primecount 0.2.1

Rust wrapper for https://github.com/kimwalisch/primecount
Documentation 
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///
/// @file  SieveTables.hpp
/// @brief Static lookup tables for unsetting bits in the sieve array.
///        These bits correspond to numbers that are either below
///        or above some threshold and hence need to be discarded.
///
/// Copyright (C) 2019 Kim Walisch, <kim.walisch@gmail.com>
///
/// This file is distributed under the BSD License. See the COPYING
/// file in the top level directory.
///

#ifndef SIEVETABLES_HPP
#define SIEVETABLES_HPP

#include <stdint.h>
#include <array>

namespace {

/// The 8 bits in each byte of the sieve array correspond
/// to the offsets { 1, 7, 11, 13, 17, 19, 23, 29 }.
///
constexpr int left_shift(int n)
{
  return   (n % 30 <= 1)  ? (n / 30 * 8) + 0
         : (n % 30 <= 7)  ? (n / 30 * 8) + 1
         : (n % 30 <= 11) ? (n / 30 * 8) + 2
         : (n % 30 <= 13) ? (n / 30 * 8) + 3
         : (n % 30 <= 17) ? (n / 30 * 8) + 4
         : (n % 30 <= 19) ? (n / 30 * 8) + 5
         : (n % 30 <= 23) ? (n / 30 * 8) + 6
         : (n / 30 * 8) + 7;
}

/// The 8 bits in each byte of the sieve array correspond
/// to the offsets { 1, 7, 11, 13, 17, 19, 23, 29 }.
///
constexpr int right_shift(int n)
{
  return   (n % 30 >= 29) ? 56 - (n / 30 * 8)
         : (n % 30 >= 23) ? 57 - (n / 30 * 8)
         : (n % 30 >= 19) ? 58 - (n / 30 * 8)
         : (n % 30 >= 17) ? 59 - (n / 30 * 8)
         : (n % 30 >= 13) ? 60 - (n / 30 * 8)
         : (n % 30 >= 11) ? 61 - (n / 30 * 8)
         : (n % 30 >= 7)  ? 62 - (n / 30 * 8)
         : (n % 30 >= 1)  ? 63 - (n / 30 * 8)
         : 64 - (n / 30 * 8);
}

#if defined(__BYTE_ORDER__) && \
    defined(__ORDER_BIG_ENDIAN__) && \
    __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__

/// Since the sieve array is a byte array but the
/// unset_smaller and unset_larger arrays are of
/// type uint64_t we need to reverse the byte order
/// of the unset_smaller and unset_larger arrays
/// on big endian CPU architectures.

/// Reverse byte order of 64-bit integer
constexpr uint64_t bswap64(uint64_t i, uint64_t j = 0, uint64_t n = 0)
{
  return (n == sizeof(uint64_t)) ? j :
    bswap64(i >> 8, (j << 8) | (i & 0xff), n + 1);
}

/// Bitmask to unset bits <= n
constexpr uint64_t unset_s(int n)
{
  return bswap64(~0ull << left_shift(n));
}

/// Bitmask to unset bits >= n
constexpr uint64_t unset_l(int n)
{
  return bswap64((n == 0) ? 0 : ~0ull >> right_shift(n));
}

#else

/// Bitmask to unset bits <= n
constexpr uint64_t unset_s(int n)
{
  return ~0ull << left_shift(n);
}

/// Bitmask to unset bits >= n
constexpr uint64_t unset_l(int n)
{
  return (n == 0) ? 0 : ~0ull >> right_shift(n);
}

