ClickHouse/dbms/src/Functions/FunctionsRandom.cpp

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#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.
}
}