playback_buffer.h

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/*
 * Copyright (C) 2000 Paul Davis & Benno Senoner
 * Copyright (C) 2019 Robin Gareus <robin@gareus.org>
 *
 * 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 <cstdint>
#include <cstring>
 
#include "pbd/libpbd_visibility.h"
#include "pbd/mutex.h"
#include "pbd/spinlock.h"
 
namespace PBD {
 
template<class T>
class /*LIBPBD_API*/ PlaybackBuffer
{
public:
        static size_t power_of_two_size (size_t sz) {
                int32_t power_of_two;
                for (power_of_two = 1; 1U << power_of_two < sz; ++power_of_two);
                return 1U << power_of_two;
        }
 
        PlaybackBuffer (size_t sz, size_t res = 8191)
        : reservation (res)
        {
                sz += reservation;
                size = power_of_two_size (sz);
                size_mask = size - 1;
                buf = new T[size];
 
                read_idx.store (0);
                reset ();
        }
 
        virtual ~PlaybackBuffer () {
                delete [] buf;
        }
 
        /* init (mlock) */
        T *buffer () { return buf; }
        /* init (mlock) */
        size_t bufsize () const { return size; }
 
        /* write-thread */
        void reset () {
                /* writer, when seeking, may block */
                PBD::Mutex::Lock lm (_reset_lock);
                SpinLock sl (_reservation_lock);
                read_idx.store (0);
                write_idx.store (0);
                reserved.store (0);
        }
 
        /* called from rt (reader) thread for new buffers */
        void align_to (PlaybackBuffer const& other) {
                PBD::Mutex::Lock lm (_reset_lock);
                read_idx.store (other.read_idx.load());
                write_idx.store (other.write_idx.load());
                reserved.store (other.reserved.load());
                memset (buf, 0, size * sizeof (T));
        }
 
        /* write-thread */
        size_t write_space () const {
                size_t w, r;
 
                w = write_idx.load ();
                r = read_idx.load ();
 
                size_t rv;
 
                if (w > r) {
                        rv = ((r + size) - w) & size_mask;
                } else if (w < r) {
                        rv = (r - w);
                } else {
                        rv = size;
                }
                /* it may hapen that the read/invalidation-pointer moves backwards
                 * e.g. after rec-stop, declick fade-out.
                 * At the same time the butler may already have written data.
                 * (it's safe as long as the disk-reader does not move backwards by more
                 * than reservation)
                 * XXX disk-reading de-click should not move the invalidation-pointer
                 */
                if (rv > reservation) {
                        return rv - 1 - reservation;
                }
                return 0;
        }
 
        /* read-thread */
        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 + size) - r) & size_mask;
                }
        }
 
        /* write thread */
        size_t overwritable_at (size_t r) const {
                size_t w;
 
                w = write_idx.load ();
 
                if (w > r) {
                        return w - r;
                }
                return (w - r + size) & size_mask;
        }
 
        /* read-thead */
        size_t read (T *dest, size_t cnt, bool commit = true, size_t offset = 0);
 
        /* write-thead */
        size_t write (T const * src, size_t cnt);
        /* write-thead */
        size_t write_zero (size_t cnt);
        /* read-thead */
        size_t increment_write_ptr (size_t cnt)
        {
                cnt = std::min (cnt, write_space ());
                write_idx.store ((write_idx.load () + cnt) & size_mask);
                return cnt;
        }
 
        /* read-thead */
        size_t decrement_read_ptr (size_t cnt)
        {
                SpinLock sl (_reservation_lock);
                size_t r = read_idx.load ();
                size_t res = reserved.load ();
 
                cnt = std::min (cnt, res);
 
                r = (r + size - cnt) & size_mask;
                res -= cnt;
 
                read_idx.store (r);
                reserved.store (res);
 
                return cnt;
        }
 
        /* read-thead */
        size_t increment_read_ptr (size_t cnt)
        {
                cnt = std::min (cnt, read_space ());
 