#endif

/// Unset bits < start
const std::array<uint64_t, 240> unset_smaller =
{
  unset_s(0), unset_s(1), unset_s(2), unset_s(3), unset_s(4),
  unset_s(5), unset_s(6), unset_s(7), unset_s(8), unset_s(9),
  unset_s(10), unset_s(11), unset_s(12), unset_s(13), unset_s(14),
  unset_s(15), unset_s(16), unset_s(17), unset_s(18), unset_s(19),
  unset_s(20), unset_s(21), unset_s(22), unset_s(23), unset_s(24),
  unset_s(25), unset_s(26), unset_s(27), unset_s(28), unset_s(29),
  unset_s(30), unset_s(31), unset_s(32), unset_s(33), unset_s(34),
  unset_s(35), unset_s(36), unset_s(37), unset_s(38), unset_s(39),
  unset_s(40), unset_s(41), unset_s(42), unset_s(43), unset_s(44),
  unset_s(45), unset_s(46), unset_s(47), unset_s(48), unset_s(49),
  unset_s(50), unset_s(51), unset_s(52), unset_s(53), unset_s(54),
  unset_s(55), unset_s(56), unset_s(57), unset_s(58), unset_s(59),
  unset_s(60), unset_s(61), unset_s(62), unset_s(63), unset_s(64),
  unset_s(65), unset_s(66), unset_s(67), unset_s(68), unset_s(69),
  unset_s(70), unset_s(71), unset_s(72), unset_s(73), unset_s(74),
  unset_s(75), unset_s(76), unset_s(77), unset_s(78), unset_s(79),
  unset_s(80), unset_s(81), unset_s(82), unset_s(83), unset_s(84),
  unset_s(85), unset_s(86), unset_s(87), unset_s(88), unset_s(89),
  unset_s(90), unset_s(91), unset_s(92), unset_s(93), unset_s(94),
  unset_s(95), unset_s(96), unset_s(97), unset_s(98), unset_s(99),
  unset_s(100), unset_s(101), unset_s(102), unset_s(103), unset_s(104),
  unset_s(105), unset_s(106), unset_s(107), unset_s(108), unset_s(109),
  unset_s(110), unset_s(111), unset_s(112), unset_s(113), unset_s(114),
  unset_s(115), unset_s(116), unset_s(117), unset_s(118), unset_s(119),
  unset_s(120), unset_s(121), unset_s(122), unset_s(123), unset_s(124),
  unset_s(125), unset_s(126), unset_s(127), unset_s(128), unset_s(129),
  unset_s(130), unset_s(131), unset_s(132), unset_s(133), unset_s(134),
  unset_s(135), unset_s(136), unset_s(137), unset_s(138), unset_s(139),
  unset_s(140), unset_s(141), unset_s(142), unset_s(143), unset_s(144),
  unset_s(145), unset_s(146), unset_s(147), unset_s(148), unset_s(149),
  unset_s(150), unset_s(151), unset_s(152), unset_s(153), unset_s(154),
  unset_s(155), unset_s(156), unset_s(157), unset_s(158), unset_s(159),
  unset_s(160), unset_s(161), unset_s(162), unset_s(163), unset_s(164),
  unset_s(165), unset_s(166), unset_s(167), unset_s(168), unset_s(169),
  unset_s(170), unset_s(171), unset_s(172), unset_s(173), unset_s(174),
  unset_s(175), unset_s(176), unset_s(177), unset_s(178), unset_s(179),
  unset_s(180), unset_s(181), unset_s(182), unset_s(183), unset_s(184),
  unset_s(185), unset_s(186), unset_s(187), unset_s(188), unset_s(189),
  unset_s(190), unset_s(191), unset_s(192), unset_s(193), unset_s(194),
  unset_s(195), unset_s(196), unset_s(197), unset_s(198), unset_s(199),
  unset_s(200), unset_s(201), unset_s(202), unset_s(203), unset_s(204),
  unset_s(205), unset_s(206), unset_s(207), unset_s(208), unset_s(209),
  unset_s(210), unset_s(211), unset_s(212), unset_s(213), unset_s(214),
  unset_s(215), unset_s(216), unset_s(217), unset_s(218), unset_s(219),
  unset_s(220), unset_s(221), unset_s(222), unset_s(223), unset_s(224),
  unset_s(225), unset_s(226), unset_s(227), unset_s(228), unset_s(229),
  unset_s(230), unset_s(231), unset_s(232), unset_s(233), unset_s(234),
  unset_s(235), unset_s(236), unset_s(237), unset_s(238), unset_s(239)
};

