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/*
* Copyright (C) 2005, 2006, 2008, 2010, 2013 Apple Inc. All rights reserved.
* Copyright (C) 2010 Patrick Gansterer <paroga@paroga.com>
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Library General Public
* License as published by the Free Software Foundation; either
* version 2 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Library General Public License for more details.
*
* You should have received a copy of the GNU Library General Public License
* along with this library; see the file COPYING.LIB. If not, write to
* the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
* Boston, MA 02110-1301, USA.
*
*/
#ifndef THIRD_PARTY_BLINK_RENDERER_PLATFORM_WTF_TEXT_STRING_HASHER_H_
#define THIRD_PARTY_BLINK_RENDERER_PLATFORM_WTF_TEXT_STRING_HASHER_H_
#include <cstring>
#include <type_traits>
#include "base/containers/span.h"
#include "base/dcheck_is_on.h"
#include "base/logging.h"
#include "third_party/blink/renderer/platform/wtf/allocator/allocator.h"
#include "third_party/blink/renderer/platform/wtf/text/wtf_uchar.h"
#include "third_party/rapidhash/rapidhash.h"
namespace WTF {
class StringHasher {
DISALLOW_NEW();
public:
static const unsigned kFlagCount =
8; // Save 8 bits for StringImpl to use as flags.
// The main entry point for the string hasher. Computes the hash and returns
// only the lowest 24 bits, since that's what we have room for in StringImpl.
//
// NOTE: length is the number of bytes produced _by the reader_.
// Normally, this means that the number of bytes actually read will be
// equivalent to (length * Reader::kCompressionFactor /
// Reader::kExpansionFactor). Also note that if you are hashing something
// that is not 8-bit elements, and do _not_ use compression factors or
// similar, you'll need to multiply by sizeof(T) to get all data read.
template <class Reader = PlainHashReader>
static unsigned ComputeHashAndMaskTop8Bits(const char* data,
unsigned length) {
return MaskTop8Bits(
rapidhash<Reader>(reinterpret_cast<const uint8_t*>(data), length));
}
// Hashing can be very performance-sensitive, but the hashing function is also
// fairly big (~300 bytes on x86-64, give or take). This function is exactly
// equivalent to ComputeHashAndMaskTop8Bits(), except that it is marked as
// ALWAYS_INLINE and thus will be force-inlined into your own code. You should
// use this if all of these are true:
//
// 1. You are in a frequently-called place, i.e. you are performance
// sensitive.
// 2. You frequently hash short strings, so that the function call overhead
// dominates; for hashing e.g. 1 kB of data, this makes no sense to call.
// 3. The gain of increased performance, ideally demonstrated by benchmarks,
// outweighs the cost of the binary size increase.
//
// Note that the compiler may choose to inline even
// ComputeHashAndMaskTop8Bits() if it deems it a win; for instance, if you
// call it with length equivalent to a small constant known at compile time,
// the function may be subject to dead-code removal and thus considered small
// enough to inline. The same goes if you are the only user of your
// HashReader.
template <class Reader = PlainHashReader>
ALWAYS_INLINE static unsigned ComputeHashAndMaskTop8BitsInline(
const char* data,
unsigned length) {
return MaskTop8Bits(
rapidhash<Reader>(reinterpret_cast<const uint8_t*>(data), length));
}
static uint64_t HashMemory(base::span<const uint8_t> data) {
return rapidhash(data.data(), data.size());
}
template <size_t Extent>
static uint64_t HashMemory(base::span<const uint8_t, Extent> data) {
return rapidhash(data.data(), data.size());
}
private:
static unsigned MaskTop8Bits(uint64_t result) {
// Reserving space from the high bits for flags preserves most of the hash's
// value, since hash lookup typically masks out the high bits anyway.
result &= (1U << (32 - kFlagCount)) - 1;
// This avoids ever returning a hash code of 0, since that is used to
// signal "hash not computed yet". Setting the high bit maintains
// reasonable fidelity to a hash code of 0 because it is likely to yield
// exactly 0 when hash lookup masks out the high bits.
if (!result) {
result = 0x80000000 >> kFlagCount;
}
return static_cast<unsigned>(result);
}
};
} // namespace WTF
using WTF::StringHasher;
#endif // THIRD_PARTY_BLINK_RENDERER_PLATFORM_WTF_TEXT_STRING_HASHER_H_
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