using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;

namespace CCalc
{
    public class Token
    {
        public enum TokenKind
        {
            BadToken,
            NumberToken,
            AddOperatorToken,
            SubOperatorToken,
            MulOperatorToken,
            DivOperatorToken,
            EndOfLineToken,
            LeftParenToken,
            RightParenToken,
        };

        public double value;
        public TokenKind kind;

        public Token(TokenKind kind, double value)
        {
            this.kind = kind;
            this.value = value;
        }

        public Token(TokenKind kind)
        {
            this.kind = kind;
            this.value = 0;
        }
    }

    public class Lexer : IEnumerable<Token>
    {
        private enum LexerStatus
        {
            InitialStatus, InIntPartStatus, DotStatus, InFracPartStatus
        };

        private string line;
        public IEnumerator<Token> tokenizer;
        public Token aheadToken;

        public Lexer(string line)
        {
            this.line = line;
            this.tokenizer = this.GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return this.GetEnumerator();
        }

        public void UnGetToken(Token token)
        {
            this.aheadToken = token;
        }

        public Token GetToken()
        {
            this.MoveNext();
            return this.Current;
        }

        public bool MoveNext()
        {
            if (this.aheadToken != null)
            {
                return true;
            }
            else
            {
                return this.tokenizer.MoveNext();
            }
        }

        public Token Current
        {
            get
            {
                if (this.aheadToken != null)
                {
                    Token token = aheadToken;
                    aheadToken = null;
                    return token;
                }
                else
                {
                    return this.tokenizer.Current;
                }
            }
        }

        public IEnumerator<Token> GetEnumerator()
        {
            LexerStatus lexerStatus = LexerStatus.InitialStatus;
            string strReturn = string.Empty;

            foreach (char ch in line)
            {
                if ((lexerStatus == LexerStatus.InIntPartStatus || lexerStatus == LexerStatus.InFracPartStatus) && !char.IsDigit(ch) && ch != '.')
                {
                    if (strReturn != string.Empty)
                    {
                        double value = double.Parse(strReturn);
                        strReturn = string.Empty;
                        yield return new Token(Token.TokenKind.NumberToken, value);
                    }
                }

                if (char.IsWhiteSpace(ch))
                {
                    if (ch == '\n')
                    {
                        yield return new Token(Token.TokenKind.EndOfLineToken);
                    }
                }

                if (ch == '+')
                {
                    yield return new Token(Token.TokenKind.AddOperatorToken);
                }
                else if (ch == '-')
                {
                    yield return new Token(Token.TokenKind.SubOperatorToken);
                }
                else if (ch == '*')
                {
                    yield return new Token(Token.TokenKind.MulOperatorToken);
                }
                else if (ch == '/')
                {
                    yield return new Token(Token.TokenKind.DivOperatorToken);
                }
                else if (ch == '(')
                {
                    yield return new Token(Token.TokenKind.LeftParenToken);
                }
                else if (ch == ')')
                {
                    yield return new Token(Token.TokenKind.RightParenToken);
                }
                else if (char.IsDigit(ch))
                {
                    if (lexerStatus == LexerStatus.InitialStatus)
                    {
                        lexerStatus = LexerStatus.InIntPartStatus;
                    }
                    else if (lexerStatus == LexerStatus.DotStatus)
                    {
                        lexerStatus = LexerStatus.InFracPartStatus;
                    }
                    strReturn += ch;
                }
                else if (ch == '.')
                {
                    if (lexerStatus == LexerStatus.InIntPartStatus)
                    {
                        lexerStatus = LexerStatus.DotStatus;
                    }
                    strReturn += ch;
                }
                else
                {
                    yield return new Token(Token.TokenKind.BadToken);
                }
            }
            yield return new Token(Token.TokenKind.EndOfLineToken);
        }
    }
}