using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ShintenScript
{
    public class Lexer
    {
        static readonly (string, Token.Type)[] symbolTokens = new (string, Token.Type)[]
        {
            ("==", Token.Type.EQ),
            (">=", Token.Type.GE),
            ("<=", Token.Type.LE),
            ("!=", Token.Type.NE),
            ("+=", Token.Type.PLUSASSIGN),
            ("-=", Token.Type.MINUSASSIGN),
            ("*=", Token.Type.ASTERISKASSIGN),
            ("/=", Token.Type.SLASHASSIGN),

            (";", Token.Type.SEMICOLON),
            ("+", Token.Type.PLUS),
            ("-", Token.Type.MINUS),
            ("*", Token.Type.ASTERISK),
            ("/", Token.Type.SLASH),
            ("=", Token.Type.ASSIGN),
            ("(", Token.Type.LPAREN),
            (")", Token.Type.RPAREN),
            ("{", Token.Type.LBRACE),
            ("}", Token.Type.RBRACE),
            (">", Token.Type.GT),
            ("<", Token.Type.LT),
        };

        static readonly HashSet<string> keywords = new HashSet<string>
        {
            "if", "while", "fn"
        };

        string code;
        int index = 0;

        public Lexer(string code_)
        {
            if (code_ == null)
                throw new ArgumentNullException("code");

            code = code_;
        }

        Token current = new Token { type = Token.Type.NULL };

        public virtual Token ParseOne()
        {
            if (EOF())
                return new Token { type = Token.Type.EOF };

            while (char.IsWhiteSpace(code[index]))
            {
                index++;

                if (EOF())
                    return new Token { type = Token.Type.EOF };
            }

            foreach ((string symbol, Token.Type token) in symbolTokens)
            {
                if (code.Substring(index, symbol.Length) == symbol)
                {
                    index += symbol.Length;
                    return new Token { type = token };
                }
            }

            if (char.IsLetter(code[index]))
            {
                StringBuilder identifierBuilder = new StringBuilder();

                while (!EOF() && char.IsLetterOrDigit(code[index]))
                {
                    identifierBuilder.Append(code[index]);
                    index++;
                }

                string identifier = identifierBuilder.ToString();

                if (keywords.Contains(identifier))
                {
                    Enum.TryParse(identifier, true, out Token.Type type);
                    return new Token { type = type };
                }

                return new Token { type = Token.Type.IDENTIFIER, data = identifier };
            }

            if (char.IsNumber(code[index]))
            {
                StringBuilder numberBuilder = new StringBuilder();
                bool seenLetter = false;

                while (!EOF() && (char.IsLetterOrDigit(code[index]) || code[index] == '.' || (!seenLetter && code[index] == '-' && (code[index - 1] == 'e' || code[index - 1] == 'E'))))
                {
                    numberBuilder.Append(code[index]);

                    if (char.IsLetter(code[index]) && code[index] != 'e' && code[index] != 'E')
                        seenLetter = true;

                    index++;
                }

                string numberString = numberBuilder.ToString();
                float number;

                if (numberString.Contains("."))
                {
                    if (!float.TryParse(numberString, NumberStyles.Float, CultureInfo.InvariantCulture, out number))
                    {
                        throw new LexingException($"Invalid float literal \"{numberString}\"");
                    }
                }

                else
                {
                    if (
                        (numberString.ToLowerInvariant().StartsWith("0x") && long.TryParse(numberString.Substring(2), NumberStyles.HexNumber, CultureInfo.InvariantCulture, out long integer))
                        || long.TryParse(numberString, NumberStyles.Number, CultureInfo.InvariantCulture, out integer))
                    {
                        number = (float)integer;
                    }
                    else
                    {
                        throw new LexingException($"Invalid integer literal \"{numberString}\"");
                    }
                }

                return new Token { type = Token.Type.NUMBER, data = number };
            }

            throw new LexingException($"Encountered unexpected character '{code[index]}'");
        }

        bool EOF()
        {
            return index >= code.Length;
        }

        public Token Current()
        {
            if (current.Null())
            {
                return current = ParseOne();
            }
            else
            {
                return current;
            }
        }

        public Token Pop()
        {
            Token token = Current();
            current = new Token { type = Token.Type.NULL };
            return token;
        }

        public bool Accept(params Token.Type[] types)
        {
            Token token = Current();

            foreach (Token.Type type in types) {
                if (token.type == type)
                {
                    return true;
                }
            }

            return false;
        }

        public Token Expect(params Token.Type[] types)
        {
            Token token = Pop();

            foreach (Token.Type type in types)
            {
                if (token.type == type)
                {
                    return token;
                }
            }

            if (types.Length == 1)
            {
                throw new ParsingException($"Encountered {token.type} when expecting {types[0]}");
            }
            else if (types.Length == 2)
            {
                throw new ParsingException($"Encountered {token.type} when expecting one of {types[0]} or {types[1]}");
            }
            else
            {
                string list = string.Join(", ", types.Take(types.Length - 1));
                throw new ParsingException($"Encountered {token.type} when expecting one of {list}, or {types[types.Length - 1]}");
            }
        }

        public IEnumerable<Token> TokenStream()
        {
            while (true)
            {
                Token token = Pop();

                if (token.type == Token.Type.EOF)
                    break;

                yield return token;
            }
        }

        public void Print()
        {
            foreach (Token token in TokenStream())
                Console.WriteLine(token);
        }
    }
}