HashTableCore.C

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    Copyright (C) 2011-2012 OpenFOAM Foundation
    Copyright (C) 2017-2025 OpenCFD Ltd.
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License
    This file is part of OpenFOAM.
 
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#include "HashTableCore.H"
 
// * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //
 
namespace Foam
{
    defineTypeNameAndDebug(HashTableCore, 0);
}
 
 
// file-scope:
// Minimum internal table size (must be a power of two!)
constexpr int32_t minTableSize = 8;
 
 
// * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * * //
 
Foam::label Foam::HashTableCore::canonicalSize(const label size) noexcept
{
    // Enforce power of two for fast modulus in hash index calculations.
    // Use unsigned for these calculations.
    //
    // - The lower limit (8) is somewhat arbitrary, but if the hash table
    //   is too small, there will be many direct table collisions.
    // - The upper limit (approx. INT32_MAX/4) must be a power of two,
    //   need not be extremely large for hashing.
 
    if (size <= minTableSize)
    {
        return (size < 1 ? 0 : minTableSize);
    }
    else if (size > maxTableSize/2)
    {
        return maxTableSize;
    }
 
    // Determine power-of-two with glibc (may or may not be faster):
    //
    // return (1 << (32-__builtin_clz(int32_t(size-1))));
 
    if (!(size & (size-1)))
    {
        // Already a power-of-two...
        return size;
    }
 
    // Non-branching for 32-bit
    // [https://graphics.stanford.edu/~seander/bithacks.html]
    {
        uint32_t n(size);
        --n;
        n |= n >> 1;
        n |= n >> 2;
        n |= n >> 4;
        n |= n >> 8;
        n |= n >> 16;
        ++n;
        return n;
    }
 
    // OLD:
    // Brute-force method
    //
    // uint32_t n(minTableSize);
    // while (n < uint32_t(size))
    // {
    //     n <<= 1;
    // }
    // return n;
}
 
 
// ************************************************************************* //