Collision detection: Left border

Collision detection needs to be calculated, as the ball contacts and needs to rebound from the borders around the game board, the paddle and the bricks.

This works on the physics equation:
speed=distance/time

To calculate the movement of the ball after collision I need to know how much time it has taken to reach the collision, as a proportion of a time step. The remaining time in the time step after the collision is used by the ball travelling in the opposite direction.
x1=ball position x – minimum x + ball radius //calculate distance ball travelled before collision








t //amount of time step as proportion, between 0 and 1
y1 //distance travelled in y before collision
velx //total distance moved (real and projected) in x in time step
vely //total distance moved (real and projected) in y in time step

t=x1/velx //distance travelled in x in proportion to whole distance to be travelled in time step
y1=t*vely //calculate distance travelled in y before time step, multiply whole distance the ball would travel in time step by proportion already travelled
velx=-velx //reverse direction