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<algorithm>: Implement batched random integer generation for shuffle() #5932

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@FranciscoThiesen FranciscoThiesen commented Dec 2, 2025

Summary

  • Implement batched random number generation for shuffle based on Brackett-Rozinsky & Lemire's paper
  • For URNGs with full 64-bit range (like mt19937_64), generate 2 bounded random integers from a single 64-bit random value
  • Uses Lemire's nearly-divisionless algorithm for rejection sampling

Implementation Details

  • Added _Batched_rng_from_urng struct that extracts batches of bounded random integers
  • Added _Random_shuffle_batched function implementing Fisher-Yates with batched RNG
  • Modified shuffle() and ranges::shuffle to use batched version for eligible URNGs
  • Compile-time dispatch based on URNG characteristics (full 64-bit range)

Test Plan

  • Existing shuffle tests pass
  • Added permutation correctness tests for both 64-bit (batched) and 32-bit (non-batched) paths
  • Added edge case tests (empty, single element, 2, 3, 4, and 10000 elements)

Resolves #5736

@FranciscoThiesen
Implement batched random integer generation for shuffle
d1744fa 
Implements the algorithm from Brackett-Rozinsky & Lemire's paper
"Batched Ranged Random Integer Generation" to reduce RNG calls in
std::shuffle and ranges::shuffle for 64-bit URNGs like mt19937_64.

The batched approach extracts multiple bounded random integers from a
single 64-bit random word, using only multiplication (no division) in
the common case. This reduces the number of RNG calls by approximately
half for arrays with fewer than 2^32 elements.

Resolves microsoft#5736
@FranciscoThiesen FranciscoThiesen requested a review from a team as a code owner December 2, 2025 22:28
@github-project-automation github-project-automation bot moved this to Initial Review in STL Code Reviews Dec 2, 2025
frederick-vs-ja
frederick-vs-ja reviewed Dec 2, 2025
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FranciscoThiesen commented Dec 3, 2025

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FranciscoThiesen commented Dec 3, 2025

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@FranciscoThiesen
Address review feedback: extract URNG range check to variable template
3df8856 
Move _Has_full_64bit_range check out of _Batched_rng_from_urng class
to reduce template instantiation overhead. Use is_same_v and _Max_limit
as suggested by reviewer.
@FranciscoThiesen
Apply clang-format
8aa184a
@StephanTLavavej StephanTLavavej added the performance Must go faster label Dec 4, 2025
@StephanTLavavej StephanTLavavej changed the title Implement batched random integer generation for shuffle <algorithm>: Implement batched random integer generation for shuffle() Dec 6, 2025
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<algorithm>: Investigate further optimizations for shuffle() and sample()

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