ringbuffer.h

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/*
 * Copyright (C) 2000-2015 Paul Davis <paul@linuxaudiosystems.com>
 * Copyright (C) 2006-2007 Taybin Rutkin <taybin@taybin.com>
 * Copyright (C) 2008-2009 David Robillard <d@drobilla.net>
 * Copyright (C) 2013-2019 Robin Gareus <robin@gareus.org>
 * Copyright (C) 2013 John Emmas <john@creativepost.co.uk>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
#pragma once
 
#include <atomic>
#include <cstring>
 
#include "pbd/libpbd_visibility.h"
 
namespace PBD {
 
template<class T>
class /*LIBPBD_API*/ RingBuffer
{
public:
        RingBuffer (size_t sz) {
#if 0
                size = ffs(sz); /* find first [bit] set is a single inlined assembly instruction. But it looks like the API rounds up so... */
#endif
                size_t power_of_two;
                for (power_of_two = 1; 1U<<power_of_two < sz; power_of_two++) {}
                size = 1<<power_of_two;
                size_mask = size;
                size_mask -= 1;
                buf = new T[size];
                reset ();
        }
 
        virtual ~RingBuffer() {
                delete [] buf;
        }
 
        void reset () {
                /* !!! NOT THREAD SAFE !!! */
                write_idx.store (0);
                read_idx.store (0);
        }
 
        void set (size_t r, size_t w) {
                /* !!! NOT THREAD SAFE !!! */
                write_idx.store (w);
                read_idx.store (r);
        }
 
        size_t read  (T *dest, size_t cnt);
        size_t write (T const * src,  size_t cnt);
 
        struct rw_vector {
                T *buf[2];
                size_t len[2];
        };
 
        void get_read_vector (rw_vector *);
        void get_write_vector (rw_vector *);
 
        void decrement_read_idx (size_t cnt) {
                read_idx.store ((read_idx.load() - cnt) & size_mask);
        }
 
        void increment_read_idx (size_t cnt) {
                read_idx.store ((read_idx.load () + cnt) & size_mask);
        }
 
        void increment_write_idx (size_t cnt) {
                write_idx.store ((write_idx.load () + cnt) & size_mask);
        }
 
        size_t write_space () const {
                size_t w, r;
 
                w = write_idx.load ();
                r = read_idx.load ();
 
                if (w > r) {
                        return ((r - w + size) & size_mask) - 1;
                } else if (w < r) {
                        return (r - w) - 1;
                } else {
                        return size - 1;
                }
        }
 
        size_t read_space () const {
                size_t w, r;
 
                w = write_idx.load ();
                r = read_idx.load ();
 
                if (w > r) {
                        return w - r;
                } else {
                        return (w - r + size) & size_mask;
                }
        }
 
        T *buffer () { return buf; }
        size_t get_write_idx () const { return write_idx.load (); }
        size_t get_read_idx () const { return read_idx.load (); }
        size_t bufsize () const { return size; }
 
protected:
        T *buf;
        size_t size;
        size_t size_mask;
        mutable std::atomic<int> write_idx;
        mutable std::atomic<int> read_idx;
 
private:
        RingBuffer (RingBuffer const&);
};
 
template<class T> /*LIBPBD_API*/ size_t
RingBuffer<T>::read (T *dest, size_t cnt)
{
        size_t free_cnt;
        size_t cnt2;
        size_t to_read;
        size_t n1, n2;
        size_t priv_read_idx;
 
        priv_read_idx = read_idx.load ();
 
        if ((free_cnt = read_space ()) == 0) {
                return 0;
        }
 
        to_read = cnt > free_cnt ? free_cnt : cnt;
 
        cnt2 = priv_read_idx + to_read;
 
        if (cnt2 > size) {
                n1 = size - priv_read_idx;
                n2 = cnt2 & size_mask;
        } else {
                n1 = to_read;
                n2 = 0;
        }
 
        memcpy (dest, &buf[priv_read_idx], n1 * sizeof (T));
        priv_read_idx = (priv_read_idx + n1) & size_mask;
 
        if (n2) {
                memcpy (dest+n1, buf, n2 * sizeof (T));
                priv_read_idx = n2;
        }
 
        read_idx.store (priv_read_idx);
        return to_read;
}
 
template<class T> /*LIBPBD_API*/ size_t
RingBuffer<T>::write (T const *src, size_t cnt)
 
{
        size_t free_cnt;
        size_t cnt2;
        size_t to_write;
        size_t n1, n2;
        size_t priv_write_idx;
 
        priv_write_idx = write_idx.load ();
 
        if ((free_cnt = write_space ()) == 0) {
                return 0;
        }
 
        to_write = cnt > free_cnt ? free_cnt : cnt;
 
        cnt2 = priv_write_idx + to_write;
 
        if (cnt2 > size) {
                n1 = size - priv_write_idx;
                n2 = cnt2 & size_mask;
        } else {
                n1 = to_write;
                n2 = 0;
        }
 
        memcpy (&buf[priv_write_idx], src, n1 * sizeof (T));
        priv_write_idx = (priv_write_idx + n1) & size_mask;
 
        if (n2) {
                memcpy (buf, src+n1, n2 * sizeof (T));
                priv_write_idx = n2;
        }
 
        write_idx.store (priv_write_idx);
        return to_write;
}
 
template<class T> /*LIBPBD_API*/ void
RingBuffer<T>::get_read_vector (typename RingBuffer<T>::rw_vector *vec)
 
{
        size_t free_cnt;
        size_t cnt2;
        size_t w, r;
 
        w = write_idx.load ();
        r = read_idx.load ();
 
        if (w > r) {
                free_cnt = w - r;
        } else {
                free_cnt = (w - r + size) & size_mask;
        }
 
        cnt2 = r + free_cnt;
 
        if (cnt2 > size) {
                /* Two part vector: the rest of the buffer after the
                   current write ptr, plus some from the start of
                   the buffer.
                */
 
                vec->buf[0] = &buf[r];
                vec->len[0] = size - r;
                vec->buf[1] = buf;
                vec->len[1] = cnt2 & size_mask;
 
        } else {
 
                /* Single part vector: just the rest of the buffer */
 
                vec->buf[0] = &buf[r];
                vec->len[0] = free_cnt;
                vec->buf[1] = 0;
                vec->len[1] = 0;
        }
}
 
template<class T> /*LIBPBD_API*/ void
RingBuffer<T>::get_write_vector (typename RingBuffer<T>::rw_vector *vec)
 
{
        size_t free_cnt;
        size_t cnt2;
        size_t w, r;
 
        w = write_idx.load ();
        r = read_idx.load ();
 
        if (w > r) {
                free_cnt = ((r - w + size) & size_mask) - 1;
        } else if (w < r) {
                free_cnt = (r - w) - 1;
        } else {
                free_cnt = size - 1;
        }
 
        cnt2 = w + free_cnt;
 
        if (cnt2 > size) {
 
                /* Two part vector: the rest of the buffer after the
                   current write ptr, plus some from the start of
                   the buffer.
                */
 
                vec->buf[0] = &buf[w];
                vec->len[0] = size - w;
                vec->buf[1] = buf;
                vec->len[1] = cnt2 & size_mask;
        } else {
                vec->buf[0] = &buf[w];
                vec->len[0] = free_cnt;
                vec->len[1] = 0;
        }
}
 
} /* end namespace */