feat(#161): Drop netstandard2.0 target, support net8.0+ only

[leecampbell-codeagent]

Brief: Issue #161 — Drop netstandard2.0 target, support net8.0+ only

Summary

Remove netstandard2.0 from the main library's <TargetFrameworks>, retaining only net10.0;net9.0;net8.0.
This eliminates all conditional compilation guards that existed solely to support the older target, simplifies Bitwise.cs to use BitOperations.LeadingZeroCount unconditionally, and cleans up a branching code path in HistogramLogReader.cs.
The three per-framework Release PropertyGroup conditions in the csproj are collapsed into a single unconditional PropertyGroup as part of this change.
The spec file build-system.md must be updated to reflect the new target list.

Previous NuGet releases remain available on NuGet.org for consumers on .NET Framework or older runtimes.

Category

chore

This is a maintenance and simplification task.
The hardware-intrinsic LeadingZeroCount path (#if NET5_0_OR_GREATER) is already compiled in for all net8.0+ builds; removing the fallback does not change the compiled output for any net8.0, net9.0, or net10.0 consumer.
No performance improvement is delivered to existing net8.0+ consumers — the change reduces build artefacts, removes dead code, and simplifies the codebase.

Affected Files (confirmed by exploration)

File	Change required
HdrHistogram/HdrHistogram.csproj	Remove netstandard2.0 from <TargetFrameworks> (line 4); delete the netstandard2.0 PropertyGroup condition (lines 36-39); collapse the three per-framework Release PropertyGroup conditions (lines 24-34) into a single Condition="'$(Configuration)' == 'Release'" block
HdrHistogram/Utilities/Bitwise.cs	Remove #if NET5_0_OR_GREATER guards (lines 25-29, 32-44); inline IntrinsicNumberOfLeadingZeros body directly into NumberOfLeadingZeros; delete the entire Bitwise.Imperative class (lines 55-109)
HdrHistogram/HistogramLogReader.cs	Remove #if NETSTANDARD2_0 block in IsComment() (lines 241-248); keep the char overload unconditionally
spec/tech-standards/build-system.md	Remove netstandard2.0 from the TargetFrameworks code block (line 25) and from the target table (line 33)
HdrHistogram.Benchmarking/LeadingZeroCount/LeadingZeroCountBenchmarkBase.cs	Remove the ImperativeImplementation benchmark method (references deleted Bitwise.Imperative class)

No other files in the codebase contain #if NETSTANDARD or #if NET5_0_OR_GREATER blocks.
Unit-test and benchmarking projects already target net10.0;net9.0;net8.0 only — no changes needed there.

Acceptance Criteria

• Library targets net10.0;net9.0;net8.0 only; netstandard2.0 is absent from HdrHistogram.csproj
• No #if NETSTANDARD or #if NET5_0_OR_GREATER conditional compilation remains anywhere in the library
• Bitwise.Imperative class is fully removed
• Bitwise.NumberOfLeadingZeros calls System.Numerics.BitOperations.LeadingZeroCount unconditionally
• HistogramLogReader.IsComment uses line.StartsWith('#') (char overload) unconditionally
• The three per-framework Release PropertyGroup conditions are collapsed into a single Condition="'$(Configuration)' == 'Release'" block
• LeadingZeroCountBenchmarkBase.cs no longer references Bitwise.Imperative; the benchmarking project compiles cleanly
• All unit tests pass on net8.0, net9.0, and net10.0
• spec/tech-standards/build-system.md no longer references netstandard2.0

Test Strategy

Existing tests

No unit tests directly target Bitwise or HistogramLogReader.IsComment in isolation.
The existing suite exercises both paths indirectly via histogram recording and log-reading tests.
Run the full unit-test suite across all three target frameworks after the changes:

dotnet test -c Release

All tests must pass; no new test failures are acceptable.

New tests (optional but recommended)

Consider adding a focused unit test in HdrHistogram.UnitTests/ that asserts Bitwise.NumberOfLeadingZeros returns correct values for a range of inputs (including 0, 1, powers-of-two, and long.MaxValue).
This is low-risk but provides a regression anchor if Bitwise.cs is touched again.

There is no need to add tests for IsComment — it is a private static helper with trivial logic.

Risks and Open Questions

1. Benchmark class references Bitwise.Imperative — deleting the class will break the ImperativeImplementation benchmark at compile time.
   The benchmark must be updated in the same PR (listed in Affected Files above).

2. NuGet package surface — removing a target framework is a breaking change for consumers on netstandard2.0.
   This is explicitly accepted: "Previous NuGet releases will remain available."
   The PR description should note this prominently and suggest a major-version bump or release notes entry.

3. No other conditional compilation found — the grep over the codebase confirmed only two files contain the relevant #if blocks.
   Low risk of missing anything.

Task breakdown

Task List: Issue #161 — Drop netstandard2.0 target, support net8.0+ only

Implementation Changes

• HdrHistogram/HdrHistogram.csproj line 4 — Remove netstandard2.0 from <TargetFrameworks>, leaving net10.0;net9.0;net8.0.

  • Why: Drops the legacy target entirely.
  • Verify: <TargetFrameworks> value contains no netstandard2.0 token.

