diff --git a/maths/odd_sieve.py b/maths/odd_sieve.py new file mode 100644 index 000000000..60e92921a --- /dev/null +++ b/maths/odd_sieve.py @@ -0,0 +1,42 @@ +from itertools import compress, repeat +from math import ceil, sqrt + + +def odd_sieve(num: int) -> list[int]: + """ + Returns the prime numbers < `num`. The prime numbers are calculated using an + odd sieve implementation of the Sieve of Eratosthenes algorithm + (see for reference https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes). + + >>> odd_sieve(2) + [] + >>> odd_sieve(3) + [2] + >>> odd_sieve(10) + [2, 3, 5, 7] + >>> odd_sieve(20) + [2, 3, 5, 7, 11, 13, 17, 19] + """ + + if num <= 2: + return [] + if num == 3: + return [2] + + # Odd sieve for numbers in range [3, num - 1] + sieve = bytearray(b"\x01") * ((num >> 1) - 1) + + for i in range(3, int(sqrt(num)) + 1, 2): + if sieve[(i >> 1) - 1]: + i_squared = i**2 + sieve[(i_squared >> 1) - 1 :: i] = repeat( + 0, ceil((num - i_squared) / (i << 1)) + ) + + return [2] + list(compress(range(3, num, 2), sieve)) + + +if __name__ == "__main__": + import doctest + + doctest.testmod()