this assignment combines both parts of assignment 1 into one big-bang program
one additional difference: the cat moves left-to-right and right-to-left at different times, changing direction when it hits a wall (instead of wrapping around)
we can no longer represent the state of the world with just a single number
this big-bang program uses a structure to represent the state of the world
the state of the world keeps track of the cat's x coordinate, the cat's direction (dx), and the cat's percent of happiness.