heatshrink Integration

Overview

Use this skill to integrate heatshrink (github.com/atomicobject/heatshrink) by fixing memory budget, window_sz2/lookahead_sz2 sizes, the sink/poll/finish streaming loop, and decompression safety. Compression bugs usually come from mismatched encoder/decoder settings, an unfinished poll loop, or output buffers that are too small.

When To Use

Use this skill when:

• The user wants heatshrink compression/decompression on an MCU.
• The task involves compressed logs, resources, firmware chunks, OTA payloads, or serial transfer compression.
• Decompression fails, output is truncated, memory is too high, or compressed data is incompatible.
• The project uses heatshrink_encoder.h, heatshrink_decoder.h, heatshrink_config.h, or the heatshrink command-line tool.

Do not use this skill for general archive formats like gzip/zip unless heatshrink is specifically involved.

First Questions

Ask for:

• Use case: logs, UI assets, firmware chunks, telemetry, or storage.
• Encoder/decoder settings: window_sz2 (4-15) and lookahead_sz2 (3 to window_sz2 - 1). The CLI's compiled default is -w 11 -l 4; its help text recommends -w 8 -l 4 for embedded systems.
• Allocation model: HEATSHRINK_DYNAMIC_ALLOC (heap via HEATSHRINK_MALLOC/HEATSHRINK_FREE) or static (HEATSHRINK_STATIC_WINDOW_BITS, HEATSHRINK_STATIC_LOOKAHEAD_BITS, HEATSHRINK_STATIC_INPUT_BUFFER_SIZE).
• Streaming or one-shot mode, and input/output chunk sizes plus decompressed size bound.
• Whether compressed data comes from host tools or target firmware.
• Current return code or mismatch symptom.

Integration Checklist

1. Match settings across the boundary.

Decoder expansion_buffer_sz2 and lookahead_sz2 must match the encoder's window_sz2/lookahead_sz2. Under static alloc, both sides must share the same HEATSHRINK_STATIC_WINDOW_BITS/HEATSHRINK_STATIC_LOOKAHEAD_BITS.

1. Choose the allocation model.

Dynamic: heatshrink_encoder_alloc(window_sz2, lookahead_sz2) / heatshrink_decoder_alloc(input_buffer_size, expansion_buffer_sz2, lookahead_sz2), freed with *_free. Static: set HEATSHRINK_DYNAMIC_ALLOC 0 in heatshrink_config.h and use a stack/BSS instance, *_reset before reuse.

1. Drive the streaming loop correctly.

Feed with heatshrink_encoder_sink/heatshrink_decoder_sink (check *input_size for bytes consumed; re-sink the remainder). Then loop *_poll until it returns HSER_POLL_EMPTY/HSDR_POLL_EMPTY before sinking more.

1. Flush at end of stream.

After the last input, call heatshrink_encoder_finish/heatshrink_decoder_finish; while it returns HSER_FINISH_MORE/HSDR_FINISH_MORE, keep calling *_poll to drain output. Done is HSER_FINISH_DONE/HSDR_FINISH_DONE. Call *_reset before reusing the instance for a new stream.

1. Validate data integrity.

Pair compression with CRC/hash when corruption matters. heatshrink carries no checksum or length header and does not reliably detect corrupt or truncated input, so integrity must be checked externally.

1. Test worst case.

Some data expands; do not assume size savings. Size the output buffer/store for expansion, not just the compressed happy path.

Common Failures

• Host encoder uses different window_sz2/lookahead_sz2 than the MCU decoder (or mismatched HEATSHRINK_STATIC_*).
• Caller stops after one *_poll and treats partial output as complete, instead of looping until HSER_POLL_EMPTY/HSDR_POLL_EMPTY.
• *_finish never called (or its HSER_FINISH_MORE/HSDR_FINISH_MORE output not drained), so the tail of the stream is lost.
• Sinking more data on the encoder after heatshrink_encoder_finish without heatshrink_encoder_reset (returns HSER_SINK_ERROR_MISUSE).
• Ignoring *input_size from *_sink and dropping the unconsumed bytes.
• Truncated/corrupt compressed input produces silent garbage rather than an error, because heatshrink has no integrity check; *_SINK_ERROR_NULL/*_POLL_ERROR_NULL only guard null handles or buffers, and HSDR_POLL_ERROR_UNKNOWN is an internal-state guard, not a corruption detector.
• Compressed firmware lacks CRC before flash write; worst-case expansion exceeds allocated storage.

Verification

Before claiming heatshrink works:

• State window_sz2/lookahead_sz2, allocation model (HEATSHRINK_DYNAMIC_ALLOC), decoder input_buffer_size, and buffer sizes.
• Confirm the poll loop drains to *_POLL_EMPTY and the finish loop drains *_FINISH_MORE to *_FINISH_DONE.
• Confirm a round-trip test for representative and worst-case (incompressible) data.
• Confirm that corrupt/truncated input is caught by an external integrity check, since heatshrink itself may emit silent garbage without an error code.
• Confirm the integrity check if used for firmware or persistent assets.

Example

User:

用 heatshrink 压缩资源，设备端解压出来缺尾巴。

Agent:

1. Asks for window_sz2/lookahead_sz2, decoder input_buffer_size/expansion_buffer_sz2, chunk sizes, and whether heatshrink_decoder_finish is called.
2. Checks that *_poll loops until HSDR_POLL_EMPTY and that finish is drained until HSDR_FINISH_DONE (the missing tail is almost always an undrained finish loop).
3. Verifies round-trip and truncated-input behavior, and that heatshrink_decoder_reset runs before reusing the instance.