def trace_back(x, y, vx, vy, clr):
x0 = x - vx
y0 = y - vy
if x0 < 0 or x0 >= pic.size[0]:
return None
if y0 < 0 or y0 >= pic.size[1]:
return None
a, b, c = get_abc(x0, y0)
return x0, y0, vx^a, vy^b, clr^c
def char_applies(x0, y0, char):
x = x0
y = y0
prev_dx = prev_dy = None
for dx, dy, clr in char:
if x < 0 or y < 0 or x >= pic.size[0] or y >= pic.size[1]:
return None
if prev_dx is not None:
a, b, c = get_abc(x, y)
if dx ^ prev_dx != a or dy ^ prev_dy != b:
return None
x += dx
y += dy
prev_dx = dx
prev_dy = dy
vx, vy, clr = char[0]
a, b, c = get_abc(x0, y0)
return (x0, y0, vx^a, vy^b, 0)
