Cycles and stuff - What does this program do?

Question

I just found one of my first C++ programs in some deep caves on my disk...

#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <sstream>
#include <string>

using namespace std;

int     a , b , c , d , e , f , g , h , i , j , x , n , N , S;
enum    cycle { A , B , C };
string  Str_;
cycle Cy;

void next_Cy();

int main() {
S = 0;
Cy = A;     // Set first cycle

cout << "Enter n:\n";

getline ( cin, Str_ );
stringstream (Str_) >> x; // take x

if ( x >= 10 ) {
    n  = (x / 10)-1;    // defining n

    for ( N = 0; N <= n; N++ ) {
        a  = N*10+0;        // defining the digits
        b  = N*10+1;
        c  = N*10+2;
        d  = N*10+3;
        e  = N*10+4;
        f  = N*10+5;
        g  = N*10+6;
        h  = N*10+7;
        i  = N*10+8;
        j  = N*10+9;

        switch ( Cy ) {          // switch cycle cases
            case A:
                S += a + d + f + g + j;
                break;
            case B:
                S += a + c + f + i;
                break;
            case C:
                S += a + b + e + f + h;
                break;
            }
            next_Cy();          // next cycle ..
        }
        cout << S << endl;
    }
return 0;
}

// function to define next cycle
// A becomes B
// B becomes C
// C becomes A again ,,
void next_Cy() {
    if ( Cy == A ) {
        Cy = B;
    }
    else {
        if ( Cy == B ) {
            Cy = C;
        }
        else {
            Cy = A;
        }
    }
}

(I can't believe that I wrote this shit some day in my life.)

CHALLENGE: Try to find out what this program does. (Yes, it does something "useful".)

Compiling and executing the code is of course allowed. (Yes, it does compile and run.)

% g++ puzzle.cpp -o puzzle
% ./puzzle
Enter n:
9000
18895500
%

So basically, you just enter a number (which should be greater than 9 because reasons) and then you will get some result, whatever that is.

By the way, I didn't modify the code or anything, these variables were really labeled like that.

PS: My favorite comment is // take x.

The first to find out what this code does, wins.

The explanation having the most upvotes wins.

Use the spoil tag on your answer!

Answer 1

This program sums all natural numbers strictly below x (rounded down to the nearest multiple of 10, I don't think this was intended), that are divisible by either 5 or 3. For example: x=30 returns: 0+3+5+6+9+10+12+15+18+20+21+24+25+27 = 195

PS what the heck? Seriously,

for(int i = 0;i < x;i++) S+= (i%3||i%5)?i:0;

would've been easier to write down, to read, and to execute.

Explanation arrives as soon as I get on my actual PC.