import math


def directions(o, t):
    """Compute a list of directions o -> t unit length."""

    assert len(o) == len(t)

    result = []

    for j in range(len(o)):
        dx = t[j][0] - o[j][0]
        dy = t[j][1] - o[j][1]
        l = math.sqrt(dx * dx + dy * dy)
        result.append((dx / l, dy / l))

    return result