex), CreateLocation(_index));
    }

    private Optional<char> Peek(int offset = 0)
    {
        if (_index + offset < _sourceText.Content.Length)
        {
            return _sourceText.Content[_index + offset];
        }

        return Optional<char>.Empty();
    }

    private Optional<char> Next()
    {
        if (_index < _sourceText.Content.Length)
        {
            return _sourceText.Content[_index++];
        }

        _index++;
        return Optional<char>.Empty();
    }
}