Index Bitwidth Pack

The IndexBitwidthPack transform compresses arrays of indices that do not require a full 8 bits into a tightly packed bitstream.

Theory

When representing cluster assignments or codebook indices (for example, downstream of a K-means quantization mechanism), the indices often belong to a small domain (e.g., 0..15 requiring only 4 bits). Storing them as full u8 bytes wastes significant space.

This transform takes the bits parameter and packs the stream of indices MSB-first into a dense byte array, successfully recovering the bit budget.

Inverse

The inverse operation unpacks the dense bitstream back into standard u8 elements by extracting the appropriate bits intervals, yielding the original indices.