Module bitstream

This file exposes a single struct that can decode a huffman encoded Bitstream in a JPEG file

This code is optimized for speed. It’s meant to be super duper super fast, because everyone else depends on this being fast. It’s (annoyingly) serial hence we cant use parallel bitstreams(it’s variable length coding.)

Furthermore, on the case of refills, we have to do bytewise processing because the standard decided that we want to support markers in the middle of streams(seriously few people use RST markers).

So we pull in all optimization steps:

• use inline[always]? ✅ ,
• pre-execute most common cases ✅,
• add random comments ✅
• fast paths ✅.

Speed-wise: It is probably the fastest JPEG BitStream decoder to ever sail the seven seas because of a couple of optimization tricks.

1. Fast refills from libjpeg-turbo
2. As few as possible branches in decoder fast paths.
3. Accelerated AC table decoding borrowed from stb_image.h written by Fabian Gissen (@ rygorous), improved by me to handle more cases.
4. Safe and extensible routines(e.g. cool ways to eliminate bounds check)
5. No unsafe here

Readability comes as a second priority(I tried with variable names this time, and we are wayy better than libjpeg).

Anyway if you are reading this it means your cool and I hope you get whatever part of the code you are looking for (or learn something cool)

Knock yourself out.

Macros

decode_huff 🔒

Structs

BitStream 🔒

A BitStream struct, a bit by bit reader with super powers

Functions

has_byte 🔒

has_zero 🔒

huff_extend 🔒

Do the equivalent of JPEG HUFF_EXTEND