[efficiency-improver] perf: cache newline+color string in SimpleAnsiTerminal

[Evangelink]

🤖 Efficiency Improver — automated AI assistant focused on reducing the energy consumption and computational footprint of this repository.

Goal and Rationale

Eliminate a per-call heap allocation in SimpleAnsiTerminal.SetColorPerLine. This method is called on every Append and AppendLine invocation while a foreground color is active — which is the common case during any colored test run in a CI environment (Azure DevOps, GitHub Actions, etc.).

Focus Area

Code-Level Efficiency — unnecessary object allocation per terminal write.

Approach

Previously, SetColorPerLine constructed the replacement string $"\n{_foregroundColor}" on every call:

// Before: allocates "\n" + _foregroundColor on every Append/AppendLine
private string? SetColorPerLine(string value)
    => _foregroundColor == null ? value : value.Replace("\n", $"\n{_foregroundColor}");

Since _foregroundColor only changes when SetColor or ResetColor is called, the "\n" + _foregroundColor string can be cached at color-change time and reused for all subsequent writes:

// After: compute once in SetColor, reuse on every Append/AppendLine
private string? _newlineAndColor; // cached "\n" + _foregroundColor

public override void SetColor(TerminalColor color)
{
    string setColor = $"{AnsiCodes.CSI}{(int)color}{AnsiCodes.SetColor}";
    _foregroundColor = setColor;
    _newlineAndColor = "\n" + setColor; // computed once per color change
    ...
}

public override void ResetColor()
{
    _foregroundColor = null;
    _newlineAndColor = null; // cleared on reset
    ...
}

private string? SetColorPerLine(string value)
    => _newlineAndColor == null ? value : value.Replace("\n", _newlineAndColor); // no allocation

Energy Efficiency Evidence

Proxy metric: heap allocation count.

• Before: 1 short-lived string object allocated per Append/AppendLine call when a color is active
• After: 0 allocations per call — the replacement string is computed once per SetColor invocation

SimpleAnsiTerminal is used in CI environments that support ANSI but not full cursor control (Azure DevOps, GitHub Actions, etc.). A test run producing 1 000 colored output lines would previously allocate ~1 000 unnecessary small strings (~10–15 bytes each). These all become GC garbage immediately, adding pressure on the GC and wasting CPU cycles reclaiming them.

GSF — Hardware Efficiency: fewer short-lived heap objects → less GC work → better utilization of CPU and DRAM already in use.

Trade-offs

• An additional string? field (_newlineAndColor) is added to SimpleAnsiTerminal. This is negligible — one pointer-sized field on a per-session singleton.
• No behaviour change: _newlineAndColor is always "\n" + _foregroundColor when non-null, matching the previous inline computation exactly.

Reproducibility

./build.sh          # verify build succeeds

Unit test coverage: TerminalTestReporterTests.SimpleAnsiTerminal_OutputFormattingIsCorrect exercises colored output through SimpleAnsiTerminal.

Test Status

✅ Build succeeded (0 errors, 0 warnings)
✅ Microsoft.Testing.Platform.UnitTests — all tests passed

Generated by Efficiency Improver · sonnet46 7.7M · ◷

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[Copilot]

Pull request overview

This PR optimizes Microsoft.Testing.Platform’s ANSI terminal output by removing a per-call string allocation in SimpleAnsiTerminal.SetColorPerLine, which is invoked for every Append/AppendLine while a foreground color is active (common during colored CI logs).

Changes:

• Cache the computed newline+color prefix ("\n" + setColor) once per SetColor call.
• Reuse the cached value in SetColorPerLine to avoid allocating the same replacement string on every append.
• Clear the cached value in ResetColor to keep state consistent.

Show a summary per file

File	Description
src/Platform/Microsoft.Testing.Platform/OutputDevice/Terminal/SimpleAnsiTerminal.cs	Adds _newlineAndColor cache and uses it in SetColorPerLine to eliminate repeated newline+color string allocations during colored output.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0