etl/include/etl/bip_buffer_spsc_atomic.h

///\file

/******************************************************************************
The MIT License(MIT)

Embedded Template Library.
https://github.com/ETLCPP/etl
https://www.etlcpp.com

Copyright(c) 2021 Benedek Kupper, John Wellbelove

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/**
 * @note Based on the works of Andrea Lattuada and James Munns:
 * https://blog.systems.ethz.ch/blog/2019/the-design-and-implementation-of-a-lock-free-ring-buffer-with-contiguous-reservations.html
 * Whose design was inspired by Simon Cooke:
 * https://www.codeproject.com/Articles/3479/The-Bip-Buffer-The-Circular-Buffer-with-a-Twist
 */

#ifndef ETL_BIP_BUFFER_SPSC_ATOMIC_INCLUDED
#define ETL_BIP_BUFFER_SPSC_ATOMIC_INCLUDED

#include "platform.h"
#include "alignment.h"
#include "atomic.h"
#include "error_handler.h"
#include "file_error_numbers.h"
#include "integral_limits.h"
#include "memory.h"
#include "memory_model.h"
#include "parameter_type.h"
#include "span.h"
#include "utility.h"

#include <stddef.h>

#if ETL_HAS_ATOMIC

namespace etl
{
  //***************************************************************************
  /// Base exception for a bip buffer.
  //***************************************************************************
  class bip_buffer_exception : public exception
  {
  public:

    bip_buffer_exception(string_type reason_, string_type file_name_, numeric_type line_number_)
      : exception(reason_, file_name_, line_number_)
    {
    }
  };

  //***************************************************************************
  /// Exception for an invalid reserve.
  //***************************************************************************
  class bip_buffer_reserve_invalid : public bip_buffer_exception
  {
  public:

    bip_buffer_reserve_invalid(string_type file_name_, numeric_type line_number_)
      : bip_buffer_exception(ETL_ERROR_TEXT("bip_buffer:reserve", ETL_BIP_BUFFER_SPSC_ATOMIC_FILE_ID"A"), file_name_, line_number_)
    {
    }
  };

  //***************************************************************************
  /// The common base for a bip_buffer_spsc_atomic_base.
  //***************************************************************************
  template <size_t Memory_Model = etl::memory_model::MEMORY_MODEL_LARGE>
  class bip_buffer_spsc_atomic_base
  {
  public:

    /// The type used for determining the size of buffer.
    typedef typename etl::size_type_lookup<Memory_Model>::type size_type;

    //*************************************************************************
    /// Returns true if the buffer is empty.
    //*************************************************************************
    bool empty() const
    {
      return size() == 0;
    }

    //*************************************************************************
    /// Returns true if the buffer is full.
    //*************************************************************************
    bool full() const
    {
      return available() == 0;
    }

    //*************************************************************************
    /// Returns the total used size, which may be split in two blocks
    /// so the size will always be smaller after a read commit.
    //*************************************************************************
    size_type size() const
    {
      size_type write_index = write.load(etl::memory_order_acquire);
      size_type read_index  = read.load(etl::memory_order_acquire);

      // no wraparound
      if (write_index >= read_index)
      {
        // size is distance between read and write
        return write_index - read_index;
      }
      else
      {
        size_type last_index = last.load(etl::memory_order_acquire);

        // size is distance between beginning and write, plus read and last
        return (write_index - 0) + (last_index - read_index);
      }
    }

    //*************************************************************************
    /// Returns the largest contiguous available block size.
    //*************************************************************************
    size_type available() const
    {
      size_type write_index = write.load(etl::memory_order_acquire);
      size_type read_index  = read.load(etl::memory_order_acquire);

      // no wraparound
      if (write_index >= read_index)
      {
        size_type forward_size = capacity() - write_index;

        // check if there's more space if wrapping around
        if (read_index > (forward_size + 1))
        {
          return read_index - 1;
        }
        else
        {
          return forward_size;
        }
      }
      else // read_index > write_index
      {
        return read_index - write_index - 1;
      }
    }

