Cleanup and optimize CalculateGeneratorPolynom

[KrisVandermotten]

Summary

I set out to simplify the calculation of the generator polynomial for the error correction codes. Especially the polynomial multiplication was overly complex and inefficient. So I wrote a new clean implementation, running it in parallel with the existing one to make sure I got the same results. But then I realized that we don't need runtime calculation of these polynomials at all: they're always the same!

So I generated 30 polynomials, and embedded their coefficients in the assembly. I turned a simple version of the rewritten generation code into a unit test, thereby also documenting the generator code. I did have to make the Polynom and PolynomItem structs internal to make that possible, but I guess that's OK.

To be honest, only 18 polynomials will ever be used, but it turned out to be simpler (also for runtime performance) to include all 30, than to write code to select the correct coefficients for each of the 18 ones we need.

Other than replacing a few redundant range checks by Debug.Assert statements, I did not change the Polynom and PolynomItem data structure (yet).

The result is a significant reduction in the amount of code. More than 200 lines of complex code could be removed. On master, a .NET 6 release build is 201 KiB. On the PR branch, a .NET 6 release build is 198.5 KiB. That's a reduction of 2.5 KiB, despite the addition of the 0.5 KiB coefficient table.

Test plan

All code is covered by existing tests. During development, I ran both old an new code, comparing results with Debug.Assert checks.

In addition, a test has been added to validate the correctness of the generator polynomial coefficients.

Update: despite what coderabbit.ai claims below, no bugs were fixed.

Summary by CodeRabbit

• Bug Fixes

  • Improved QR code error-correction calculations for more reliable generation.
  • Added safeguards for invalid error-correction settings and polynomial values.

• Quality Improvements

  • Improved consistency across QR code error-correction levels.
  • Strengthened validation during QR code generation.
  • Expanded automated checks for error-correction and Galois field calculations.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 5fa96cb7-84ae-490c-b43d-9577176d9076

📥 Commits

Reviewing files that changed from the base of the PR and between 768c9c0 and b8968a1.

📒 Files selected for processing (2)

• QRCoder/QRCodeGenerator/ECCInfo.cs
• QRCoder/QRCodeGenerator/Polynom.cs

Included review availability: Your plan provides up to 8 included reviews per hour; 6 remain after this review.

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📝 Walkthrough

Walkthrough

QRCodeGenerator now uses the shared Polynom.CreateGeneratorPolynom implementation. Galois-field APIs use byte values. Polynomial and ECC invariants use debug assertions. Tests validate generator polynomials against a reference implementation.

Changes

QR generator refactor

Layer / File(s)	Summary
Field and value contracts QRCoder/QRCodeGenerator/GaloisField.cs, QRCoder/QRCodeGenerator/ECCInfo.cs, QRCoder/QRCodeGenerator/PolynomItem.cs	Galois-field conversions now use byte values. ECCInfo is readonly. Debug assertions validate ECC counts and polynomial coefficients.
Shared generator polynomial QRCoder/QRCodeGenerator/Polynom.cs	Polynom now creates generator polynomials from precomputed coefficients. Bounds and capacity checks use debug assertions.
QR generation integration QRCoder/QRCodeGenerator.cs, QRCoderTests/QRGeneratorTests.cs	QR generation uses the shared polynomial and fixed 8-bit conversion. Tests validate lookup tables and generator polynomials against a reference implementation.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to b8968

The PR replaces runtime generator-polynomial calculation with embedded coefficients and adds validation coverage; no actionable merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 78.95% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 19 functions across 6 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: cleaning up and optimizing generator polynomial calculation.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

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Inline comments:
In `@QRCoder/QRCodeGenerator/Polynom.cs`:
- Around line 58-60: Update the assertion in CreateGeneratorPolynom to validate
the complete supported numEccWords range, requiring values greater than zero and
less than 32 before calculating startIndex.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 83eef00e-a529-431a-878a-e5b7a379b39c

📥 Commits

Reviewing files that changed from the base of the PR and between 544a0be and 768c9c0.

📒 Files selected for processing (6)

• QRCoder/QRCodeGenerator.cs
• QRCoder/QRCodeGenerator/ECCInfo.cs
• QRCoder/QRCodeGenerator/GaloisField.cs
• QRCoder/QRCodeGenerator/Polynom.cs
• QRCoder/QRCodeGenerator/PolynomItem.cs
• QRCoderTests/QRGeneratorTests.cs

Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review.

[Shane32]

So if I understand this correctly, CalculateGeneratorPolynom(int numEccWords) always returns one of a small set of polynominals (since numEccWords is a small set) which can be precalculated and embedded, which is what eliminates most of the code in this PR. Correct?

[KrisVandermotten]

So if I understand this correctly, CalculateGeneratorPolynom(int numEccWords) always returns one of a small set of polynominals (since numEccWords is a small set) which can be precalculated and embedded, which is what eliminates most of the code in this PR. Correct?

Correct.

On top of that, the now eliminated code was very complex and inefficient. Compare it to the equivalent that now lives in a unit test.

[Shane32]

This now has eliminated a substantial amount of calculation code. It seems that if we merge this PR, then if we want to support additional QR Code variants (see https://www.qrcode.com/en/codes/ ) then we may need to manually recalculate these values if the input changes. Would it be wise to move this into the tests, such that the test calculates and verifies the precalculated values are correct? Or just move it into a console app to generate the values or something?

[KrisVandermotten]

Would it be wise to move this into the tests, such that the test calculates and verifies the precalculated values are correct?

That's exactly what I did.

[Shane32]

In other words, it seems that it would be a good idea to retain the code that was used to generate the values that are now constants embedded into the library.

[Shane32]

Would it be wise to move this into the tests, such that the test calculates and verifies the precalculated values are correct?

That's exactly what I did.

Ok let me review further.

[Shane32]

I see; so it's just greatly simplified in the tests

[Shane32]

This is great and makes complete sense - thanks !!!!