3182da3965
Claude Code keeps per-project memory at <home>/.claude/projects/<key>/,
where <key> is the project's absolute path with every separator and the
drive colon turned into a dash. That makes it both rename-fragile and
machine-specific, and this project has hit each of those once:
- D:\DEV\Schunix -> D:\DEV\SchOS changed the key, and 43 memories were
invisible until moved across by hand.
- Development happens from Windows and from Linux, and each looks in a
different place for the same checkout.
Committing .claude/memory/ fixes both -- git carries it across renames
and between hosts -- and `import` puts it wherever the local machine
expects. .claude/settings.local.json stays ignored: those are per-machine
permissions.
The colon maps to a dash rather than being dropped. Deleting it gives
D-DEV-SchOS, which matches nothing and silently finds zero memories; that
was the first attempt. On Git Bash the key comes from `cygpath -w`,
because Claude Code is a Windows program and sees D:\DEV\SchOS, not
/d/DEV/SchOS.
71 lines
4 KiB
Markdown
71 lines
4 KiB
Markdown
---
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name: schunix-qr-encoder
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description: "kernel/core/qr.c — the crash-report QR encoder, how it's verified, and the segno padding bug that is NOT ours."
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metadata:
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node_type: memory
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type: project
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originSessionId: 9d8a2aef-bcf0-47a0-96c6-f706efab4a95
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modified: 2026-08-08T16:22:01.468Z
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---
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Built 2026-08-08. `kernel/core/qr.c` + `include/qr.h`, the Phase 8 crash channel
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(see [[schunix-crash-screen]] and `docs/roadmap.md`).
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**Wired into the crash screen the same day.** `panic.c` builds a compact payload
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and paints a white card on the right of PSOGM. Verified end-to-end with
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`make crashshot && make crashqr`, which decodes the symbol out of the captured
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framebuffer — not out of the encoder. Payload format `SXPSOGM1`: magic, reason,
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`c<cpu> p<pid> t<ticks> <name>`, `b<kernel base>`, then call-chain frames as
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**offsets from the kernel base** (8 hex digits, not 16 — that is what makes a
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useful number of frames fit in 134 bytes). The reason is written against a
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buffer end held back by 34 bytes, so a long message costs frames but never all
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of them. Deliberately not a URL: it encodes the report itself and reaches
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nothing.
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**Scope, deliberately narrow:** byte mode only, EC level L only, versions 1–6
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only (max 134 bytes). Version 7 is where QR starts needing an 18-bit
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version-information block in two extra places; version 10 is where the character
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count grows to 16 bits and the alignment grid becomes 2-D. Stopping at 6 avoids
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both. If a payload ever needs more, shorten the report rather than add the
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tables. Freestanding: no allocation, no libc beyond memset/memmove, GF(256)
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multiply by shift-and-add so there is no table init to depend on — all of which
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matters because the caller is a panic handler.
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**How it is verified — this is the important part.** A wrong QR still looks
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exactly like a QR, so "it renders" proves nothing.
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- `make qrtest` — self-checks (table arithmetic, RS generator polynomials,
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finder/timing placement) and writes `build/qr.png` plus a terminal rendering
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a phone can read straight off the screen. `-q` suppresses the ANSI art,
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`-m TEXT` dumps the module matrix, `-f TEXT` dumps the function-pattern map.
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- `make qrcheck` (`tools/qrcheck.py`) — the real oracle. Decodes with OpenCV,
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*and* compares module-for-module against segno. Needs
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`pip install --target <dir> opencv-python-headless numpy segno` and
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PYTHONPATH; not wired into `make`.
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**Two traps found doing this, both worth remembering:**
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1. **segno adds a spurious zero pad codeword to every byte-mode symbol.** Its
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`write_padding_bits()` does `extend([0] * (8 - (length % 8)))`, which emits a
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whole byte when the stream is already codeword-aligned — and in byte mode it
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always is (8L+16 bits after the terminator). ISO/IEC 18004 makes it
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conditional and segno quotes that exact sentence above the line. Harmless
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(decoders read `count` bytes and ignore padding) but it means the two
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encoders **cannot** agree bit-for-bit on any padded payload. qrcheck compares
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full matrices only for exact-fill payloads and decodes the rest. Our
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behaviour is correct; do not "fix" it to match.
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2. **OpenCV's QRCodeDetector fails on perfectly valid symbols**, depending on
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scale and quiet-zone width. qrcheck tries six renderings, and if one still
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fails it re-tests segno's symbol for the same payload — if that fails too it
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reports `[detector]`, not `[FAIL]`. Without that step the check produces
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false failures, which is worse than not running it.
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**Debugging technique that cracked it:** dump the module matrix *and* the
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function map, run the zigzag backwards in Python to recover the codewords from
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both encoders, and diff those. "First differing codeword index 18" localised it
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instantly; comparing matrices only ever said "108 of 441 modules differ". Note
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the mask must be read from the format bits and un-applied first — un-masking
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with the wrong mask makes your own output look like noise, which cost a round.
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Windows note: `subprocess` cannot launch a relative path with forward slashes;
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`os.path.abspath` it.
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