using System;

using System.Collections.Generic;

​

public class Program

{

    //Create static instances

    static string NextToken;

    static Stack<string> TokenStack;

    static List<string> InputBuffer;

    static Stack<string> ActionStack = new Stack<string>();

    static int TokenCounter;

    static int State = 0;

    public static void Main()

    {

            Console.WriteLine("\n\nEnter Expression");

            List<string> InputBuffer = Lexer(Console.ReadLine());

            Stack<string> TokenStack = new Stack<string>();

            for(int i = InputBuffer.Count-1; i >= 0; i--) {

                TokenStack.Push(InputBuffer[i]);

            }

            while(TokenStack.Count > 0) {

                Shift(0);

            }

            TokenStack.Push("$");

            Console.WriteLine("\nPress any key to exit.");

    }

    public static List<string> Lexer(string input)

    {

        //LookupTable contains special characters known by the compiler

        SortedList<string, string> LookupTable = new SortedList<string, string>();

        LookupTable.Add("+","+");

        LookupTable.Add("-","-");

        LookupTable.Add("=","=");

        LookupTable.Add("*","*");

        LookupTable.Add("/","/");

        LookupTable.Add("(","(");

        LookupTable.Add(")",")");

​

        //CharacterStream contains every character from input expression

        char[] CharacterStream = input.ToCharArray();

        //InputBuffer contains every token found by Lexer

        InputBuffer = new List<string>();

​

        //i and j used to iterate through the InputBuffer and CharacterStream

        int i = 0;

        int j = 0;

        //While loop iterates over CharacterStream until it reaches the end

        while(j < CharacterStream.Length && j < CharacterStream.Length)

        {

            string CurChar = Char.ToString(CharacterStream[j]);

            //If next character is a letter or digit the token will be id

            if(Char.IsLetter(CharacterStream[j]) || Char.IsDigit(CharacterStream[j]))

            {

                string Lexeme = CurChar;

                //While there is another char and it's either a letter or digit

                while(j < CharacterStream.Length-1 && (Char.IsDigit(CharacterStream[j+1]) || Char.IsLetter(CharacterStream[j+1])))

                {

                    j++;

                    Lexeme += CharacterStream[j];

                }

                //Add token id

                InputBuffer.Add("id");

                //LexStream.Add(Lexeme);

                Console.WriteLine(Lexeme+":"+InputBuffer[i]);

                i++;

            }

            //If character is in LookupTable

            else if(LookupTable.ContainsKey(CurChar))

            {

                InputBuffer.Add(LookupTable[CurChar]);

                i++;

                //LexStream.Add(CurChar);

                Console.WriteLine(CurChar+":"+LookupTable[CurChar]);

            }

            //If character is not white space

            else if(CurChar != " ")

            {

                //Set InputBuffer to null

                InputBuffer = null;

                Console.WriteLine("Error: Character "+CurChar+" not recognized.");

                break;

            }

            j++;

        }

        //If there was no unrecognized character errors, add EOF token

        if(InputBuffer != null) InputBuffer.Add("EOF");

        return InputBuffer;

    }

    private static void E() {

    }

    private static void T() {

    }

    private static void F() {

    }

    private static void Error() {

    }

    private static void Shift(int NewState) {

        State = NewState;

        NextToken = TokenStack.Pop();

        ActionStack.Push(NextToken);

    }

    private static void NextStep() {

        NextToken = TokenStack.Peek();

        switch(NextToken) {

            case "id":

                if(State == 0 || State == 4 || State == 6 || State == 7) Shift(5);

                else Error();

                break;

            case "+":

                switch(State) {

                    case 1:

                        Shift(6);

                        break;

                    case 2:

                        Reduce(2);

                        break;

                }

                break;

        }

    }

    private static void Reduce(int Rule) {

    }

}

​