libcruft-util/bitwise.hpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2011-2018 Danny Robson <danny@nerdcruft.net>
 */

#pragma once

#include "debug/assert.hpp"

#include <type_traits>
#include <cstdint>


namespace cruft {
    ///////////////////////////////////////////////////////////////////////////
    /// Rotate `value` left by `magnitude` bits.
    ///
    /// `magnitude` must not be greater than the bit count of `value`.
    template <typename T>
    constexpr T
    rotatel [[gnu::pure]] (const T value, std::size_t magnitude)
    {
        CHECK_LT (magnitude, sizeof (value) * 8u);

        return (value << magnitude) | (value >> (sizeof (value) * 8 - magnitude));
    }


    ///------------------------------------------------------------------------
    /// Rotate `value` right by `magnitude` bits.
    ///
    /// `magnitude` must not be greater than the bit count of `value`.
    template <typename T>
    constexpr T
    rotater [[gnu::pure]] (const T value, std::size_t magnitude)
    {
        CHECK_LT (magnitude, sizeof (value) * 8);

        return (value >> magnitude) | (value << (sizeof (value) * 8 - magnitude));
    }


    ///////////////////////////////////////////////////////////////////////////
    /// Reverse the order of bits in the supplied value.
    ///
    /// Adapted from 'bit twiddling hacks'
    template <typename T>
    constexpr
    std::enable_if_t<std::is_integral<T>::value, T>
    reverse (T src)
    {
        T dst = src; // dst will be reversed bits of v; first get LSB of v

        int bits = sizeof (src) * 8 - 1; // extra shift needed at end
        for (src >>= 1; src; src >>= 1) {
            dst <<= 1;
            dst  |= src & 1;
            bits--;
        }

        dst <<= bits; // shift when src's highest bits are zero
        return dst;
    }


    ///////////////////////////////////////////////////////////////////////////
    /// Reverse the order of bits in the supplied value.
    ///
    /// Adapted from 'bit twiddling hacks'
    template <>
    constexpr
    uint8_t
    reverse (uint8_t val)
    {
        return ((val * 0x80200802ULL) & 0x0884422110ULL) * 0x0101010101ULL >> 32;
    }


    ///////////////////////////////////////////////////////////////////////////
    /// Calculates the number of bits that are set.
    constexpr unsigned
    popcount (unsigned char t)
    {
        return __builtin_popcount (t);
    }


    ///------------------------------------------------------------------------
    /// Calculates the number of bits that are set.
    constexpr unsigned
    popcount (unsigned short t)
    {
        return __builtin_popcount (t);
    }


    ///------------------------------------------------------------------------
    /// Calculates the number of bits that are set.
    constexpr unsigned
    popcount (unsigned t)
    {
        return __builtin_popcount (t);
    }


    ///------------------------------------------------------------------------
    /// Calculates the number of bits that are set.
    constexpr unsigned long
    popcount (unsigned long t)
    {
        return __builtin_popcountl (t);
    }


    ///------------------------------------------------------------------------
    /// Calculates the number of bits that are set.
    constexpr unsigned long long
    popcount (unsigned long long t)
    {
        return __builtin_popcountll (t);
    }


    ///////////////////////////////////////////////////////////////////////////
    /// Count trailing zeroes
    constexpr unsigned int
    ctz (unsigned int x) noexcept
    {
        return __builtin_ctz (x);
    }


    //-------------------------------------------------------------------------
    constexpr unsigned long
    ctz (unsigned long x) noexcept
    {
        return __builtin_ctzl (x);
    }


    //-------------------------------------------------------------------------
    constexpr unsigned long long
    ctz (unsigned long long x) noexcept
    {
        return __builtin_ctzll (x);
    }


    ///////////////////////////////////////////////////////////////////////////
    /// returns the integral value composed of the bits from `val' in the
    /// inclusive range [lo, hi].
    template <typename ValueT, typename IndexT>
    constexpr ValueT
    from_bits (ValueT val, IndexT hi, IndexT lo)
    {
        CHECK_LT (hi, IndexT (sizeof (ValueT) * 8));
        CHECK_LT (lo, IndexT (sizeof (ValueT) * 8));
        CHECK_GE (hi, 0);
        CHECK_GE (lo, 1);
        CHECK_GE (hi, lo);

        ValueT zero = 0;
        ValueT ones = ~zero;

        return (val >> lo) & (ones >> (sizeof (ValueT) * 8 - (hi - lo + 1)));
    }


    /// A convenience wrapper that provides access to a range of bits in an
    /// underlying integral value.
    ///
    /// This class is most useful as part of a union where each has differing
    /// offsets and sizes that cover the underlying type.
    ///
    /// A conversion operator is supplied for accessing the bit value.
    ///
    /// ValueT must support bit shifting and bitwise operators (primarily
    /// bitwise and).
    ///
    /// \tparam ValueT   the underlying integral type
    /// \tparam OffsetV  the index of the first bit of the range (where 0 is
    ///                  least significant)
    /// \tparam SizeV    the number of bits in the range. Must be strictly
    ///                  positive.
    template <
        typename ValueT,
        size_t OffsetV,
        size_t SizeV
    >
    class [[gnu::packed]] bitfield {
    public:
        static_assert (SizeV > 0);
        static_assert (OffsetV + SizeV <= sizeof (ValueT) * 8);

        constexpr ValueT get (void) const
        {
            auto const MASK = ~(~0u << SizeV);
            ValueT shifted = value >> OffsetV;
            ValueT masked = shifted & MASK;
            return masked;
        }

        operator ValueT () const { return get (); }

        auto operator+ () const { return +get (); }

        ValueT value;
    };

    template <typename OperandT, typename BitfieldT, size_t OffsetV, size_t SizeV>
    bool operator== (bitfield<BitfieldT,OffsetV,SizeV> const &a, OperandT const &b)
    {
        return a.get () == b;
    }


    template <typename ValueT, size_t Offset, size_t Size>
    std::ostream& operator<< (std::ostream &os, bitfield<ValueT,Offset,Size> const &val)
    {
        return os << +ValueT(val);
    }
}