ping-pong game

I am trying to write the ping-pong game.
The problem is that I cannot make the ball bounce from the top and the bottom sides. When it is near one of the sides it just continues going in the same direction.
Help me to write the right condition in the calculate () function.
This is algorithm:
print the start screen
endless cycle:
if user pressed certain key then calculate new positions of objects
print them


use up arrow or down arrow to move the right player and a or q to move the left player
escape to exit

Code:

---

/* PING-PONG GAME */


#include <conio.h> /* getch () */
#include <stdio.h> /* putchar () */
#include <stdlib.h> /* system () */


#define UP 72 /* up arrow key code */
#define ESC 27 /* escape key code */
#define DOWN 80 /* down arrow key code */


void reverse_direction (); /* reverses the direction of the ball */
void refresh (); /* prints game process */
int calculate (int c); /* calculates next positions of objects (player1, player2, ball, etc.) */
void init (); /* initializes the player1, player2, ball structures */


struct
{
    int x, y; /* positions */
} player1, player2; /* left and right players */


struct
{
    int x, y; /* positions */
    struct
    {
        int x, y; /* how fast the ball moves */
    } speed;
    enum
    {
        LEFT_DOWN = 1,
        LEFT_UP,
        RIGHT_DOWN,
        RIGHT_UP
    } direction; /* contains information about what direction the ball should move in */
} ball;


int main (void)
{
    int c; /* stores code of a pressed key */
    init (); /* initializes the structures */
    refresh (); /* clears the screen and prints characters */
    while (c = getch ())
        if (calculate (c)) /* refresh () only if one if certain keys was pressed, if the escape key was pressed it calls exit (0) */
            refresh ();
    return 0;
}


void refresh ()
{
    int i = 0, j = 0;
    system ("cls");
    while (i < 25)
    {
        j = 0;
        while (j < 80)
        {
            if ((j == player1.x && i == player1.y) || (j == player2.x && i == player2.y) || (j == ball.x && i == ball.y))
                putchar (219);
            else
                putchar (' ');
            ++j;
        }
        ++i;
    }
    return;
}


void init ()
{
    player1.x = 0;
    player1.y = 0;
    player2.x = 79;
    player2.y = 0;
    ball.x = 50;
    ball.y = 15;
    ball.speed.x = 1;
    ball.speed.y = 1;
    ball.direction = RIGHT_DOWN;
    return;
}


int calculate (int c)
{
    if ((((ball.x == (player1.x + 1)) && (ball.y == player1.y)) || (ball.x == (player2.x - 1) && (ball.y == player2.y))) || (ball.y == 0 || ball.y == 24))
        reverse_direction ();
    if (ball.direction == LEFT_DOWN)
    {
        ball.x -= ball.speed.x;
        ball.y += ball.speed.y;
    }
    else if (ball.direction == LEFT_UP)
    {
        ball.x -= ball.speed.x;
        ball.y -= ball.speed.y;
    }
    else if (ball.direction == RIGHT_UP)
    {
        ball.x += ball.speed.x;
        ball.y -= ball.speed.y;
    }
    else if (ball.direction == RIGHT_DOWN)
    {
        ball.x += ball.speed.x;
        ball.y += ball.speed.y;
    }
    if (c == UP)
        player1.y--;
    else if (c == DOWN)
        player1.y++;
    else if (c == 'q')
        player2.y--;
    else if (c == 'a')
        player2.y++;
    else if (c == ESC)
    {
        system ("cls");
        exit (0);
    }
    else
        return 0;
    return 1;
}


void reverse_direction (void)
{
    if (ball.direction == RIGHT_UP)
        ball.direction = RIGHT_DOWN;
    if (ball.direction == RIGHT_DOWN)
        ball.direction = RIGHT_UP;
    if (ball.direction == LEFT_DOWN)
        ball.direction = LEFT_UP;
    if (ball.direction == LEFT_UP)
        ball.direction = LEFT_DOWN;
    return;
}

---

If you just do this
ball.x += ball.speed.x;

And to make it bounce (on either wall), you do this
ball.speed.x = -ball.speed.x;

Adding a negative number is just like subtraction.

Then it will automatically toggle between going right and going left, so long as you can bounce it at the right moment.

No need to code complicated direction information :)