Eliminates exceptions when characters are outside ISO-8859-1 range

[Shane32]

@gfoidl Although it only takes a few nanoseconds (billionths of a second) to run, it is 30% slower than the original code assuming all characters can be encoded. On the other hand, it is a thousand times faster with zero allocations for a string that requires UTF-8.

Method	Mean	Error	StdDev	Gen0	Allocated
OriginalAsciiValid	6.222 ns	0.4550 ns	1.3415 ns	-	-
OriginalInvalidLastChar	9,625.746 ns	190.4809 ns	247.6790 ns	0.1373	888 B
OptimizedAsciiValid	8.275 ns	0.1907 ns	0.2119 ns	-	-
OptimizedInvalidLastChar	11.375 ns	0.2461 ns	0.2302 ns	-	-

I could not find a more optimized approach.

[coderabbitai]

📝 Walkthrough

Walkthrough

Replaced exception-based ISO-8859-1 validation with a direct per-character range check in QRCoder/QRCodeGenerator.cs, removing a static Encoding fallback and associated GetByteCount call.

Changes

Cohort / File(s)	Change summary
ISO-8859-1 validation refactor QRCoder/QRCodeGenerator.cs	Removed encoder-fallback based validation; implemented IsValidISO to iterate characters and return false if any > 0xFF; eliminated static readonly fallback Encoding and exception-driven flow.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Suggested labels

performance

Suggested reviewers

• gfoidl

Pre-merge checks and finishing touches

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title Check	✅ Passed	The title succinctly summarizes the removal of exception-throwing behavior for characters outside the ISO-8859-1 range by highlighting the core change to direct range checks. It aligns directly with the PR’s objective of replacing exception-driven validation with a more efficient implementation. The phrasing is clear and specific, avoiding unnecessary technical detail while conveying the primary functionality. It is concise and understandable to team members scanning the change history.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch iso_no_exception

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[gfoidl]

Avoiding the potential allocation is better than trading the few nano seconds here.

Speeding this up could be done by vectorized code, but it gets ugly and long -- especially it can't be implemented for all TFMs.

.NET (in recent versions) provides Ascii.IsValid (and Utf8.IsValid), which use a vectorized approach, so we could check for valid ascii (the majority of inputs will be ascii) first, and when not fallback to the manual loop. In the worst case that's $O(2n)$.
But for what? Just a few ns in trade for complicated code...

I'm fine with the solution you have here.