On Wed, Jul 21, 2021 at 2:40 PM Neal Becker <ndbecker2@gmail.com> wrote:

In my application I need to pack bits of a specified group size into
integral values.
Currently np.packbits only packs into full bytes.
For example, I might have a string of bits encoded as a np.uint8
vector with each uint8 item specifying a single bit 1/0. I want to
encode them 4 bits at a time into a np.uint32 vector.

python code to implement this:

---------------
def pack_bits (inp, bits_per_word, dir=1, dtype=np.int32):
assert bits_per_word <= np.dtype(dtype).itemsize * 8
assert len(inp) % bits_per_word == 0
out = np.empty (len (inp)//bits_per_word, dtype=dtype)
i = 0
o = 0
while i < len(inp):
ret = 0
for b in range (bits_per_word):
if dir > 0:
ret |= inp[i] << b
else:
ret |= inp[i] << (bits_per_word - b - 1)
i += 1
out[o] = ret
o += 1
return out
---------------

Can't you just `packbits` into a uint8 array and then convert that to uint32? If I change `dtype` in your code from `np.int32` to `np.uint32` (as you mentioned in your email) I can do this:

    rng = np.random.default_rng()
    arr = (rng.uniform(size=32) < 0.5).astype(np.uint8)
    group_size = 4
    original = pack_bits(arr, group_size, dtype=np.uint32)
    new = np.packbits(arr.reshape(-1, group_size), axis=-1, bitorder='little').ravel().astype(np.uint32)
    print(np.array_equal(new, original))
    # True

There could be edge cases where the result dtype is too small, but I haven't thought about that part of the problem. I assume this would work as long as `group_size <= 8`.

András

It looks like unpackbits has a "count" parameter but packbits does not.
Also would be good to be able to specify an output dtype.
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