HashTableCore.C

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    Copyright (C) 2017-2019 OpenCFD Ltd.
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License
    This file is part of OpenFOAM.
 
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#include "HashTableCore.H"
#include "uLabel.H"
 
// * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //
 
namespace Foam
{
    defineTypeNameAndDebug(HashTableCore, 0);
}
 
// Approximately labelMax/4
const Foam::label Foam::HashTableCore::maxTableSize(1L << (sizeof(label)*8-3));
 
 
// * * * * * * * * * * * * * * Member Functions  * * * * * * * * * * * * * * //
 
Foam::label Foam::HashTableCore::canonicalSize(const label requested_size)
{
    if (requested_size < 1)
    {
        return 0;
    }
    else if (requested_size >= maxTableSize)
    {
        return maxTableSize;
    }
 
    // 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. labelMax/4) must be a power of two,
    //   need not be extremely large for hashing.
 
    uLabel powerOfTwo = 8u; // lower-limit
 
    const uLabel size = requested_size;
    if (size <= powerOfTwo)
    {
        return powerOfTwo;
    }
 
    if (size & (size-1))  // <- Modulus of i^2
    {
        // Determine power-of-two. Brute-force is fast enough.
        while (powerOfTwo < size)
        {
            powerOfTwo <<= 1;
        }
 
        return powerOfTwo;
    }
 
    return size;
}
 
 
// ************************************************************************* //