• HdrHistogram/HdrHistogram.csproj lines 24–34 — Collapse the three per-framework Release PropertyGroup blocks (one each for net8.0, net9.0, net10.0) into a single PropertyGroup Condition="'$(Configuration)' == 'Release'" block with a single <DocumentationFile> element that resolves via $(TargetFramework).

  • Why: Eliminates duplicated XML and the framework-specific condition strings.
  • Verify: Only one Release PropertyGroup exists; it contains no $(TargetFramework) literals in the condition string.

• HdrHistogram/HdrHistogram.csproj lines 36–39 — Delete the netstandard2.0 PropertyGroup block (the one setting DefineConstants to RELEASE;NETSTANDARD2_0).

  • Why: The constant NETSTANDARD2_0 is no longer needed once the target is removed.
  • Verify: No PropertyGroup referencing netstandard2.0 or NETSTANDARD2_0 remains in the file.

• HdrHistogram/Utilities/Bitwise.cs lines 23–44 — Simplify NumberOfLeadingZeros(long) to call System.Numerics.BitOperations.LeadingZeroCount((ulong)value) directly; remove the #if NET5_0_OR_GREATER / #else / #endif guards and delete the private IntrinsicNumberOfLeadingZeros helper.

  • Why: All supported targets (net8.0+) provide BitOperations.LeadingZeroCount; the conditional dispatch is dead code.
  • Verify: NumberOfLeadingZeros body is a single return System.Numerics.BitOperations.LeadingZeroCount((ulong)value); statement; no #if directives remain in the method or immediately around it.

• HdrHistogram/Utilities/Bitwise.cs lines 55–109 — Delete the entire Bitwise.Imperative nested public static class (including the Lookup table, NumberOfLeadingZeros, NumberOfLeadingZerosLong, and Log2 methods).

  • Why: The imperative fallback path is unreachable on net8.0+; removing it eliminates dead code and the public surface that the benchmark references.
  • Verify: No class Imperative or Bitwise.Imperative identifier exists anywhere in the solution.

• HdrHistogram/HistogramLogReader.cs lines 241–248 — Remove the #if NETSTANDARD2_0 / #else / #endif block inside IsComment(string line), keeping only return line.StartsWith('#');.

  • Why: The char overload of StartsWith is available on all net8.0+ targets; the string-overload fallback is dead code.
  • Verify: IsComment contains no #if directives; the method body is return line.StartsWith('#');.

• HdrHistogram.Benchmarking/LeadingZeroCount/LeadingZeroCountBenchmarkBase.cs — Remove the "Imperative" entry from the validation dictionary (line 56) and delete the ImperativeImplementation() benchmark method (lines 124–133).

  • Why: Both reference Bitwise.Imperative which will no longer exist; leaving them causes a compile error.
  • Verify: dotnet build HdrHistogram.Benchmarking/ -c Release exits with code 0; no reference to Bitwise.Imperative remains in the file.

Unit Tests

• HdrHistogram.UnitTests/ — Add a focused unit test class BitwiseTests (e.g. HdrHistogram.UnitTests/Utilities/BitwiseTests.cs) that asserts Bitwise.NumberOfLeadingZeros returns correct results for representative inputs: 0, 1, 2, powers of two up to 2⁶², and long.MaxValue.

  • Why: No existing test directly covers Bitwise; this provides a regression anchor if the method is ever touched again.
  • Verify: Test class exists; dotnet test -c Release reports the new tests as passing on all three target frameworks.

• Run the full unit-test suite — Execute dotnet test -c Release across all three target frameworks (net8.0, net9.0, net10.0).

  • Why: Confirms that removing the netstandard2.0 conditional paths has not broken any indirect consumer of Bitwise or HistogramLogReader.
  • Verify: Zero test failures; zero skipped tests that were previously passing.

Documentation

• spec/tech-standards/build-system.md line 25 — Remove netstandard2.0 from the <TargetFrameworks> code block example.

  • Why: The spec must reflect the actual supported targets.
  • Verify: The code block contains only net10.0;net9.0;net8.0.

• spec/tech-standards/build-system.md line 33 — Delete the | \netstandard2.0` | Broad compatibility (.NET Framework 4.6.1+, .NET Core 2.0+) |` row from the target table.

  • Why: The target no longer exists; the row is misleading.
  • Verify: No mention of netstandard2.0 remains anywhere in build-system.md.

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Acceptance Criteria Cross-Reference

Acceptance criterion (from brief)	Covered by task
Library targets net10.0;net9.0;net8.0 only; netstandard2.0 absent from .csproj	Task 1 (TargetFrameworks)
No #if NETSTANDARD or #if NET5_0_OR_GREATER conditional compilation in library	Tasks 4, 6 (Bitwise.cs, HistogramLogReader.cs)
Bitwise.Imperative class fully removed	Task 5
Bitwise.NumberOfLeadingZeros calls BitOperations.LeadingZeroCount unconditionally	Task 4
HistogramLogReader.IsComment uses line.StartsWith('#') unconditionally	Task 6
Three per-framework Release PropertyGroup conditions collapsed into one	Task 2
LeadingZeroCountBenchmarkBase.cs no longer references Bitwise.Imperative; benchmarking project compiles	Task 7
All unit tests pass on net8.0, net9.0, net10.0	Tasks 8 (new tests), 9 (full suite)
spec/tech-standards/build-system.md no longer references netstandard2.0	Tasks 10, 11

Closes #161