Finding the location of ones in a bit mask - Julia

Question

I have a series of values that are each being stored as UInt16. Each of these numbers represents a bitmask - these numbers are commands that have been sent to a microprocessor telling it which pins to set high or low. I would like to parse this arrow of commands to find out which pins were being set high each time in such a way that is easier to analyse later.

Consider the example value 0x3c00, which in decimal is 15360 and in binary is 0011110000000000. Currently I have the following function

function read_message(hex_rep)
    return findall.(x -> x .== '1',bitstring(hex_rep))
end

Which gets called on every element of the array of UInt16. Is there a better/more efficient way of doing this?

Answer 1

The best approach probably depends on how you want to handle vectors of hex-values. But here's an approach for processing a single hex which is much faster than the one in the OP:

 function readmsg(x::UInt16)
    N = count_ones(x)
    inds = Vector{Int}(undef, N)
    if N == 0
        return inds
    end
    k = trailing_zeros(x)
    x >>= k + 1
    i = N - 1
    inds[N] = n = 16 - k
    while i >= 1
        (x, r) = divrem(x, 0x2)
        n -= 1
        if r == 1
            inds[i] = n
            i -= 1
        end
    end
    return inds
end