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include/asio/detail/consuming_buffers.hpp

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+//
+// detail/consuming_buffers.hpp
+// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+//
+// Copyright (c) 2003-2025 Christopher M. Kohlhoff (chris at kohlhoff dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef ASIO_DETAIL_CONSUMING_BUFFERS_HPP
+#define ASIO_DETAIL_CONSUMING_BUFFERS_HPP
+
+#if defined(_MSC_VER) && (_MSC_VER >= 1200)
+# pragma once
+#endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
+
+#include "asio/detail/config.hpp"
+#include <cstddef>
+#include "asio/buffer.hpp"
+#include "asio/detail/buffer_sequence_adapter.hpp"
+#include "asio/detail/limits.hpp"
+#include "asio/registered_buffer.hpp"
+
+#include "asio/detail/push_options.hpp"
+
+namespace asio {
+namespace detail {
+
+// Helper template to determine the maximum number of prepared buffers.
+template <typename Buffers>
+struct prepared_buffers_max
+{
+  enum { value = buffer_sequence_adapter_base::max_buffers };
+};
+
+template <typename Elem, std::size_t N>
+struct prepared_buffers_max<boost::array<Elem, N>>
+{
+  enum { value = N };
+};
+
+template <typename Elem, std::size_t N>
+struct prepared_buffers_max<std::array<Elem, N>>
+{
+  enum { value = N };
+};
+
+// A buffer sequence used to represent a subsequence of the buffers.
+template <typename Buffer, std::size_t MaxBuffers>
+struct prepared_buffers
+{
+  typedef Buffer value_type;
+  typedef const Buffer* const_iterator;
+
+  enum { max_buffers = MaxBuffers < 16 ? MaxBuffers : 16 };
+
+  prepared_buffers() : count(0) {}
+  const_iterator begin() const { return elems; }
+  const_iterator end() const { return elems + count; }
+
+  Buffer elems[max_buffers];
+  std::size_t count;
+};
+
+// A proxy for a sub-range in a list of buffers.
+template <typename Buffer, typename Buffers, typename Buffer_Iterator>
+class consuming_buffers
+{
+public:
+  typedef prepared_buffers<Buffer, prepared_buffers_max<Buffers>::value>
+    prepared_buffers_type;
+
+  // Construct to represent the entire list of buffers.
+  explicit consuming_buffers(const Buffers& buffers)
+    : buffers_(buffers),
+      total_consumed_(0),
+      next_elem_(0),
+      next_elem_offset_(0)
+  {
+    using asio::buffer_size;
+    total_size_ = buffer_size(buffers);
+  }
+
+  // Determine if we are at the end of the buffers.
+  bool empty() const
+  {
+    return total_consumed_ >= total_size_;
+  }
+
+  // Get the buffer for a single transfer, with a size.
+  prepared_buffers_type prepare(std::size_t max_size)
+  {
+    prepared_buffers_type result;
+
+    Buffer_Iterator next = asio::buffer_sequence_begin(buffers_);
+    Buffer_Iterator end = asio::buffer_sequence_end(buffers_);
+
+    std::advance(next, next_elem_);
+    std::size_t elem_offset = next_elem_offset_;
+    while (next != end && max_size > 0 && (result.count) < result.max_buffers)
+    {
+      Buffer next_buf = Buffer(*next) + elem_offset;
+      result.elems[result.count] = asio::buffer(next_buf, max_size);
+      max_size -= result.elems[result.count].size();
+      elem_offset = 0;
+      if (result.elems[result.count].size() > 0)
+        ++result.count;
+      ++next;
+    }
+
+    return result;
+  }
+
+  // Consume the specified number of bytes from the buffers.
