Keyboard-Calculator/Calculator/Calc.cs

using NCalc;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using FormsKey = System.Windows.Forms.Keys;

namespace Calculator
{
    public class Calc
    {
        private const int Precision = 3;
        private readonly List<CalculationElement> calculation;
        private bool FirstKeyPressed = true;

        public Calc()
        {
            calculation = new List<CalculationElement>();
        }

        public CalculationResponse KeyPressed(FormsKey key)
        {
            switch (key)
            {
                case FormsKey.Enter:
                    return Calculate();

                case FormsKey.Delete:
                    return Delete();

                case FormsKey.NumPad0:
                    return AddCalculationElement(new NumberElement(0));

                case FormsKey.NumPad1:
                    return AddCalculationElement(new NumberElement(1));

                case FormsKey.NumPad2:
                    return AddCalculationElement(new NumberElement(2));

                case FormsKey.NumPad3:
                    return AddCalculationElement(new NumberElement(3));

                case FormsKey.NumPad4:
                    return AddCalculationElement(new NumberElement(4));

                case FormsKey.NumPad5:
                    return AddCalculationElement(new NumberElement(5));

                case FormsKey.NumPad6:
                    return AddCalculationElement(new NumberElement(6));

                case FormsKey.NumPad7:
                    return AddCalculationElement(new NumberElement(7));

                case FormsKey.NumPad8:
                    return AddCalculationElement(new NumberElement(8));

                case FormsKey.NumPad9:
                    return AddCalculationElement(new NumberElement(9));

                case FormsKey.Add:
                    return AddCalculationElement(new OperationElement(Operation.Add));

                case FormsKey.Subtract:
                    return AddCalculationElement(new OperationElement(Operation.Subtract));

                case FormsKey.Multiply:
                    return AddCalculationElement(new OperationElement(Operation.Muliply));

                case FormsKey.Divide:
                    return AddCalculationElement(new OperationElement(Operation.Divide));

                case FormsKey.Decimal:
                    return AddCalculationElement(new DecimalElement());

                default:
                    return CalculationResponse.WrongInput;
            }
        }

        private CalculationResponse Delete()
        {
            calculation.Clear();
            return CalculationResponse.ClearedMemory;
        }

        private bool CheckIfOperationIsAllowed(CalculationElement element)
        {
            if (calculation.Count == 0 && (element as OperationElement).Operation != Operation.Subtract)
                return false;
            if (calculation.LastOrDefault() is OperationElement)
                return false;
            return true;
        }

        private bool CheckIfNumberIsAlreadyDecimal()
        {
            bool decimalFound = false;
            foreach (CalculationElement item in calculation)
            {
                if (decimalFound)
                {
                    if (item is OperationElement)
                        decimalFound = false;
                }
                else
                    if (item is DecimalElement)
                    decimalFound = true;
            }
            return decimalFound;
        }

        private bool CheckIfDecimalIsAllowed()
        {
            if (FirstKeyPressed)
            {
                calculation.Clear();
                FirstKeyPressed = false;
                calculation.Add(new NumberElement(0));
                return true;
            }
            CalculationElement last = calculation.LastOrDefault();
            if (last is DecimalElement)
                return false;
            else if (last is NumberElement)
            {  //check if another decimal exists before we encounter an OperationElement
                if (CheckIfNumberIsAlreadyDecimal()) return false;
            }
            else if (last == null || last is OperationElement)
                calculation.Add(new NumberElement(0));
            return true;
        }

        private CalculationResponse AddCalculationElement(CalculationElement element)
        {
            if (element is OperationElement)
                if (!CheckIfOperationIsAllowed(element))
                    return CalculationResponse.WrongInput;
            if (element is DecimalElement)
                if (!CheckIfDecimalIsAllowed())
                    return CalculationResponse.WrongInput;
            if (element is NumberElement && FirstKeyPressed)
                calculation.Clear();
            FirstKeyPressed = false;
            calculation.Add(element);
            return CalculationResponse.Ok;
        }

        private CalculationResponse Calculate()
        {
            if (calculation.Count == 0) return new CalculationResponse(Response.Result, "0");
            StringBuilder calcString = new StringBuilder();
            calculation.ForEach(element => calcString.Append(element.ToString()));
            Expression e = new Expression(calcString.ToString().TrimEnd('.', '/', '*', '-', '+'));
            double solution = Convert.ToDouble(e.Evaluate());
            ClearCalculation();
            string result = solution.ToString(string.Format("F{0}", Precision)); //Region Specific!!
            result = result.TrimEnd('0', ',');
            AddResultToCalculation(result);
            return new CalculationResponse(Response.Result, result);
        }

        private void ClearCalculation()
        {
            calculation.Clear();
            FirstKeyPressed = true;
        }

        private string DecimalsZero()
        {
            return ',' + new string('0', Precision);
        }

        private void AddResultToCalculation(string result)
        {
            for (int i = 0; i < result.Length; i++)
            {
                if (result[i] == ',')
                    calculation.Add(new DecimalElement());
                else if (result[i] == '-')
                    calculation.Add(new OperationElement(Operation.Subtract));
                else
                    calculation.Add(new NumberElement((byte)char.GetNumericValue(result[i])));
            }
        }
    }
}