uint_fast_div.hpp

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#ifndef ROCPRIM_DEVICE_DETAIL_UINT_FAST_DIV_HPP_
#define ROCPRIM_DEVICE_DETAIL_UINT_FAST_DIV_HPP_

#include "../../config.hpp"

BEGIN_ROCPRIM_NAMESPACE

namespace detail
{

struct uint_fast_div
{
    unsigned int magic; // Magic number
    unsigned int shift; // shift amount
    unsigned int add;   // "add" indicator

    ROCPRIM_HOST_DEVICE inline
    uint_fast_div() = default;

    ROCPRIM_HOST_DEVICE inline
    uint_fast_div(unsigned int d)
    {
        // Must have 1 <= d <= 2**32-1.

        if(d == 1)
        {
            magic = 0;
            shift = 0;
            add = 0;
            return;
        }

        int p;
        unsigned int p32 = 1, q, r, delta;
        add = 0;                // Initialize "add" indicator.
        p = 31;                 // Initialize p.
        q = 0x7FFFFFFF/d;       // Initialize q = (2**p - 1)/d.
        r = 0x7FFFFFFF - q*d;   // Init. r = rem(2**p - 1, d).
        do {
            p = p + 1;
            if(p == 32) p32 = 1;     // Set p32 = 2**(p-32).
            else p32 = 2*p32;
            if(r + 1 >= d - r)
            {
                if(q >= 0x7FFFFFFF) add = 1;
                q = 2*q + 1;
                r = 2*r + 1 - d;
            }
            else
            {
                if(q >= 0x80000000) add = 1;
                q = 2*q;
                r = 2*r + 1;
            }
            delta = d - 1 - r;
        } while (p < 64 && p32 < delta);
        magic = q + 1;         // Magic number and
        shift = p - 32;        // shift amount

        if(add) shift--;
    }
};

ROCPRIM_HOST_DEVICE inline
unsigned int operator/(unsigned int n, const uint_fast_div& divisor)
{
    if(divisor.magic == 0)
    {
        // Special case for 1
        return n;
    }

    // Higher 32-bit of 64-bit multiplication
    unsigned int q = (static_cast<unsigned long long>(divisor.magic) * static_cast<unsigned long long>(n)) >> 32;
    if(divisor.add)
    {
        q = ((n - q) >> 1) + q;
    }
    return q >> divisor.shift;
}

} // end of detail namespace

END_ROCPRIM_NAMESPACE

#endif // ROCPRIM_DEVICE_DETAIL_UINT_FAST_DIV_HPP_