+  void consume(std::size_t size)
+  {
+    total_consumed_ += size;
+
+    Buffer_Iterator next = asio::buffer_sequence_begin(buffers_);
+    Buffer_Iterator end = asio::buffer_sequence_end(buffers_);
+
+    std::advance(next, next_elem_);
+    while (next != end && size > 0)
+    {
+      Buffer next_buf = Buffer(*next) + next_elem_offset_;
+      if (size < next_buf.size())
+      {
+        next_elem_offset_ += size;
+        size = 0;
+      }
+      else
+      {
+        size -= next_buf.size();
+        next_elem_offset_ = 0;
+        ++next_elem_;
+        ++next;
+      }
+    }
+  }
+
+  // Get the total number of bytes consumed from the buffers.
+  std::size_t total_consumed() const
+  {
+    return total_consumed_;
+  }
+
+private:
+  Buffers buffers_;
+  std::size_t total_size_;
+  std::size_t total_consumed_;
+  std::size_t next_elem_;
+  std::size_t next_elem_offset_;
+};
+
+// Base class of all consuming_buffers specialisations for single buffers.
+template <typename Buffer>
+class consuming_single_buffer
+{
+public:
+  // Construct to represent the entire list of buffers.
+  template <typename Buffer1>
+  explicit consuming_single_buffer(const Buffer1& buffer)
+    : buffer_(buffer),
+      total_consumed_(0)
+  {
+  }
+
+  // Determine if we are at the end of the buffers.
+  bool empty() const
+  {
+    return total_consumed_ >= buffer_.size();
+  }
+
+  // Get the buffer for a single transfer, with a size.
+  Buffer prepare(std::size_t max_size)
+  {
+    return asio::buffer(buffer_ + total_consumed_, max_size);
+  }
+
+  // Consume the specified number of bytes from the buffers.
+  void consume(std::size_t size)
+  {
+    total_consumed_ += size;
+  }
+
+  // Get the total number of bytes consumed from the buffers.
+  std::size_t total_consumed() const
+  {
+    return total_consumed_;
+  }
+
+private:
+  Buffer buffer_;
+  std::size_t total_consumed_;
+};
+
+template <>
+class consuming_buffers<mutable_buffer, mutable_buffer, const mutable_buffer*>
+  : public consuming_single_buffer<mutable_buffer>
+{
+public:
+  explicit consuming_buffers(const mutable_buffer& buffer)
+    : consuming_single_buffer<mutable_buffer>(buffer)
+  {
+  }
+};
+
+template <>
+class consuming_buffers<const_buffer, mutable_buffer, const mutable_buffer*>
+  : public consuming_single_buffer<const_buffer>
+{
+public:
+  explicit consuming_buffers(const mutable_buffer& buffer)
+    : consuming_single_buffer<const_buffer>(buffer)
+  {
+  }
+};
+
+template <>
+class consuming_buffers<const_buffer, const_buffer, const const_buffer*>
+  : public consuming_single_buffer<const_buffer>
+{
+public:
+  explicit consuming_buffers(const const_buffer& buffer)
+    : consuming_single_buffer<const_buffer>(buffer)
+  {
+  }
+};
+
+template <>
+class consuming_buffers<mutable_buffer,
+    mutable_registered_buffer, const mutable_buffer*>
+  : public consuming_single_buffer<mutable_registered_buffer>
+{
+public:
+  explicit consuming_buffers(const mutable_registered_buffer& buffer)
+    : consuming_single_buffer<mutable_registered_buffer>(buffer)
+  {
+  }
+};
+
+template <>
+class consuming_buffers<const_buffer,
+    mutable_registered_buffer, const mutable_buffer*>
+  : public consuming_single_buffer<mutable_registered_buffer>
+{
+public:
+  explicit consuming_buffers(const mutable_registered_buffer& buffer)
+    : consuming_single_buffer<mutable_registered_buffer>(buffer)
+  {
+  }
+};
+
+template <>
+class consuming_buffers<const_buffer,
+    const_registered_buffer, const const_buffer*>
+  : public consuming_single_buffer<const_registered_buffer>
+{
+public:
+  explicit consuming_buffers(const const_registered_buffer& buffer)
+    : consuming_single_buffer<const_registered_buffer>(buffer)
+  {
+  }
+};
+
+template <typename Buffer, typename Elem>
+class consuming_buffers<Buffer, boost::array<Elem, 2>,
+    typename boost::array<Elem, 2>::const_iterator>
+{
+public:
+  // Construct to represent the entire list of buffers.
