folly/folly/compression/Select64.h

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#pragma once

#include <array>

#include <glog/logging.h>

#include <folly/Portability.h>
#include <folly/compression/Instructions.h>

namespace folly {

namespace detail {

//  kSelectInByte
//
//  Described in:
//    http://dsiutils.di.unimi.it/docs/it/unimi/dsi/bits/Fast.html#selectInByte
//
//  A precomputed tabled containing the positions of the set bits in the binary
//  representations of all 8-bit unsigned integers.
//
//  For i: [0, 256) ranging over all 8-bit unsigned integers and for j: [0, 8)
//  ranging over all 0-based bit positions in an 8-bit unsigned integer, the
//  table entry kSelectInByte[i][j] is the 0-based bit position of the j-th set
//  bit in the binary representation of i, or 8 if it has fewer than j set bits.
//
//  Example: i: 17 (b00010001), j: [0, 8)
//    kSelectInByte[b00010001][0] = 0
//    kSelectInByte[b00010001][1] = 4
//    kSelectInByte[b00010001][2] = 8
//    ...
//    kSelectInByte[b00010001][7] = 8
extern std::array<std::array<std::uint8_t, 256>, 8> const kSelectInByte;

} // namespace detail

/**
 * Returns the position of the k-th 1 in the 64-bit word x.
 * k is 0-based, so k=0 returns the position of the first 1.
 *
 * Uses the broadword selection algorithm by Vigna [1], improved by Gog
 * and Petri [2] and Vigna [3].
 *
 * [1] Sebastiano Vigna. Broadword Implementation of Rank/Select
 *     Queries. WEA, 2008
 *
 * [2] Simon Gog, Matthias Petri. Optimized succinct data structures
 *     for massive data. Softw. Pract. Exper., 2014
 *
 * [3] Sebastiano Vigna. MG4J 5.2.1. http://mg4j.di.unimi.it/
 */
template <class Instructions>
inline uint64_t select64(uint64_t x, uint64_t k) { … }

#if FOLLY_X64 || defined(__i386)
template <>
FOLLY_ALWAYS_INLINE uint64_t
select64<compression::instructions::Haswell>(uint64_t x, uint64_t k) {
#if defined(__GNUC__)
  // GCC and Clang won't inline the intrinsics.
  uint64_t result = uint64_t(1) << k;

  asm("pdep %1, %0, %0\n\t"
      "tzcnt %0, %0"
      : "+r"(result)
      : "r"(x));

  return result;
#else
  return _tzcnt_u64(_pdep_u64(1ULL << k, x));
#endif
}
#endif

} // namespace folly