    //*************************************************************************
    /// Returns the maximum capacity of the buffer.
    //*************************************************************************
    size_type capacity() const
    {
      return Reserved;
    }

    //*************************************************************************
    /// Returns the maximum size of the buffer.
    //*************************************************************************
    size_type max_size() const
    {
      return Reserved;
    }

  protected:

    //*************************************************************************
    /// Constructs the buffer.
    //*************************************************************************
    bip_buffer_spsc_atomic_base(size_type reserved_)
      : read(0)
      , write(0)
      , last(0)
      , Reserved(reserved_)
    {
    }

    //*************************************************************************
    void reset()
    {
      read.store(0, etl::memory_order_release);
      write.store(0, etl::memory_order_release);
      last.store(0, etl::memory_order_release);
    }

    //*************************************************************************
    size_type get_write_reserve(size_type* psize, size_type fallback_size = numeric_limits<size_type>::max())
    {
      size_type write_index = write.load(etl::memory_order_relaxed);
      size_type read_index  = read.load(etl::memory_order_acquire);

      // No wraparound
      if (write_index >= read_index)
      {
        size_type forward_size = capacity() - write_index;

        // We still fit in linearly
        if (*psize <= forward_size)
        {
          return write_index;
        }
        // There isn't more space even when wrapping around,
        // or the linear size is good enough as fallback
        else if ((read_index <= (forward_size + 1)) || (fallback_size <= forward_size))
        {
          *psize = forward_size;
          return write_index;
        }
        // Better wrap around now
        else
        {
          // Check if size fits.
          // When wrapping, the write index cannot reach read index,
          // then we'd not be able to distinguish wrapped situation from linear.
          if (*psize >= read_index)
          {
            if (read_index > 0)
            {
              *psize = read_index - 1;
            }
            else
            {
              *psize = 0;
            }
          }

          return 0;
        }
      }
      else // read_index > write_index
      {
        // Doesn't fit
        if (*psize >= read_index - write_index)
        {
          *psize = read_index - write_index - 1;
        }

        return write_index;
      }
    }

    //*************************************************************************
    void apply_write_reserve(size_type windex, size_type wsize)
    {
      if (wsize > 0)
      {
        size_type write_index = write.load(etl::memory_order_relaxed);
        size_type read_index  = read.load(etl::memory_order_acquire);

        // Wrapped around already
        if (write_index < read_index)
        {
          ETL_ASSERT_OR_RETURN((windex == write_index) && ((wsize + 1) <= read_index), ETL_ERROR(bip_buffer_reserve_invalid));
        }
        // No wraparound so far, also not wrapping around with this block
        else if (windex == write_index)
        {
          ETL_ASSERT_OR_RETURN(wsize <= (capacity() - write_index), ETL_ERROR(bip_buffer_reserve_invalid));

          // Move both indexes forward
          last.store(windex + wsize, etl::memory_order_release);
        }
        // Wrapping around now
        else
        {
          ETL_ASSERT_OR_RETURN((windex == 0) && ((wsize + 1) <= read_index), ETL_ERROR(bip_buffer_reserve_invalid));

          // Correct wrapping point
          last.store(write_index, etl::memory_order_release);
        }

        // Always update write index
        write.store(windex + wsize, etl::memory_order_release);
      }
    }

    //*************************************************************************
    size_type get_read_reserve(size_type* psize)
    {
      size_type read_index  = read.load(etl::memory_order_relaxed);
      size_type write_index = write.load(etl::memory_order_acquire);

      if (read_index > write_index)
      {
        // Writer has wrapped around
        size_type last_index = last.load(etl::memory_order_relaxed);

        if (read_index == last_index)
        {
          // Reader reached the end, start read from 0
          read_index = 0;
        }
        else // (read_index < last_index)
        {
          // Use the remaining buffer at the end
          write_index = last_index;
        }
      }
      else
      {
        // No wraparound, nothing to adjust
      }

