The sieve bit vector deals with numbers in chunks of 30. Therefore,
the number 90 (say) is the 91st 'element' of the vector. Each byte
deals with some range {0,1,...,29}+30n so to have the number 90, you
need four bytes.
Rather pleasingly, I bumped into this bug and it reduced to the
incantation:
2010 2010 sieve marked-prime?
In any given 30 successive integers greater than 5, there are at most
8 prime numbers. Use this to tightly pack the result of the Eratostene
sieve. This lets us store more prime numbers than before in less space.
John Benediktsson
Jon Harper
Rupert Swarbrick
Slava Pestov
Doug Coleman
Samuel Tardieu