Question

As an exercise I am working on a parallel implementation of the Sieve of Eratosthenes. As part of that I am implementing a sequence of bitmaps, using one bit per number to save memory. Reading bits one at a time appear to work fine, but setting them is slow, especially when I use large binaries.

getBit(Bin, N, Size)->
    R=Size-N-1,
    <<_:N,Bit:1,_:R>> = Bin,
    Bit.

setBit(Bin, N, Size)->
    R=Size-N-1,
    <<A:N,_:1,B:R>> = Bin,
    <<A:N,1:1,B:R>>.

Is there a way to do this well in functional Erlang, perhaps similar to how Arrays work?
I have read about hipe_bifs:bytearray_update but would prefer to keep my coding style functional.

Answer 1

You can store your bitmaps as integers (if they are not longer than ~1000 bits):

-module(z).
-export([get_bit/2, set_bit/2]).

get_bit(N, B) -> N band bit_to_int(B) > 0.
set_bit(N, B) -> N bor bit_to_int(B).

bit_to_int(B) -> 1 bsl B.

---

You might try your version w/o passing the Size, and padding to bytes:

getBit(Bin, N)->
    <<_:N/bitstring,Bit:1,_/bitstring>> = Bin,
    Bit.

setBit(Bin, N)->
    <<A:N,_:1,B/bitstring>> = Bin,
    <<A:N,1:1,B>>.

%OR:
setBit(Bin, N)->
    PSize = 8 - ((N + 1) rem 8),
    <<A:N,_:1, Pad:PSize,B/bytes>> = Bin,
    <<A:N,1:1,Pad:PSize,B>>.

Can be better or worse 🙂