      // Limit to max available size
      if ((write_index - read_index) < *psize)
      {
        *psize = write_index - read_index;
      }

      return read_index;
    }

    //*************************************************************************
    void apply_read_reserve(size_type rindex, size_type rsize)
    {
      if (rsize > 0)
      {
        size_type rsize_checker = rsize;
        ETL_ASSERT_OR_RETURN((rindex == get_read_reserve(&rsize_checker)) && (rsize == rsize_checker), ETL_ERROR(bip_buffer_reserve_invalid));

        read.store(rindex + rsize, etl::memory_order_release);
      }
    }

  private:

    etl::atomic<size_type> read;
    etl::atomic<size_type> write;
    etl::atomic<size_type> last;
    const size_type        Reserved;

  #if defined(ETL_POLYMORPHIC_SPSC_BIP_BUFFER_ATOMIC) || defined(ETL_POLYMORPHIC_CONTAINERS)

  public:

    virtual ~bip_buffer_spsc_atomic_base() {}
  #else

  protected:

    ~bip_buffer_spsc_atomic_base() {}
  #endif
  };

  //***************************************************************************
  /// A fixed capacity bipartite buffer.
  //***************************************************************************
  template <typename T, const size_t Memory_Model = etl::memory_model::MEMORY_MODEL_LARGE>
  class ibip_buffer_spsc_atomic : public bip_buffer_spsc_atomic_base<Memory_Model>
  {
  private:

    typedef typename etl::bip_buffer_spsc_atomic_base<Memory_Model> base_t;
    using base_t::apply_read_reserve;
    using base_t::apply_write_reserve;
    using base_t::get_read_reserve;
    using base_t::get_write_reserve;
    using base_t::reset;

  public:

    typedef T        value_type;      ///< The type stored in the buffer.
    typedef T&       reference;       ///< A reference to the type used in the buffer.
    typedef const T& const_reference; ///< A const reference to the type used in the buffer.
  #if ETL_USING_CPP11
    typedef T&& rvalue_reference; ///< An rvalue_reference to the type used in the buffer.
  #endif
    typedef typename base_t::size_type size_type; ///< The type used for determining the size of the buffer.

    using base_t::max_size;

    //*************************************************************************
    // Reserves a memory area for reading (up to the max_reserve_size).
    //*************************************************************************
    span<T> read_reserve(size_type max_reserve_size = numeric_limits<size_type>::max())
    {
      size_type reserve_size = max_reserve_size;
      size_type rindex       = get_read_reserve(&reserve_size);

      return span<T>(p_buffer + rindex, reserve_size);
    }

    //*************************************************************************
    // Commits the previously reserved read memory area
    // the reserve can be trimmed at the end before committing.
    // Throws bip_buffer_reserve_invalid
    //*************************************************************************
    void read_commit(const span<T>& reserve)
    {
      size_type rindex = static_cast<size_type>(etl::distance(p_buffer, reserve.data()));
      apply_read_reserve(rindex, reserve.size());
    }

    //*************************************************************************
    // Reserves a memory area for writing up to the max_reserve_size.
    //*************************************************************************
    span<T> write_reserve(size_type max_reserve_size)
    {
      size_type reserve_size = max_reserve_size;
      size_type windex       = get_write_reserve(&reserve_size);

      return span<T>(p_buffer + windex, reserve_size);
    }

    //*************************************************************************
    // Reserves an optimal memory area for writing. The buffer will only wrap
    // around if the available forward space is less than min_reserve_size.
    //*************************************************************************
    span<T> write_reserve_optimal(size_type min_reserve_size = 1U)
    {
      size_type reserve_size = numeric_limits<size_type>::max();
      size_type windex       = get_write_reserve(&reserve_size, min_reserve_size);

      return span<T>(p_buffer + windex, reserve_size);
    }

    //*************************************************************************
    // Commits the previously reserved write memory area
    // the reserve can be trimmed at the end before committing.
    // Throws bip_buffer_reserve_invalid
    //*************************************************************************
    void write_commit(const span<T>& reserve)
    {
      size_type windex = static_cast<size_type>(etl::distance(p_buffer, reserve.data()));
      apply_write_reserve(windex, reserve.size());
    }

