"""
Stdio Server Transport Module

This module provides functionality for creating an stdio-based transport layer
that can be used to communicate with an MCP client through standard input/output
streams.

Example usage:
```
    async def run_server():
        async with stdio_server() as (read_stream, write_stream):
            # read_stream contains incoming JSONRPCMessages from stdin
            # write_stream allows sending JSONRPCMessages to stdout
            server = await create_my_server()
            await server.run(read_stream, write_stream, init_options)

    anyio.run(run_server)
```
"""

import os
import sys
from contextlib import asynccontextmanager
from io import TextIOWrapper

import anyio
import anyio.lowlevel
from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream

from mcp import types
from mcp.shared.message import SessionMessage

# Chunk size for non-blocking stdout writes.  Small enough to avoid
# filling the OS pipe buffer (64 KB on macOS) in a single syscall.
_WRITE_CHUNK = 4096


async def _write_nonblocking(fd: int, data: bytes) -> None:
    """Write *data* to a non-blocking fd, yielding on EAGAIN.

    Writes in small chunks so the event loop stays responsive even when
    the MCP client reads slowly and the pipe buffer fills up.
    """
    mv = memoryview(data)
    while mv:
        try:
            n = os.write(fd, mv[:_WRITE_CHUNK])
            mv = mv[n:]
        except BlockingIOError:
            # Pipe full — yield to event loop and retry.
            await anyio.sleep(0.005)


@asynccontextmanager
async def stdio_server(
    stdin: anyio.AsyncFile[str] | None = None,
    stdout: anyio.AsyncFile[str] | None = None,
):
    """
    Server transport for stdio: this communicates with an MCP client by reading
    from the current process' stdin and writing to stdout.
    """
    # Purposely not using context managers for these, as we don't want to close
    # standard process handles. Encoding of stdin/stdout as text streams on
    # python is platform-dependent (Windows is particularly problematic), so we
    # re-wrap the underlying binary stream to ensure UTF-8.
    if not stdin:
        stdin = anyio.wrap_file(TextIOWrapper(sys.stdin.buffer, encoding="utf-8"))

    # For the default stdout (no custom override), use non-blocking I/O
    # directly on the file descriptor to prevent the event loop from
    # blocking when the OS pipe buffer is full (macOS: 64 KB).
    stdout_fd: int | None = None
    if not stdout:
        stdout_fd = sys.stdout.buffer.fileno()
        os.set_blocking(stdout_fd, False)
        # Still create the wrapped stdout for the type signature / fallback
        stdout = anyio.wrap_file(TextIOWrapper(sys.stdout.buffer, encoding="utf-8"))

    read_stream: MemoryObjectReceiveStream[SessionMessage | Exception]
    read_stream_writer: MemoryObjectSendStream[SessionMessage | Exception]

    write_stream: MemoryObjectSendStream[SessionMessage]
    write_stream_reader: MemoryObjectReceiveStream[SessionMessage]

    read_stream_writer, read_stream = anyio.create_memory_object_stream(0)
    write_stream, write_stream_reader = anyio.create_memory_object_stream(0)

    async def stdin_reader():
        try:
            async with read_stream_writer:
                async for line in stdin:
                    try:
                        message = types.JSONRPCMessage.model_validate_json(line)
                    except Exception as exc:  # pragma: no cover
                        await read_stream_writer.send(exc)
                        continue

                    session_message = SessionMessage(message)
                    await read_stream_writer.send(session_message)
        except anyio.ClosedResourceError:  # pragma: no cover
            await anyio.lowlevel.checkpoint()

    async def stdout_writer():
        try:
            async with write_stream_reader:
                async for session_message in write_stream_reader:
                    json_str = session_message.message.model_dump_json(
                        by_alias=True, exclude_none=True
                    )
                    if stdout_fd is not None:
                        # Non-blocking write directly to fd — never blocks
                        # the event loop, yields on pipe-full (EAGAIN).
                        await _write_nonblocking(
                            stdout_fd, (json_str + "\n").encode("utf-8")
                        )
                    else:
                        # Custom stdout provided — use original path.
                        await stdout.write(json_str + "\n")
                        await stdout.flush()
        except anyio.ClosedResourceError:  # pragma: no cover
            await anyio.lowlevel.checkpoint()

    async with anyio.create_task_group() as tg:
        tg.start_soon(stdin_reader)
        tg.start_soon(stdout_writer)
        yield read_stream, write_stream
