asio_coroutine.hpp

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// SPDX-FileCopyrightText: 2026 Jarosław Pelczar <jarek@jpelczar.com>
//
// SPDX-License-Identifier: BSD-2-Clause
 
#pragma once
 
#include "../microfmt.hpp"
#include "../reloco.hpp"
#include "asio_sink.hpp"
#include <boost/asio/awaitable.hpp>
#include <boost/asio/use_awaitable.hpp>
#include <boost/asio/write.hpp>
#include <cstddef>
#include <utility>
 
namespace microfmt::coro {
 
template <typename Stream, std::size_t StackSize = 256, typename... Args>
[[nodiscard]] boost::asio::awaitable<std::size_t> async_format_to(Stream &stream, string_view fmt, Args &&...args) {
  // Allocate a temporary buffer entirely on the coroutine frame / stack.
  char stack_buffer[StackSize];
 
  // Wrap it in our zero-allocation Boost.Asio mutable buffer sink; the sink
  // only borrows stack_buffer, so it must not outlive this coroutine frame.
  asio_mutable_buffer_sink sink(boost::asio::buffer(stack_buffer));
 
  // Perform zero-heap formatting directly into the stack buffer.
  microfmt::format_to(sink.as_sink(), fmt, std::forward<Args>(args)...);
 
  // Asynchronously stream the exact written bytes using C++20 co_await.
  co_return co_await boost::asio::async_write(stream, sink.written_buffer(), boost::asio::use_awaitable);
}
 
} // namespace microfmt::coro