Micro-RLE

https://www.CoreLathe.com

≤264-byte stream compressor with drift-aware RLE-XOR—halves MCU UART bandwidth on repetitive logs.

Specs

• Flash: 252 B (M4) / 264 B (M0+) @ -Os -mthumb -nostdlib
• RAM: 36 B state (no heap, no initialized data)
• Compression: 40-60 % on repetitive firmware traces (status lines, hex dumps, etc.)
  – Plain English or random data: 0-10 % expansion
• Timing: 6-14 cycles/byte deterministic
• Boot: < 600 µs @ 24 MHz
• Lossless: Full 8-bit delta support via escape sequence

Measured Footprint (Cortex-M4)

Function      Size    Notes
log_byte      196 B   core compression + escape handling
log_init       30 B   state initialization
log_flush      24 B   pending run flush
emit (weak)     2 B   output hook
Total         252 B flash, 36 B RAM

Encoding Format

• Token: [run:3][delta:5]
• Escape: If delta > 0x1F, emit [run:3][0x1F] followed by full 8-bit delta
• Cost: +1 byte per large delta (rare for sensor streams)
• Benefit: Lossless for all input patterns

API

void log_init(void);       // Initialize state
void log_byte(uint8_t b);  // Encode single byte
void log_flush(void);      // Flush pending run

Integration

Replace emit() in micro_rle.c with your output (UART/DMA/ringbuf).

Build

arm-none-eabi-gcc -Os -mthumb -mcpu=cortex-m0plus -nostdlib \
  -c micro_rle.c -o micro_rle.o

Parameters

• WIN=32: Drift observation window (must be power-of-2)
• WIN_MSK=31: Bitmask for modulo (WIN - 1)
• STUFF_TH=2: Hamming threshold for frame sync
• RLE_MAX=7: Max run length (3 bits)
• STUFF_BYTE=0x7E: Frame marker (consecutive occurrences suppressed)

All constants tuned for 115200 bps serial @ 24 MHz.

Decoder Notes

• Receiver should ignore consecutive STUFF_BYTE markers
• Drift detection uses 1-cycle AND mask instead of 2-cycle MUL on M0+

License

MIT License - See LICENSE file
No warranty. Test thoroughly before production use.

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Contributing

Issues and PRs welcome for:

• Bug fixes
• Documentation improvements
• Additional test cases
• Platform-specific examples

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Support

• GitHub Issues: Bug reports and questions
• Email: sales@corelathe.com
• Website: http://www.corelathe.com