                SpinLock sl (_reservation_lock);
                read_idx.store ((read_idx.load () + cnt) & size_mask);
                reserved.store (std::min (reservation, reserved.load () + cnt));
 
                return cnt;
        }
 
        /* read-thead */
        bool can_seek (int64_t cnt) {
                if (cnt > 0) {
                        return read_space() >= (size_t) cnt;
                } else if (cnt < 0) {
                        return reserved.load () >= (size_t) -cnt;
                } else {
                        return true;
                }
        }
 
        size_t read_ptr() const { return read_idx.load (); }
        size_t write_ptr() const { return write_idx.load (); }
        size_t reserved_size() const { return reserved.load (); }
        size_t reservation_size() const { return reservation; }
 
private:
        T *buf;
        const size_t reservation;
        size_t size;
        size_t size_mask;
 
        mutable std::atomic<size_t> write_idx;
        mutable std::atomic<size_t> read_idx;
        mutable std::atomic<size_t> reserved;
 
        /* spinlock will be used to update write_idx and reserved in sync */
        spinlock_t _reservation_lock;
        /* reset_lock is used to prevent concurrent reading and reset (seek, transport reversal etc). */
        PBD::Mutex _reset_lock;
};
 
template<class T> /*LIBPBD_API*/ size_t
PlaybackBuffer<T>::write (T const *src, size_t cnt)
{
        size_t w = write_idx.load ();
        const size_t free_cnt = write_space ();
 
        if (free_cnt == 0) {
                return 0;
        }
 
        const size_t to_write = cnt > free_cnt ? free_cnt : cnt;
        const size_t cnt2 = w + to_write;
 
        size_t n1, n2;
        if (cnt2 > size) {
                n1 = size - w;
                n2 = cnt2 & size_mask;
        } else {
                n1 = to_write;
                n2 = 0;
        }
 
        memcpy (&buf[w], src, n1 * sizeof (T));
        w = (w + n1) & size_mask;
 
        if (n2) {
                memcpy (buf, src+n1, n2 * sizeof (T));
                w = n2;
        }
 
        write_idx.store (w);
        return to_write;
}
 
template<class T> /*LIBPBD_API*/ size_t
PlaybackBuffer<T>::write_zero (size_t cnt)
{
        size_t w = write_idx.load ();
        const size_t free_cnt = write_space ();
 
        if (free_cnt == 0) {
                return 0;
        }
 
        const size_t to_write = cnt > free_cnt ? free_cnt : cnt;
        const size_t cnt2 = w + to_write;
 
        size_t n1, n2;
        if (cnt2 > size) {
                n1 = size - w;
                n2 = cnt2 & size_mask;
        } else {
                n1 = to_write;
                n2 = 0;
        }
 
        memset (&buf[w], 0, n1 * sizeof (T));
        w = (w + n1) & size_mask;
 
        if (n2) {
                memset (buf, 0, n2 * sizeof (T));
                w = n2;
        }
 
        write_idx.store (w);
        return to_write;
}
 
template<class T> /*LIBPBD_API*/ size_t
PlaybackBuffer<T>::read (T *dest, size_t cnt, bool commit, size_t offset)
{
        PBD::Mutex::Lock lm (_reset_lock, PBD::Mutex::TryLock);
        if (!lm.locked ()) {
                /* seek, reset in progress */
                return 0;
        }
 
        size_t r = read_idx.load ();
        const size_t w = write_idx.load ();
 
        size_t free_cnt = (w > r) ? (w - r) : ((w - r + size) & size_mask);
 
        if (!commit && offset > 0) {
                if (offset > free_cnt) {
                        return 0;
                }
                free_cnt -= offset;
                r = (r + offset) & size_mask;
        }
 
        const size_t to_read = cnt > free_cnt ? free_cnt : cnt;
 
        const size_t cnt2 = r + to_read;
 
        size_t n1, n2;
        if (cnt2 > size) {
                n1 = size - r;
                n2 = cnt2 & size_mask;
        } else {
                n1 = to_read;
                n2 = 0;
        }
 
        memcpy (dest, &buf[r], n1 * sizeof (T));
        r = (r + n1) & size_mask;
 
        if (n2) {
                memcpy (dest + n1, buf, n2 * sizeof (T));
                r = n2;
        }
 
        if (commit) {
                SpinLock sl (_reservation_lock);
                read_idx.store (r);
                reserved.store (std::min (reservation, reserved.load () + to_read));
        }
        return to_read;
}
 
} /* end namespace */