"""
@file lattice.py
@brief multiplication of two numbers
"""
def lattice(valueA, valueB):
""" the lattice calculation bases on a matrix writing
the digits of the first value as column headers and
writing the digits of the second value as row labels.
Each matrix cell has a diagonal. Multiplicating the
column digit with the row digit the result will be
splitted like this: 9 x 8 = 72 -> 7 will written above
the diagonal and 2 will be written below the diagonal.
When each cell is filled - looking at the diagonals -
you can see that - more or less - at the top right
of a cell and at the bottom left the diagonal can be
continued when there is a further cell. Starting at
the bottom right of the matrix we sum up each digit
of same diagonal.
The last step is to adjust each sum that way that the
value > 9 is transfered as "too much" to the next sum.
Each time a digit remains being part of the final
product of the multiplication. You have to start with
the last diagonal (bottom right of matrix)
"""
diagonals = [0] * (len(valueA) + len(valueB))
for indexA, digitA in enumerate(valueA):
for indexB, digitB in enumerate(valueB):
value = int(digitA) * int(digitB)
diagonals[indexA+indexB+0] += value // 10
diagonals[indexA+indexB+1] += value % 10
digits = []
rest = 0
for value in reversed(diagonals):
value += rest
if value > 9:
rest = value // 10
digits.insert(0, value % 10)
else:
rest = 0
digits.insert(0, value)
if rest > 0:
digits.insert(0, rest)
if digits[0] == 0:
del digits[0]
return digits
def test():
""" verifying lattice calculation """
stringA = "1234567890"
stringB = "1234567890"
# python calculates:
resultA = "%s" % (int(stringA) * int(stringB))
print("%s x %s = %s" % (stringA, stringB, resultA))
# string digits as input:
resultB = lattice(stringA, stringB)
assert resultB == list(map(int, resultA))
# list of integer digits as input:
resultC = lattice(list(map(int, stringA)), list(map(int, stringB)))
assert resultC == list(map(int, resultA))
if __name__ == "__main__":
test()
