python # find x s.t. f(x) = y
x = 0
for i in range(30 - 1, -1, -1):
dy = fs[i]
if y >= dy:
y -= dy
x += 2 ** ipython for i in range(30 - 1, -1, -1):
up = db_ups[i][cur % D]
if countdown >= up:
countdown -= up
cur = db_next[i][cur % D]
ret += 2 ** i