Why it matters

Random selection powers surveys, ML train/val splits, and load testing. Correctness matters.

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The architecture

Reservoir for streams.

Partial Fisher-Yates for arrays with k<

Sort-by-random-key alternative.

Random selectionStreamreservoir samplingArray k<partial Fisher-YatesAlternativesort by random keyPartial Fisher-Yates is O(k); full shuffle is O(n) — pick by use case
Random selection.
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How it works end to end

Reservoir for streams.

Partial Fisher-Yates O(k).

Sort-by-key O(n log n).