ClickHouse/dbms/Functions/FunctionsRandom.cpp

#include <Functions/FunctionFactory.h>
#include <Functions/FunctionsRandom.h>
#include <Common/HashTable/Hash.h>
#include <Common/randomSeed.h>
#include <common/unaligned.h>


namespace DB
{

namespace
{
    /// NOTE Probably
    ///    http://www.pcg-random.org/
    /// or http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/SFMT/
    /// or http://docs.yeppp.info/c/group__yep_random___w_e_l_l1024a.html
    /// could go better.

    struct LinearCongruentialGenerator
    {
        /// Constants from `man lrand48_r`.
        static constexpr UInt64 a = 0x5DEECE66D;
        static constexpr UInt64 c = 0xB;

        /// And this is from `head -c8 /dev/urandom | xxd -p`
        UInt64 current = 0x09826f4a081cee35ULL;

        void seed(UInt64 value)
        {
            current = value;
        }

        UInt32 next()
        {
            current = current * a + c;
            return current >> 16;
        }
    };

    void seed(LinearCongruentialGenerator & generator, intptr_t additional_seed)
    {
        generator.seed(intHash64(randomSeed() ^ intHash64(additional_seed)));
    }
}

void RandImpl::execute(char * output, size_t size)
{
    LinearCongruentialGenerator generator0;
    LinearCongruentialGenerator generator1;
    LinearCongruentialGenerator generator2;
    LinearCongruentialGenerator generator3;

    seed(generator0, 0xfb4121280b2ab902ULL + reinterpret_cast<intptr_t>(output));
    seed(generator1, 0x0121cf76df39c673ULL + reinterpret_cast<intptr_t>(output));
    seed(generator2, 0x17ae86e3a19a602fULL + reinterpret_cast<intptr_t>(output));
    seed(generator3, 0x8b6e16da7e06d622ULL + reinterpret_cast<intptr_t>(output));

    for (const char * end = output + size; output < end; output += 16)
    {
        unalignedStore<UInt32>(output, generator0.next());
        unalignedStore<UInt32>(output + 4, generator1.next());
        unalignedStore<UInt32>(output + 8, generator2.next());
        unalignedStore<UInt32>(output + 12, generator3.next());
    }

    /// It is guaranteed (by PaddedPODArray) that we can overwrite up to 15 bytes after end.
}

}