# Digits are chosen to be the least ambiguous
# They all have unique seven-segment glyphs, and cannot be easily confused by humans
digits = '123456789abcdehjnrtyACEFGHJLOPUY'
radix = len(digits)

def encode(v):
	if not len(v):
		return ''
	n = 0
	bits = 0
	o = []
	pad = (len(v) * 8) % 5
	if pad:
		v = b'\0' + v
	v = bytearray(v)  # For Python 2.7 compatibility
	for i in range(len(v) - 1, -1, -1):
		n |= v[i] << bits
		bits += 8
		while bits >= 5:
			o.insert(0, digits[n & 0x1f])
			n >>= 5
			bits -= 5
			if i == 0 and pad:
				break
	return ''.join(o)

def decode(s):
	n = 0
	bits = 0
	o = bytearray()
	for i in range(len(s) - 1, -1, -1):
		n |= digits.index(s[i]) << bits
		bits += 5
		while bits >= 8:
			o.insert(0, n & 0xff)
			n >>= 8
			bits -= 8
	return bytes(o)

def test():
	from math import ceil
	assert '' == encode(b'')
	for (i, oc) in (
		(1, '8'),
		(2, '2'),
		(3, 'j'),
		(4, '4'),
		(5, 'Y'),
		(6, '8'),
		(7, '2'),
		(8, 'j'),
		(9, '4'),
	):
		ol = int(ceil(i * 8 / 5.))
		try:
			inzero = b'\0' * i
			inone = b'\xff' * i
			ezero = encode(inzero)
			eone = encode(inone)
			dzero = decode(ezero)
			done = decode(eone)
			
			assert ezero == '1' * ol
			assert eone == oc + ('Y' * (ol - 1))
			assert dzero == inzero
			assert done == inone
		except AssertionError:
			raise AssertionError('Input of length %s failed test' % (i,))
	try:
		for c in range(1024):
			decode('111' + chr(c))
	except ValueError:
		pass
	else:
		raise AssertionError('Invalid decode input (%02x) did not throw a ValueError' % (c,))

if __name__ == '__main__':
	test()
	print("Tests passed")