[perf-improver] perf: eliminate per-call string allocation in AnsiTerminal.SetColor and use HashSet for KnownFileExtensions

[Evangelink]

🤖 This is an automated contribution from Perf Improver.

Goal and Rationale

Two allocation hotspots in the terminal output path:

1. AnsiTerminal.SetColor — Every call to SetColor(TerminalColor) built a new string via $"{AnsiCodes.CSI}{(int)color}{AnsiCodes.SetColor}". This happens during every live-progress render frame (once per assembly × 3 color changes per row × every 500 ms). There are only 17 possible TerminalColor values, so the strings can be pre-computed.

2. KnownFileExtensions — The 13-element string[] was scanned linearly by Array.Contains on every AppendLink call (triggered for each failed-test file link in the run summary). A HashSet<string> gives O(1) lookup.

Approach

1. Added AnsiCodes.GetSetColorEscapeCode(TerminalColor) — a switch expression that returns interned compile-time string literals (e.g. "\x1b[32m") for every known color. No heap allocation for common cases. Falls back to string interpolation for any unexpected future value.

2. AnsiTerminal.SetColor now calls AnsiCodes.GetSetColorEscapeCode(color) instead of building the string inline.

3. Changed KnownFileExtensions from private static readonly string[] to private static readonly HashSet<string> initialized with StringComparer.Ordinal (preserving the existing case-sensitive behaviour).

Performance Evidence

Site	Before	After
AnsiTerminal.SetColor (per call)	1 × string allocation (~6–8 bytes, formatting)	0 — returns interned literal
KnownFileExtensions.Contains	O(n) — up to 13 comparisons	O(1) HashSet lookup
Frame-level colour allocations (10 assemblies, 500 ms cadence)	~30 strings/sec	0

Methodology: code inspection + diff review. The switch expression returns literal strings which the runtime interns; no heap allocation occurs.

Trade-offs

• AnsiCodes gains one method (~20 lines). The switch explicitly enumerates all 17 TerminalColor values; adding a new value without updating the switch falls back to the interpolated string (no regression).
• KnownFileExtensions is now a HashSet (one allocation at class init) instead of an array. The comparer is Ordinal to preserve existing case-sensitivity.

Test Status

• Microsoft.Testing.Platform.UnitTests (net8.0): 995 passed, 0 failed, 3 skipped ✅

Reproducibility

./build.sh
artifacts/bin/Microsoft.Testing.Platform.UnitTests/Debug/net8.0/Microsoft.Testing.Platform.UnitTests

Generated by Perf Improver · sonnet46 7.6M · ◷

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[Copilot]

Pull request overview

This PR optimizes the Microsoft.Testing.Platform (MTP) ANSI terminal output path by removing two small but frequent allocation/lookup hotspots during progress rendering and link generation.

Changes:

• Avoid per-call string interpolation in AnsiTerminal.SetColor by routing through a precomputed escape-code mapping.
• Replace linear extension lookup (string[] + Contains) with a HashSet<string> for O(1) membership checks in AppendLink.

Show a summary per file

File	Description
src/Platform/Microsoft.Testing.Platform/OutputDevice/Terminal/AnsiTerminal.cs	Uses HashSet<string> for known file extensions and calls the new escape-code helper in SetColor.
src/Platform/Microsoft.Testing.Platform/OutputDevice/Terminal/AnsiCodes.cs	Adds GetSetColorEscapeCode(TerminalColor) returning precomputed ANSI color escape sequences.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 0