from pathlib import Path
import sys
import threading
import subprocess
from typing import AnyStr, IO, Callable


def _chunk_transfer(chunk_file: Path, eof: bool):
    print(f'Transferring chunk {chunk_file} to ... (This is the default method, it has no effect)')
    if eof:
        print(f'The last chunk has been transferred.')


def _rotate_chunk(chunk_file: Path, eof: bool, chunk_transfer_fun: Callable, chunk_transfer_args: tuple):
    print(f'Transferring chunk {chunk_file}')
    chunk_transfer_fun(chunk_file, eof, *chunk_transfer_args)

    print(f'Removing chunk {chunk_file}')
    chunk_file.unlink(missing_ok=False)


def _save_chunk(chunk: bytes, chunk_file: Path):
    print(f'Saving chunk {chunk_file}')
    # Fails if file does already exist.
    with open(chunk_file, 'xb') as f:
        f.write(chunk)


def _save_output(pipe: IO[AnyStr], stdout_dir: Path):
    stdout_dir.mkdir(parents=True, exist_ok=False)

    b: bytes
    ct: int = 1
    for b in pipe:
        stdout_dir.joinpath(str(ct)).write_bytes(b)
        ct += 1

    # TODO: Has this any effect?
    # pipe.close()


def _save_output_rotating_chunks(pipe: IO[AnyStr], chunk_file_tmpl: Path, chunk_size,
                                 chunk_transfer_fun: Callable, chunk_transfer_args: tuple):
    chunk_file_tmpl.parent.mkdir(parents=True, exist_ok=True)

    ct: int = 1
    remaining_bytes = chunk_size
    chunk: bytes = b''
    while True:
        # https://docs.python.org/3/library/io.html#io.RawIOBase.read
        # If 0 bytes are returned, and size was not 0, this indicates end of file.
        # If the object is in non-blocking mode and no bytes are available, None is returned.
        b = pipe.read(remaining_bytes)
        if len(b) == 0:
            # EOF reached.
            chunk_file = chunk_file_tmpl.parent.joinpath(f'{chunk_file_tmpl.name}.{ct}')

            _save_chunk(chunk, chunk_file)
            _rotate_chunk(chunk_file, True, chunk_transfer_fun, chunk_transfer_args)

            break
        chunk += b

        chunk_len = len(chunk)
        if chunk_len == chunk_size:
            # Next chunk is full.
            chunk_file = chunk_file_tmpl.parent.joinpath(f'{chunk_file_tmpl.name}.{ct}')

            _save_chunk(chunk, chunk_file)
            _rotate_chunk(chunk_file, False, chunk_transfer_fun, chunk_transfer_args)

            chunk = b''
            remaining_bytes = chunk_size
            ct += 1
        elif chunk_len < chunk_size:
            remaining_bytes = chunk_size - chunk_len
        else:
            raise ValueError('Invalid state')

    # TODO: Has this any effect?
    # pipe.close()


def _print_output(pipe: IO[AnyStr]):
    line: str
    for line in pipe:
        sys.stderr.write(f'[STDERR] {line}')

    # TODO: Has this any effect?
    # pipe.close()


def execute_print_transfer_chunks(command: list[str],
                                  chunk_file_tmpl: Path,
                                  chunk_transfer_fun: Callable = _chunk_transfer,
                                  chunk_transfer_args: tuple = None,
                                  chunk_size=1024 * 1024,
                                  ) -> int:
    """
    Executes the given command.

    Until the command has finished:

    - saves a small part of the commands stdout to chunk_file = f'{chunk_file_tmpl.name}.{chunk_number}'
    - calls chunk_transfer_fun with arguments (chunk_file, eof_reached, *chunk_transfer args)
      - eof_reached is True for the last chunk
      - the last chunk may be smaller than chunk_size
    - deletes chunk_file

    During command execution: Forwards stderr output of the command to stderr.

    :param chunk_file_tmpl: Chunks are saved as this file before they are transferred.
    :param command: Command to execute, e.g. ['cat', '/some/large/file']
    :param chunk_transfer_fun:
    :param chunk_transfer_args:
    :param chunk_size: Defaults to 1MB (1024*1024).
    :return: returncode of executed command
    """
    if chunk_transfer_args is None:
        chunk_transfer_args = tuple()

    process = subprocess.Popen(
        command,
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE,
        close_fds=True,
    )

    t_out = threading.Thread(
        target=_save_output_rotating_chunks,
        args=(process.stdout, chunk_file_tmpl, chunk_size, chunk_transfer_fun, chunk_transfer_args))
    t_err = threading.Thread(
        target=_print_output, args=(process.stderr,))

    for t in (t_out, t_err):
        t.daemon = True
        t.start()

    returncode = process.wait()
    for t in (t_out, t_err):
        t.join()

    return returncode