heatshrink

A data compression/decompression library for embedded/real-time systems.

Key Features:

• 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.

Getting Started:

There is a standalone command-line program, heatshrink, but the
encoder and decoder can also be used as libraries, independent of each
other. To do so, copy heatshrink_common.h, heatshrink_config.h, and
either heatshrink_encoder.c or heatshrink_decoder.c (and their
respective header) into your project. For projects that use both,
static libraries are built that use static and dynamic allocation.

Dynamic allocation is used by default, but in an embedded context, you
probably want to statically allocate the encoder/decoder. Set
HEATSHRINK_DYNAMIC_ALLOC to 0 in heatshrink_config.h.

Basic Usage

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.

Configuration

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.

Recommended Defaults

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.

More Information and Benchmarks:

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.

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