cassidoo_problem_09212020.py

'''
Given an array of integers representing asteroids in a row, for each asteroid, the absolute value represents its size, and the sign represents its direction (positive = right, negative = left).
Return the state of the asteroids after all collisions (assuming they are moving at the same speed).
If two asteroids meet, the smaller one will explode.
When they are the same size, they both explode. Asteroids moving in the same direction will never meet.

Example:
$ asteroids([5, 8, -5]) -> [5, 8] // The 8 and -5 collide, 8 wins. The 5 and 8 never collide.
$ asteroids([10, -10]) -> [] // The 10 and -10 collide and they both explode.

Assumptions:
- No asteroids have 0 speed, so there are no 3 way collisions
- After colliding, the surviving asteroid doesn't keep colliding
'''

def asteroids(l):
    survivors = []
    for i in range(len(l)):
        size = abs(l[i])
        direction = int(l[i] / size)
        if 0 <= i + direction < len(l):
            neighbor_size = abs(l[i + direction])
            neighbor_direction = int(l[i + direction] / neighbor_size)
            if neighbor_direction == direction or size > neighbor_size:
                survivors.append(l[i])
        else:
            survivors.append(l[i])
    return survivors

if __name__ == "__main__":
    assert(asteroids([5, 8, -5]) == [5, 8])
    assert(asteroids([10, -10]) == [])
    assert(asteroids([-10, -10]) == [-10, -10]) # No collision
    assert(asteroids([1, -0.5]) == [1]) # Simple collision
    assert(asteroids([-1, 1]) == [-1, 1]) # Diverge
    print("All tests passed!")