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Improve Random{NumberGenerator}.GetItems/String for non-power of 2 choices #107988

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Improve Random{NumberGenerator}.GetItems/String for non-power of 2 choices #107988

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Commits on Sep 19, 2024

  1. Improve Random{NumberGenerator}.GetItems/String for non-power of 2 ch… 

    …oices

In .NET 9, we added an optimization to Random.GetItems and RandomNumberGenerator.GetItems/GetString that special-cases a power-of-2 number of choices that's <= 256. In such a case, we can avoid many trips to the RNG by requesting bytes in bulk, rather than requesting an Int32 per element. Each byte is masked to produce the index into the choices.

This PR extends that optimization to also cover non-power-of-2 choices. It can't just mask off the bits as in the power-of-2 case, but it can mask off some bits and then do rejection sampling, which on average still yields big wins.
    @stephentoub
    stephentoub committed Sep 19, 2024
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