+  explicit consuming_buffers(const boost::array<Elem, 2>& buffers)
+    : buffers_(buffers),
+      total_consumed_(0)
+  {
+  }
+
+  // Determine if we are at the end of the buffers.
+  bool empty() const
+  {
+    return total_consumed_ >=
+      Buffer(buffers_[0]).size() + Buffer(buffers_[1]).size();
+  }
+
+  // Get the buffer for a single transfer, with a size.
+  boost::array<Buffer, 2> prepare(std::size_t max_size)
+  {
+    boost::array<Buffer, 2> result = {{
+      Buffer(buffers_[0]), Buffer(buffers_[1]) }};
+    std::size_t buffer0_size = result[0].size();
+    result[0] = asio::buffer(result[0] + total_consumed_, max_size);
+    result[1] = asio::buffer(
+        result[1] + (total_consumed_ < buffer0_size
+          ? 0 : total_consumed_ - buffer0_size),
+        max_size - result[0].size());
+    return result;
+  }
+
+  // Consume the specified number of bytes from the buffers.
+  void consume(std::size_t size)
+  {
+    total_consumed_ += size;
+  }
+
+  // Get the total number of bytes consumed from the buffers.
+  std::size_t total_consumed() const
+  {
+    return total_consumed_;
+  }
+
+private:
+  boost::array<Elem, 2> buffers_;
+  std::size_t total_consumed_;
+};
+
+template <typename Buffer, typename Elem>
+class consuming_buffers<Buffer, std::array<Elem, 2>,
+    typename std::array<Elem, 2>::const_iterator>
+{
+public:
+  // Construct to represent the entire list of buffers.
+  explicit consuming_buffers(const std::array<Elem, 2>& buffers)
+    : buffers_(buffers),
+      total_consumed_(0)
+  {
+  }
+
+  // Determine if we are at the end of the buffers.
+  bool empty() const
+  {
+    return total_consumed_ >=
+      Buffer(buffers_[0]).size() + Buffer(buffers_[1]).size();
+  }
+
+  // Get the buffer for a single transfer, with a size.
+  std::array<Buffer, 2> prepare(std::size_t max_size)
+  {
+    std::array<Buffer, 2> result = {{
+      Buffer(buffers_[0]), Buffer(buffers_[1]) }};
+    std::size_t buffer0_size = result[0].size();
+    result[0] = asio::buffer(result[0] + total_consumed_, max_size);
+    result[1] = asio::buffer(
+        result[1] + (total_consumed_ < buffer0_size
+          ? 0 : total_consumed_ - buffer0_size),
+        max_size - result[0].size());
+    return result;
+  }
+
+  // Consume the specified number of bytes from the buffers.
+  void consume(std::size_t size)
+  {
+    total_consumed_ += size;
+  }
+
+  // Get the total number of bytes consumed from the buffers.
+  std::size_t total_consumed() const
+  {
+    return total_consumed_;
+  }
+
+private:
+  std::array<Elem, 2> buffers_;
+  std::size_t total_consumed_;
+};
+
+// Specialisation for null_buffers to ensure that the null_buffers type is
+// always passed through to the underlying read or write operation.
+template <typename Buffer>
+class consuming_buffers<Buffer, null_buffers, const mutable_buffer*>
+  : public asio::null_buffers
+{
+public:
+  consuming_buffers(const null_buffers&)
+  {
+    // No-op.
+  }
+
+  bool empty()
+  {
+    return false;
+  }
+
+  null_buffers prepare(std::size_t)
+  {
+    return null_buffers();
+  }
+
+  void consume(std::size_t)
+  {
+    // No-op.
+  }
+
+  std::size_t total_consumed() const
+  {
+    return 0;
+  }
+};
+
+} // namespace detail
+} // namespace asio
+
+#include "asio/detail/pop_options.hpp"
+
+#endif // ASIO_DETAIL_CONSUMING_BUFFERS_HPP