Refactor DataSegment from struct to class and add chained segment capability

[Shane32]

Next step in supporting multi mode QR codes. It requires an additional allocation, but it's slightly faster presumably due to less copying of data between methods.

Before

Method	Mean	Error	StdDev	Gen0	Allocated
CreateQRCode	47.33 us	0.903 us	1.508 us	0.1831	4.07 KB
CreateQRCodeLong	1,200.37 us	10.287 us	9.623 us	-	11.05 KB
CreateQRCodeLongest	6,762.80 us	38.097 us	35.636 us	-	46.41 KB

After

Method	Mean	Error	StdDev	Gen0	Allocated
CreateQRCode	46.69 us	0.928 us	1.140 us	0.1831	4.12 KB
CreateQRCodeLong	1,184.45 us	19.989 us	18.698 us	-	11.09 KB
CreateQRCodeLongest	6,716.87 us	21.365 us	18.940 us	-	46.45 KB

Summary by CodeRabbit

• New Features
  • Added support for chaining multiple QR data segments, enabling encoding of mixed content in a single QR code.
  • Improved handling of ECI (Extended Channel Interpretation) by preserving mode information across chained segments.
  • Ensured accurate bit-length calculation and writing across chained segments for more reliable QR generation.
  • Streamlined bit array creation to seamlessly serialize data from consecutive segments without user intervention.

[coderabbitai]

📝 Walkthrough

Walkthrough

Converts the private nested DataSegment from a readonly struct to a private class with get-only properties, adds a Next reference for chaining, and updates GetBitLength, ToBitArray, and WriteTo to traverse chained segments and accumulate length and bit writes across the chain.

Changes

Cohort / File(s)	Change Summary
Data segment refactor and chaining QRCoder/QRCodeGenerator/DataSegment.cs	Change type from readonly struct to class; introduce get-only properties (EncodingMode, CharacterCount, Data, EciMode) and HasEciMode; add mutable Next for chaining; update GetBitLength to include Next; modify ToBitArray to use WriteTo; update WriteTo to recursively process Next and return final index.

Sequence Diagram(s)

sequenceDiagram
    autonumber
    participant G as QRCodeGenerator
    participant DS as DataSegment (current)
    participant N as Next DataSegment (optional)
    participant BA as BitArray

    G->>DS: ToBitArray()
    DS->>BA: Create BitArray with total length (via GetBitLength incl. chain)
    DS->>DS: WriteTo(BA, startIndex)
    note over DS: Encode mode, length, data bits
    alt Has Next
        DS->>N: WriteTo(BA, updatedIndex)
        note over N: Encode its bits and recurse further if chained
    end
    DS-->>G: Return BitArray

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• Refactor conversion of text to data segments #670 — Introduced/modified DataSegment; current PR builds on it by converting to class and adding chaining.
• Refactor BuildBitArrayFromSegment #671 — Adjusts DataSegment serialization (ToBitArray/WriteTo); current PR extends these for chained segments.

Suggested labels

refactoring

Suggested reviewers

• gfoidl

Pre-merge checks and finishing touches

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title Check	✅ Passed	The title concisely and accurately summarizes the primary change by indicating the conversion of DataSegment from a struct to a class and the addition of chained segment capability, which aligns directly with the implemented refactoring and new chaining support in the code.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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📜 Recent review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 4cb1f83 and 4f31d52.

📒 Files selected for processing (1)

• QRCoder/QRCodeGenerator/DataSegment.cs (5 hunks)

🧰 Additional context used

🧬 Code graph analysis (1)

QRCoder/QRCodeGenerator/DataSegment.cs (1)

QRCoder/QRCodeGenerator.cs (3)

• DataSegment (126-132)
• EncodingMode (743-758)
• GetCountIndicatorLength (886-946)

⏰ 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). (5)

• GitHub Check: Test .NET 5.0 Windows
• GitHub Check: Test .NET Core 3.1
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🔇 Additional comments (3)

QRCoder/QRCodeGenerator/DataSegment.cs (3)

8-28: LGTM! Well-designed struct-to-class refactor with chaining support.

The conversion from struct to class is appropriate here because:

• Enables the self-referencing Next property for chaining segments
• Reduces copying overhead (as noted in the PR's performance benchmarks)
• The slight increase in allocation (4.12 KB vs 4.07 KB) is offset by performance gains

The Next property with a public setter allows flexible chain construction while keeping the other properties immutable through get-only accessors.

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61-72: Chaining logic in GetBitLength is correct.

The recursive accumulation properly includes the bit length of all chained segments. The calculation for modeIndicatorLength at line 63 correctly accounts for both ECI mode (when present: 4 bits ECI indicator + 8 bits ECI value + 4 bits mode = 16 total) and regular mode (4 bits).

Note: The recursive implementation assumes chains will not be deeply nested. Given QR code capacity constraints, this is a reasonable assumption, but be aware that circular references (e.g., A.Next = B, B.Next = A) would cause a stack overflow. Since DataSegment is private, such misuse should be preventable at the call sites.

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94-120: Chaining logic in WriteTo is correct.

The method properly writes the current segment's data and then recursively delegates to Next.WriteTo when a chained segment exists. The index is correctly advanced after each write operation, and the final index after all chained segments is returned.

Same note as for GetBitLength: the recursive implementation could overflow if chains contain cycles. This is acceptable given the private scope and expected usage patterns.

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