libcruft-util/cruft/util/hash/buzhash.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 2019 Danny Robson <danny@nerdcruft.net>
 */

#pragma once

#include "table.hpp"
#include "../bitwise.hpp"
#include "../std.hpp"
#include "../view.hpp"

#include <cstddef>


///////////////////////////////////////////////////////////////////////////////
namespace cruft::hash {
    /// Implements a rolling hash by using a cyclic polynomial, aka buzhash.
    ///   see: https://en.wikipedia.org/wiki/Rolling_hash#Cyclic_polynomial
    ///
    /// \tparam StateT  The hash state size, and the result type.
    /// \tparam WordT   The data type for each term of the polynomial
    /// \tparam HashT   The object used to hash the words prior to mixing.
    ///     This needs to be very fast for most clients of the algorithm;
    ///     although functionally it could be any functor that maps WordT
    ///     onto StateT.
    template <
        typename StateT,
        typename WordT = u08,
        typename HashT = table<StateT>
    >
    class buzhash {
    public:
        static_assert (sizeof (StateT) >= sizeof (WordT));

        using digest_type = StateT;

        template <typename ...Args>
        buzhash (
            std::size_t _width,
            cruft::view<WordT const*> _init,
            Args &&...args
        )
            : m_width (_width)
            , m_hash (std::forward<Args> (args)...)
        {
            // Zero width would make for a constant zero hash.
            CHECK_NEZ (_width);
            // Rotations greater than data type size are often undefined.
            CHECK_LT  (_width, sizeof (StateT) * 8);

            if (_init.size () < m_width)
                throw std::out_of_range ("buzhash input too small");

            // Prime the initial window
            auto cursor = _init.begin ();
            for (std::size_t i = 1; i <= m_width; i++, cursor++) {
                m_state ^= rotatel (m_hash (*cursor), m_width - i);
            }
        }

        buzhash (buzhash const&) = default;
        buzhash (buzhash &&) noexcept = default;
        buzhash& operator= (buzhash const&) = default;
        buzhash& operator= (buzhash &&) noexcept = default;

        /// Rotate the hash over a pointer to the buffer we've been operating
        /// on.
        ///
        /// The previous `width` bytes _must_ be dereferencable and identical
        /// to the previously observed values.
        StateT operator() (WordT const *cursor)
        {
            return (*this) (cursor[0], *(cursor - m_width));
        }


        /// Rotate a word into the hash, and the corresponding word out of the
        /// hash.
        ///
        /// The `removal` value _must_ be the same as the value seen `width`
        /// values previously.
        StateT operator() (WordT const addition, WordT const removal)
        {
                       // Shift the polynomial
            m_state  = rotatel (m_state, 1)
                       // Remove the data that's about to leave our window.
                    ^  rotatel (m_hash (removal), m_width)
                       // Mix in the new data
                    ^  m_hash (addition);

            return m_state;
        }


        /// An observer for the hash state/value.
        ///
        /// Provided for symmetry with other hash objects.
        constexpr StateT digest (void) const noexcept { return m_state; }
        operator StateT () const { return digest (); }

    private:
        std::size_t m_width;
        HashT m_hash;
        StateT m_state = 0;
    };
}