1a_checksum.py

import unittest

def checksum_helper(plate):
	"""
	This helper function converts a license plate comprising an alphabetical 
	prefix (which can have one to three letters) and a numeral suffix num (which 
	can have one to four digits) into a tuple of six numbers.
	"""
	def split_plate(plate):
		i = 0
		for i in range(len(plate)):
			if plate[i].isdigit():
				break
		return plate[:i], plate[i:]
	
	def numerise(c):
		return ord(c) - ord('A') + 1
	def digits(num):
		ds = list(map(int, num))
		return [0] * (4 - len(ds)) + ds

	cs, num = split_plate(plate)
	c1 = numerise(cs[-2]) if len(cs) > 1 else 0
	c2 = numerise(cs[-1])
	d1, d2, d3, d4 = digits(num)
	return (c1, c2, d1, d2, d3, d4)

def checksum(plate):
	"""
	Courtesy of Ryan Ch’ng, 16S47 - TA will be Wei En.

	A license plate comprises an alphabetical prefix (which can have one to three 
	letters) and a numeral suffix (which can have one to four digits). The last 
	letter of a license plate number is a checksum, which is a letter generated 
	from the rest of the license plate number. Checksums are used to detect errors 
	in data.

	The checksum letter is calculated by converting the letters into numbers, 
	i.e., where A=1 and Z=26, potentially giving six individual numbers from each 
	registration plate. A helper function checksum_helper has been provided to 
	convert the license plate number into a tuple of numbers. Through this 
	conversion, SBS3229, for example, would give the tuple (2, 19, 3, 2, 2, 9).

	Each individual number is then multiplied by 6 fixed numbers (9, 4, 5, 4, 3, 
	2). These are added up, then divided by 19. The remainder corresponds to one 
	of the 19 letters used (A, Z, Y, X, U, T, S, R, P, M, L, K, J, H, G, E, D, C, 
	B), with 'A' corresponding to a remainder of 0, 'Z' corresponding to 1, 'Y' 
	corresponding to 2 and so on. In the case of SBS3229, the final letter should 
	be a P. (Note that letters F, I, N, O, Q, V and W are not used as checksum 
	letters.)

	Complete the function checksum which takes in a license plate number and 
	outputs a checksum letter.
	"""
	# ts is the 6-digit tuple obtained after conversion from the license plate
	ts = checksum_helper(plate)

	# TODO: Fill up your code here, and set checksum to the final answer.
	checksum = None
	
	# This returns the final checksum letter.
	return checksum

class TestChecksum(unittest.TestCase):
	def test_checksum(self):
		f = checksum
		self.assertEqual(f('SBS3229'), 'P')
		self.assertEqual(f('TIB838'), 'H')
		self.assertEqual(f('SG5999'), 'Z')
		self.assertEqual(f('PA2'), 'A')
		self.assertEqual(f('PC4071'), 'M')
		self.assertEqual(f('S32'), 'H')
		self.assertEqual(f('CB4650'), 'B')
		self.assertEqual(f('SG35'), 'M')

if __name__ == '__main__':
	unittest.main()