What does this piece of code does(SFML C++ game)?

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Windows

game

C++14

Hello guys,
I am trying to make a simple ping pong game using SFML.

I am stuck with a piece of code which I took out from internet, I am not able to understand the working in detail so guys please explain.

Below piece of code is to move a ball in a random direction and make it bounce when it hits the boundary or a paddle

<pre lang="c++">
void update(sf::Clock &clock, sf::Time &elapsed) {

		const float velocity = std::sqrt(direction.x * direction.x + direction.y * direction.y);

		elapsed += clock.restart();
		while (elapsed >= update_ms) {
			const auto pos = ball.getPosition();
			const auto delta = update_ms.asSeconds() * velocity;
			sf::Vector2f new_pos(pos.x + direction.x * delta, pos.y + direction.y * delta);

			if (new_pos.x - ball_radius < 0) { // left window edge
				direction.x *= -1;
				new_pos.x = 0 + ball_radius;
			}
			else if (new_pos.x + ball_radius >= 800) { // right window edge
				direction.x *= -1;
				new_pos.x = 800 - ball_radius;
			}
			else if (new_pos.y - ball_radius < 0) { // top of window
				direction.y *= -1;
				new_pos.y = 0 + ball_radius;
			}
			else if (new_pos.y + ball_radius >= 500) { // bottom of window
				direction.y *= -1;
				new_pos.y = 500 - ball_radius;
			}
			ball.setPosition(new_pos);

			elapsed -= update_ms;
		}
	}

I have edited to code to my wish a bit. I have given a reference to a clock and vector2f direction to a private variable which will make the ball shoot in a random direction.

What I have tried:

Understood the code a bit, but not in detail so need your help guys.