    //*************************************************************************
    /// Clears the buffer, destructing any elements that haven't been read.
    //*************************************************************************
    void clear()
    {
      // the buffer might be split into two contiguous blocks
      for (span<T> reader = read_reserve(); reader.size() > 0; reader = read_reserve())
      {
        destroy(reader.begin(), reader.end());
        read_commit(reader);
      }
      // now the buffer is already empty
      // resetting the buffer here is beneficial to have
      // the whole buffer available for a single block,
      // but it requires synchronization between the writer and reader threads
      reset();
    }

  protected:

    //*************************************************************************
    ibip_buffer_spsc_atomic(T* p_buffer_, size_type reserved_)
      : base_t(reserved_)
      , p_buffer(p_buffer_)
    {
    }

  private:

    // Disable copy construction and assignment.
    ibip_buffer_spsc_atomic(const ibip_buffer_spsc_atomic&) ETL_DELETE;
    ibip_buffer_spsc_atomic& operator=(const ibip_buffer_spsc_atomic&) ETL_DELETE;

  #if ETL_USING_CPP11
    ibip_buffer_spsc_atomic(ibip_buffer_spsc_atomic&&)            = delete;
    ibip_buffer_spsc_atomic& operator=(ibip_buffer_spsc_atomic&&) = delete;
  #endif

    T* const p_buffer;
  };

  //***************************************************************************
  /// A fixed capacity bipartite buffer.
  /// This buffer supports concurrent access by one producer and one consumer.
  /// \tparam T            The type this buffer should support.
  /// \tparam Size         The maximum capacity of the buffer.
  /// \tparam Memory_Model The memory model for the buffer. Determines the type
  /// of the internal counter variables.
  //***************************************************************************
  template <typename T, const size_t Size, const size_t Memory_Model = etl::memory_model::MEMORY_MODEL_LARGE>
  class bip_buffer_spsc_atomic : public ibip_buffer_spsc_atomic<T, Memory_Model>
  {
  private:

    typedef typename etl::ibip_buffer_spsc_atomic<T, Memory_Model> base_t;

  public:

    typedef typename base_t::size_type size_type;

  private:

    static ETL_CONSTANT size_type Reserved_Size = size_type(Size);

  public:

    ETL_STATIC_ASSERT((Size <= (etl::integral_limits<size_type>::max)), "Size too large for memory model");

    static ETL_CONSTANT size_type MAX_SIZE = size_type(Size);

    //*************************************************************************
    /// Default constructor.
    //*************************************************************************
    bip_buffer_spsc_atomic()
      : base_t(reinterpret_cast<T*>(buffer.raw), Reserved_Size)
    {
    }

    //*************************************************************************
    /// Destructor.
    //*************************************************************************
    ~bip_buffer_spsc_atomic()
    {
      base_t::clear();
    }

  private:

    /// The uninitialised buffer of T used in the bip_buffer_spsc.
    etl::uninitialized_buffer_of<T, Reserved_Size> buffer;
  };

  template <typename T, const size_t Size, const size_t Memory_Model>
  ETL_CONSTANT typename bip_buffer_spsc_atomic<T, Size, Memory_Model>::size_type bip_buffer_spsc_atomic<T, Size, Memory_Model>::Reserved_Size;
} // namespace etl

#endif /* ETL_HAS_ATOMIC && ETL_USING_CPP11 */

#endif /* ETL_BIP_BUFFER_SPSC_ATOMIC_INCLUDED */