Pack 12-bit buffer (very inefficient right now)

pack-12bits.py

import zlib
from io import BytesIO

# pip install bitstruct
import bitstruct
import numpy as np

arr = np.random.randint(0, 4096, size=(256, 256), dtype=np.uint16)
buf = BytesIO()
np.save(buf, arr)
buf.tell()
buf.seek(0)
compressed_buf = zlib.compress(buf.getvalue())
len(compressed_buf)
# 112635 compressed
# 131200 uncompressed
# 14% savings

flat = arr.flatten()

exp_byte_length = int(12 * 256 * 256 / 8)
data = bytearray(exp_byte_length)

# This is a horribly slow way to pack the data, but I didn't want to spend the
# time yet thinking through an efficient, vectorized approach.
offset = 0
for val in flat:
    bitstruct.pack_into('u12', data, offset, val)
    offset += 12

len(zlib.compress(data))
# 98340

# I think there's no improvement with compression because the original data was
# completely random? (Created with np.random.randint)