using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

public class Program

{

    public static void Main()

    {

        Write("\nIEnumerables:");

        Write("\n");

        //This won't generate any output, no processing happens

        var processedNumbersEnumerate = GimmeSomeNumbers(10)

            .Select(Square)

            .Where(Filter);

        Write("\nLists:");

        Write("\n");

        //This will generate output. first it will get the numbers, then square them, then filter them

        //  (in this case it's especially bad, because this is actually creating copies of the list 3 times, not modifying it in place)

        var processedNumbersList = GimmeSomeNumbers(10).ToList()

            .Select(Square).ToList()

            .Where(Filter).ToList();

        Write("\nEvaluation of the IEnumerable:");

        Write("\n");

        //Here is where the first query executes. but instead of generating, squaring, then filtering; it does them all at once.

        //  It generates the first item; squares it; and filters on it. then it tries the next item, etc.

        var tmplist = processedNumbersEnumerate.ToList();

        //Note that once the IEnumerable starts to be used by writing it to a string and comparing it to the other list,

        //  it would be much less performant to use the IEnumerable in multiple places like this, se we convert to a list

        //  But we only convert to a list once; ever.

        Write("\n" + EnumerableToString(processedNumbersList));

        Write("\n" + EnumerableToString(tmplist));

        //in the end its the same result

        Write("\n\nAre Equal: " + tmplist.SequenceEqual(processedNumbersList));

    }

    public static IEnumerable<int> GimmeSomeNumbers(int howManyNumber){

        for(int i = 0; i < howManyNumber; i++){

            Write(" Gimme " + i);

            yield return i;

        }

    }

    public static int Square(int n){

        Write(" Square " + n);

        return n * n;

    }

    public static bool Filter(int n){

        Write(" Filter " + n);

        return (n + 1) % 5 == 0;

    }

    public static string EnumerableToString(IEnumerable<int> list){

        StringBuilder b = new StringBuilder("[");

        foreach(int i in list){

            b.Append(i + ", ");

        }

        b.Append("]");

        return b.ToString();

    }

    public static void Write(string a){

        Console.Out.Write(a);

    }

}