Calculator

import 'dart:io';

bool isOp(String c)
{
    return (c == '*' || c == '/' || c == '-' || c == '+');
}

num _pow(num __n , num __p){
    if( __p == 0 ) return 1;
    if( __p % 2 == 1 ) return __n * _pow( __n ,__p - 1);
    num R = _pow( __n , __p / 2);
    return R * R;
}

num stringConverter(String s){
    num res = 0;
    for(num i = s.length - 1 ,j = 0 ; i >= 0 ; i-- , j++)
        res += _pow(10 ,j) * num.parse(s[i]);
    return res;
}

class Calculator
{
    num _result;

    Calculator()
    {
        this._result = 0.0;
    }

    num Calc(String op ,num a ,num b)
    {
        num Res = 0;

        switch( op )
        {
            case '+':
                Res = a + b;
                break;

            case '-':
                Res = a - b;
                break;

            case '*':
                Res = a * b;
                break;

            case '/':
                Res = a / b;
                break;
        }

        return Res;
    }

    num longOperation(String s ,num Size)
    {
        bool LastType = isOp( s[0] ) ? true : false;
        /// --- 1 Operation - 0 Numbers --- ///

        List< String > input = [ s[0] ];
        num idx = 1;

        if( isOp( s[0] ) )
        {
            idx += 1;
            input = [ s[1] ];
        }

        for(; idx < Size ; idx++)
        {
            bool CurType = isOp( s[idx] ) ? true : false;

            if( LastType == CurType ) input[ input.length - 1 ] += s[ idx ];
            else input.add( s[ idx ] );

            LastType = CurType;
        }

        num lastIdx = 0;
        num res   = 0.0;
        String resOperator = '+';

        for(num i = 0 ; i < input.length ; i++)
        {
            if( input[i] == "+" || input[i] == "-" )
            {
                num cur = stringConverter( input[lastIdx] );

                for(num j = lastIdx + 1 ; j < i ; j += 2)
                    cur = Calc(input[j][0] ,cur ,stringConverter( input[j + 1] ) );

                res = Calc(resOperator ,res ,cur);
                resOperator = input[i][0];
                lastIdx = i + 1;
            }
            else if( i == input.length - 1 )
            {
                num cur = stringConverter( input[lastIdx] );

                for(num j = lastIdx + 1 ; j <= i ; j += 2)
                    cur = Calc(input[j][0] ,cur ,stringConverter( input[j + 1] ) );

                res = Calc(resOperator ,res ,cur);
            }
        }

        return res;
    }

    bool isValidOperations(String s ,num size)
    {
        bool flag2 = false;
        bool flag1 = true;

        for(num i = 0 ; i < size && flag1 ; i++)
        {
            flag2 = true;

            if( !( s[i] is num ) && !isOp( s[i] ) )
                flag1 = false;

            if( i > 0 && isOp( s[i] ) )
                if( isOp( s[i - 1] ) )
                    flag1 = false;
        }

        return (flag1 && flag2);
    }

    void editRes(num x ,String op)
    {
        switch( op )
        {
            case '+':
                _result += x;
                break;

            case '-':
                _result -= x;
                break;

            case '*':
                _result *= x;
                break;

            case '/':
                _result /= x;
                break;

            case '=':
                _result = x;
                break;
        }
    }

    num getRes()
    {
        var result = _result;
        return result;
    }
}

void main()
{
    String ch;

    do
    {
        print('1 - (Enter Operation) : ');
        print('0 - (Exit !!) : ');
        print('Enter Choice : ');
        ch = stdin.readLineSync();

        switch( ch )
        {
            case '1':
            {
                String buffer;

                Calculator C;

                print('\nPress "End" To End ..\n\n');

                do  /// --- Loop Of Input --- ///
                {
                    print('Cur Res = ${C.getRes()} | New Operations (Without Spaces): ');
                    buffer = stdin.readLineSync();

                    num size = buffer.length;
                    if( isOp( buffer[size - 1] ) )
                    {
                        /// --- Ignore The Last Operation If It Alone --- ///
                        size -= 1;
                    }

                    bool flag = C.isValidOperations(buffer ,size);

                    if( flag ) /// --- The Operations Are Right --- ///
                    {
                        num res = C.longOperation(buffer ,size);
                        C.editRes(res ,isOp( buffer[0] ) ? buffer[0] : '=');
                    }
                    else
                    {
                        print('Wrong Operation In The Input !!\n\n');
                        break;
                    }

                } while( buffer != "End" );

                print('Last Ans = ${C.getRes()}\n\n');
            } break;

            case '0':
                print('\nThanks.\n_______________\n');
                break;

            default:
                print('Error !! UnValid Operation !!');
                break;
        }
    } while( ch != '0' );
}