feat: cap compressed group size at 64 KB

[jpirnay]

Add GROUP_MAX_UNCOMPRESSED_BYTES=65536 on top of the existing script- based grouping. When adding the next glyph would push a group past the cap, the group is closed and a new one started with the same script ID.

Without the cap, the size of a script group is bounded only by however many glyphs the font supplies in that block. Dense scripts (CJK, Vietnamese precomposed, user-supplied fonts with large Unicode blocks) can produce a single group whose uncompressed size exceeds what fits in the embedded decompressor's transient malloc on the ESP32-C3, which manifests as a runtime allocation failure instead of a build error.

The 64 KB ceiling is large enough to hold any single built-in script group with headroom and small enough to be a comfortable transient allocation on-device.

The check uses byte-aligned size (4-pixel-aligned row stride × height), which is what the decompressor actually consumes — not the packed on-disk length. A defensive guard rejects any single glyph whose own size exceeds the cap with a clear error pointing at the offending codepoint.

Summary

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  specific areas to focus on).

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📒 Files selected for processing (1)

• lib/EpdFont/scripts/fontconvert.py

🚧 Files skipped from review as they are similar to previous changes (1)

• lib/EpdFont/scripts/fontconvert.py

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📝 Walkthrough

Walkthrough

This PR constrains DEFLATE compression group formation in the font converter by introducing a hard 65536-byte cap on uncompressed bitmap data per group. The script now tracks byte-aligned buffer sizes, closes groups when the threshold is exceeded, and raises an error if any single glyph exceeds the cap.

Changes

DEFLATE Group Size Constraining

Layer / File(s)	Summary
Byte Cap Documentation lib/EpdFont/scripts/fontconvert.py	Documents that groups are split when a fixed maximum uncompressed byte cap would be exceeded to bound embedded decompressor transient allocation.
Size Cap Constant lib/EpdFont/scripts/fontconvert.py	Introduces GROUP_MAX_UNCOMPRESSED_BYTES = 65536 as the enforced per-group limit.
Group Formation Logic lib/EpdFont/scripts/fontconvert.py	Group construction now computes per-glyph byte-aligned sizes, validates no single glyph exceeds the cap, and closes/starts groups when appending a glyph would exceed GROUP_MAX_UNCOMPRESSED_BYTES while still using get_script_group for script boundaries.

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🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely summarizes the main change: adding a 64 KB cap to compressed group sizes.
Description check	✅ Passed	The description is substantially related to the changeset, providing clear motivation, implementation details, and the rationale for the 64 KB cap.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@lib/EpdFont/scripts/fontconvert.py`:
- Around line 845-850: The ValueError raised when glyph_aligned_size >
GROUP_MAX_UNCOMPRESSED_BYTES is not actionable; change the message in the
ValueError raised in fontconvert.py (the block referencing glyph_aligned_size,
props.code_point and GROUP_MAX_UNCOMPRESSED_BYTES) to suggest concrete remedies
(e.g., increase GROUP_MAX_UNCOMPRESSED_BYTES, reduce the font/glyph pixel size
or complexity, or switch to a different encoding/compression) and remove "split
the glyph" so users see viable next steps and can act on the error.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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Configuration used: Organization UI

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Run ID: 61824674-f2b3-4ae8-994e-7de8c2aca774

📥 Commits

Reviewing files that changed from the base of the PR and between 91de6ac and d1ccfc1.

📒 Files selected for processing (1)

• lib/EpdFont/scripts/fontconvert.py

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⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (3)

• GitHub Check: build
• GitHub Check: clang-format
• GitHub Check: cppcheck

🧰 Additional context used

🧠 Learnings (1)

📚 Learning: 2026-02-23T19:55:03.054Z

Learnt from: martinbrook
Repo: crosspoint-reader/crosspoint-reader PR: 1136
File: scripts/patch_jpegdec.py:20-23
Timestamp: 2026-02-23T19:55:03.054Z
Learning: In the crosspoint-reader project, enable Python linters (e.g., Ruff or Flake8) in CI for all Python scripts. Run linting as part of the CI workflow and fix reported issues to maintain code quality. Do not skip linting for any Python files.

Applied to files:

• lib/EpdFont/scripts/fontconvert.py

🔇 Additional comments (3)

lib/EpdFont/scripts/fontconvert.py (3)

795-803: LGTM! Clear documentation of the size cap behavior.

The comment accurately explains the layered cap on top of script-based grouping and provides good rationale for the constraint. This helps future maintainers understand why groups might split within a single script block.

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821-824: LGTM! Well-chosen constant with clear rationale.

The 64 KB cap is appropriately documented and follows Python naming conventions. The value balances accommodation of dense script groups against embedded device memory constraints.

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836-862: LGTM! Correct implementation of size-capped group formation.

The logic correctly:

• Tracks byte-aligned uncompressed size matching the to_byte_aligned() buffer format (lines 843-844 match line 769)
• Closes groups when adding the next glyph would exceed the cap (line 851)
• Handles edge cases: empty glyphs contribute 0 size, first glyph starts a new group, last group is appended after the loop
• Preserves existing script-based grouping behavior when the cap is not hit

The OR condition on line 853 ensures groups split on either script boundaries or size overflow, maintaining both constraints.

[mcrosson]

@znelson Can you review this PR