/// Unset bits > stop
const std::array<uint64_t, 240> unset_larger =
{
  unset_l(0), unset_l(1), unset_l(2), unset_l(3), unset_l(4),
  unset_l(5), unset_l(6), unset_l(7), unset_l(8), unset_l(9),
  unset_l(10), unset_l(11), unset_l(12), unset_l(13), unset_l(14),
  unset_l(15), unset_l(16), unset_l(17), unset_l(18), unset_l(19),
  unset_l(20), unset_l(21), unset_l(22), unset_l(23), unset_l(24),
  unset_l(25), unset_l(26), unset_l(27), unset_l(28), unset_l(29),
  unset_l(30), unset_l(31), unset_l(32), unset_l(33), unset_l(34),
  unset_l(35), unset_l(36), unset_l(37), unset_l(38), unset_l(39),
  unset_l(40), unset_l(41), unset_l(42), unset_l(43), unset_l(44),
  unset_l(45), unset_l(46), unset_l(47), unset_l(48), unset_l(49),
  unset_l(50), unset_l(51), unset_l(52), unset_l(53), unset_l(54),
  unset_l(55), unset_l(56), unset_l(57), unset_l(58), unset_l(59),
  unset_l(60), unset_l(61), unset_l(62), unset_l(63), unset_l(64),
  unset_l(65), unset_l(66), unset_l(67), unset_l(68), unset_l(69),
  unset_l(70), unset_l(71), unset_l(72), unset_l(73), unset_l(74),
  unset_l(75), unset_l(76), unset_l(77), unset_l(78), unset_l(79),
  unset_l(80), unset_l(81), unset_l(82), unset_l(83), unset_l(84),
  unset_l(85), unset_l(86), unset_l(87), unset_l(88), unset_l(89),
  unset_l(90), unset_l(91), unset_l(92), unset_l(93), unset_l(94),
  unset_l(95), unset_l(96), unset_l(97), unset_l(98), unset_l(99),
  unset_l(100), unset_l(101), unset_l(102), unset_l(103), unset_l(104),
  unset_l(105), unset_l(106), unset_l(107), unset_l(108), unset_l(109),
  unset_l(110), unset_l(111), unset_l(112), unset_l(113), unset_l(114),
  unset_l(115), unset_l(116), unset_l(117), unset_l(118), unset_l(119),
  unset_l(120), unset_l(121), unset_l(122), unset_l(123), unset_l(124),
  unset_l(125), unset_l(126), unset_l(127), unset_l(128), unset_l(129),
  unset_l(130), unset_l(131), unset_l(132), unset_l(133), unset_l(134),
  unset_l(135), unset_l(136), unset_l(137), unset_l(138), unset_l(139),
  unset_l(140), unset_l(141), unset_l(142), unset_l(143), unset_l(144),
  unset_l(145), unset_l(146), unset_l(147), unset_l(148), unset_l(149),
  unset_l(150), unset_l(151), unset_l(152), unset_l(153), unset_l(154),
  unset_l(155), unset_l(156), unset_l(157), unset_l(158), unset_l(159),
  unset_l(160), unset_l(161), unset_l(162), unset_l(163), unset_l(164),
  unset_l(165), unset_l(166), unset_l(167), unset_l(168), unset_l(169),
  unset_l(170), unset_l(171), unset_l(172), unset_l(173), unset_l(174),
  unset_l(175), unset_l(176), unset_l(177), unset_l(178), unset_l(179),
  unset_l(180), unset_l(181), unset_l(182), unset_l(183), unset_l(184),
  unset_l(185), unset_l(186), unset_l(187), unset_l(188), unset_l(189),
  unset_l(190), unset_l(191), unset_l(192), unset_l(193), unset_l(194),
  unset_l(195), unset_l(196), unset_l(197), unset_l(198), unset_l(199),
  unset_l(200), unset_l(201), unset_l(202), unset_l(203), unset_l(204),
  unset_l(205), unset_l(206), unset_l(207), unset_l(208), unset_l(209),
  unset_l(210), unset_l(211), unset_l(212), unset_l(213), unset_l(214),
  unset_l(215), unset_l(216), unset_l(217), unset_l(218), unset_l(219),
  unset_l(220), unset_l(221), unset_l(222), unset_l(223), unset_l(224),
  unset_l(225), unset_l(226), unset_l(227), unset_l(228), unset_l(229),
  unset_l(230), unset_l(231), unset_l(232), unset_l(233), unset_l(234),
  unset_l(235), unset_l(236), unset_l(237), unset_l(238), unset_l(239)
};

} // namespace

#endif