perf: read each source file once, restore per-file parallelism, use Foundation .strings parser (~4× faster)

[memoto]

Why

LocalizeChecker runs as a build-phase script on every Debug build of large iOS apps. Profiling showed several real, fixable inefficiencies in the implementation.

Benchmarked on Apple Silicon, corpus: 400 Swift files (~1.85 MB), 600 bundle keys, ×10 XCTest.measure runs:

	avg	min	stddev
Before (0.1.13)	8.020 s	7.944 s	1.3 %
After	1.930 s	1.895 s	1.3 %
Speed-up	≈ 4.1×

What changed

1. Read each source file once

SourceFileChecker.start() previously called fastCheck(), which loaded and UTF-8-decoded the file just to substring-check for .localized, then re-loaded the same file for SwiftParser. Now the file is read once and the string is reused for both the early-exit check and the parse.

2. Replace the hand-rolled .strings parser

The old tokenizer split on ; and used per-entry String.init + two trimmingCharacters calls — slow, and mis-parsed values that contain ;. Replaced with Foundation's NSDictionary(contentsOf:error:) — faster and correct. The bug is covered by a new correctness test in the companion PR to the consuming repo.

3. Restore real parallelism in SourceFileBatchChecker

0.1.13 made this type an actor. Every group.addTask { try await self.processBatch(...) } closure hopped back onto the actor's serial executor, so chunking added overhead without delivering any parallelism. Reverted to final class & Sendable and switched from fixed-size chunks to one task per file — Swift Concurrency's cooperative pool then load-balances CPU-bound SwiftParser work across cores.

4. Smaller cleanups

• Cache the nextToken(viewMode:) walk in LocalizeParser.visit (the original walked the syntax tree twice per node).
• Use a Set for unused-key detection (was Array.contains(where:), O(N·M) on large bundles).
• Drop debug print() calls from LocalizeBundle.init that fired on every build.
• Stream SourceFilesTraversalTrait.parseSourceDirectory with a for-in loop instead of chaining lazy filters that allocate intermediate arrays.

Correctness

All existing public API shapes are preserved. The only breaking change is that SourceFileBatchChecker is no longer an actor — callers that await its methods still compile; the async qualifier is retained where the method itself spawns tasks.

🤖 Generated with Claude Code