Zephyr integration as external module of a data compression/decompression library for embedded/real-time systems. Heatshrink has been forked from this original project.

Online documentation is available here.

• Low memory usage (as low as 50 bytes) It is useful for some cases with less than 50 bytes, and useful for many general cases with < 300 bytes.
• Incremental, bounded CPU use You can chew on input data in arbitrarily tiny bites. This is a useful property in hard real-time environments.
• Can use either static or dynamic memory allocation The library doesn't impose any constraints on memory management.
• ISC license You can use it freely, even for commercial purposes.

Add this project as external module by appending your west.yml project file as:

manifest:
  remotes:
    - ...
    - name: km
      url-base: https://github.com/kickmaker 

  projects:
    - ...
    - name: heatshrink
      remote: km
      revision: zephyr
      path: modules/lib/heatshrink

Then, update your Zephyr workspace:

west update

For sustainability, it would be prefered to target a specific version:

• Zephyr 3.5 release : 90027f015c1446a1925c3d9a7fabbc8908c26f27

  projects:
    - ...
    - name: heatshrink
      remote: km
      revision: 90027f015c1446a1925c3d9a7fabbc8908c26f27
      path: modules/lib/heatshrink

Samples exists in the samples folder.

Go to the sample folder to build and run:

west build -b <your_board> -p

This module (as original one) allows you to choose memory allocation strategy to fit footprints requirements. Heatshrink provides some settings to optimize / fine tune compression ratio VS footprint usage. These parameters window_sz2, lookahead_sz2 and input_buffer_size are explained in Configuration chapter.

In this mode (default mode, or CONFIG_HEATSHRINK_DYNAMIC_ALLOC=n), the module will reserve statically allocated memory space to process encoding and/or decoding, mainly based on parameters fixed at compilation time.

• window_sz2 : CONFIG_HEATSHRINK_STATIC_WINDOW_BITS
• lookahead_sz2 : CONFIG_HEATSHRINK_STATIC_LOOKAHEAD_BITS
• input_buffer_size : CONFIG_HEATSHRINK_STATIC_INPUT_BUFFER_SIZE

Please not that a file must be decoded with the same parameters as used for encoding. So, in this mode saving memory footprint, it comes to YOU to fix these parameters.

The most flexible solution from a software point of view (CONFIG_HEATSHRINK_DYNAMIC_ALLOC=y).

Heatshrink parameters could be changed at runtime but requires a heap.

By default, decoder and encoder are enabled. To save memory footprint you can disable them independantly:

• CONFIG_HEATSHRINK_DECODER=n
• CONFIG_HEATSHRINK_ENCODER=n

1. Allocate a heatshrink_encoder or heatshrink_decoder state machine using their alloc function, or statically allocate one and call their reset function to initialize them. (See below for configuration options.)

2. Use sink to sink an input buffer into the state machine. The input_size pointer argument will be set to indicate how many bytes of the input buffer were actually consumed. (If 0 bytes were conusmed, the buffer is full.)

3. Use poll to move output from the state machine into an output buffer. The output_size pointer argument will be set to indicate how many bytes were output, and the function return value will indicate whether further output is available. (The state machine may not output any data until it has received enough input.)

Repeat steps 2 and 3 to stream data through the state machine. Since it's doing data compression, the input and output sizes can vary significantly. Looping will be necessary to buffer the input and output as the data is processed.

4. When the end of the input stream is reached, call finish to notify the state machine that no more input is available. The return value from finish will indicate whether any output remains. if so, call poll to get more.

Continue calling finish and polling to flush remaining output until finish indicates that the output has been exhausted.

Sinking more data after finish has been called will not work without calling reset on the state machine.

heatshrink has a couple configuration options, which impact its resource usage and how effectively it can compress data. These are set when dynamically allocating an encoder or decoder, or in heatshrink_config.h if they are statically allocated.

• window_sz2, -w in the CLI: Set the window size to 2^W bytes.

The window size determines how far back in the input can be searched for repeated patterns. A window_sz2 of 8 will only use 256 bytes (2^8), while a window_sz2 of 10 will use 1024 bytes (2^10). The latter uses more memory, but may also compress more effectively by detecting more repetition.

The window_sz2 setting currently must be between 4 and 15.

• lookahead_sz2, -l in the CLI: Set the lookahead size to 2^L bytes.

The lookahead size determines the max length for repeated patterns that are found. If the lookahead_sz2 is 4, a 50-byte run of 'a' characters will be represented as several repeated 16-byte patterns (2^4 is 16), whereas a larger lookahead_sz2 may be able to represent it all at once. The number of bits used for the lookahead size is fixed, so an overly large lookahead size can reduce compression by adding unused size bits to small patterns.

The lookahead_sz2 setting currently must be between 3 and the window_sz2 - 1.

• input_buffer_size - How large an input buffer to use for the decoder. This impacts how much work the decoder can do in a single step, and a larger buffer will use more memory. An extremely small buffer (say, 1 byte) will add overhead due to lots of suspend/resume function calls, but should not change how well data compresses.

For embedded/low memory contexts, a window_sz2 in the 8 to 10 range is probably a good default, depending on how tight memory is. Smaller or larger window sizes may make better trade-offs in specific circumstances, but should be checked with representative data.

The lookahead_sz2 should probably start near the window_sz2/2, e.g. -w 8 -l 4 or -w 10 -l 5. The command-line program can be used to measure how well test data works with different settings.

heatshrink is based on LZSS, since it's particularly suitable for compression in small amounts of memory. It can use an optional, small index to make compression significantly faster, but otherwise can run in under 100 bytes of memory. The index currently adds 2^(window size 1) bytes to memory usage for compression, and temporarily allocates 512 bytes on the stack during index construction (if the index is enabled).

For more information, see the blog post for an overview, and the heatshrink_encoder.h / heatshrink_decoder.h header files for API documentation.

Contributions are welcomed : feel free to open PR. zephyr branch is the default (and targeted one) according to Zephyr External modules guidelines.

The following developers have already contributed to that module:

• @ClovisCorde
• @tyalie