perf(usenet): lazy on-demand segment loading#319
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- Move buildSegmentIndex from OpenFile to createUsenetReader, guarded by sync.Once so the O(n) index build is deferred until the file is actually read - Add lazy segmentRange: segmentRange now holds a SegmentLoader + index metadata and creates *segment objects on demand via createSegmentLocked, replacing the O(N) eager GetSegmentsInRangeFromIndex path - createUsenetReader now calls NewLazySegmentRange directly (O(1) init) instead of routing through BuildSegmentRange, since the index is always available at read time - Files that are only stat'd or listed in directories incur zero segment processing cost; first actual read triggers the one-time O(n) index build followed by O(log n) binary search per range lookup Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Summary
buildSegmentIndex: Moved the O(n) segment offset index build fromOpenFiletocreateUsenetReader, protected bysync.Once. Files that are only stat'd or listed never pay the index cost.segmentRange:segmentRangenow supports aSegmentLoader+ index metadata and creates*segmentobjects on demand (createSegmentLocked) instead of eagerly iterating all segments upfront viaGetSegmentsInRangeFromIndex.NewLazySegmentRange:createUsenetReadernow callsNewLazySegmentRangedirectly (O(1) init) instead of routing throughBuildSegmentRange, since the segment index is always available at read time.Performance Impact
OpenFilesegmentRangeinitFiles only accessed via stat/readdir/metadata operations pay zero segment processing cost.