text: /*
   * 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/experimental/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) {
    DCHECK_LT(k, Instructions::popcount(x));
  
    constexpr uint64_t kOnesStep4 = 0x1111111111111111ULL;
    constexpr uint64_t kOnesStep8 = 0x0101010101010101ULL;
    constexpr uint64_t kMSBsStep8 = 0x80ULL * kOnesStep8;
  
    auto s = x;
    s = s - ((s & 0xA * kOnesStep4) >> 1);
    s = (s & 0x3 * kOnesStep4) + ((s >> 2) & 0x3 * kOnesStep4);
    s = (s + (s >> 4)) & 0xF * kOnesStep8;
    uint64_t byteSums = s * kOnesStep8;
  
    uint64_t kStep8 = k * kOnesStep8;
    uint64_t geqKStep8 = (((kStep8 | kMSBsStep8) - byteSums) & kMSBsStep8);
    uint64_t place = Instructions::popcount(geqKStep8) * 8;
    uint64_t byteRank = k - (((byteSums << 8) >> place) & uint64_t(0xFF));
    return place + detail::kSelectInByte[byteRank][((x >> place) & 0xFF)];
  }
  
  #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
  