from __future__ import annotations

import io
from collections import deque
from typing import Callable, Deque, Generator, Generic, Iterable, TypeVar, Union


class ParseError(Exception):
    pass


class ParseEOF(ParseError):
    """End of Stream."""


class Awaitable:
    __slots__: list[str] = []


class _Read(Awaitable):
    __slots__ = ["remaining"]

    def __init__(self, count: int) -> None:
        self.remaining = count

    def __repr__(self) -> str:
        return f"_ReadBytes({self.remaining})"


class _Read1(Awaitable):
    __slots__: list[str] = []


class _ReadUntil(Awaitable):
    __slots__ = ["sep", "max_bytes"]

    def __init__(self, sep: str, max_bytes: int | None = None) -> None:
        self.sep = sep
        self.max_bytes = max_bytes


class _PeekBuffer(Awaitable):
    __slots__: list[str] = []


T = TypeVar("T")


TokenCallback = Callable[[T], None]


class Parser(Generic[T]):
    read = _Read
    read1 = _Read1
    read_until = _ReadUntil
    peek_buffer = _PeekBuffer

    def __init__(self) -> None:
        self._buffer = io.StringIO()
        self._eof = False
        self._tokens: Deque[T] = deque()
        self._gen = self.parse(self._tokens.append)
        self._awaiting: Union[Awaitable, T] = next(self._gen)

    @property
    def is_eof(self) -> bool:
        return self._eof

    def reset(self) -> None:
        self._gen = self.parse(self._tokens.append)
        self._awaiting = next(self._gen)

    def feed(self, data: str) -> Iterable[T]:
        if self._eof:
            raise ParseError("end of file reached") from None
        if not data:
            self._eof = True
            try:
                self._gen.send(self._buffer.getvalue())
            except StopIteration:
                raise ParseError("end of file reached") from None
            while self._tokens:
                yield self._tokens.popleft()

            self._buffer.truncate(0)
            return

        _buffer = self._buffer
        pos = 0
        tokens = self._tokens
        popleft = tokens.popleft
        data_size = len(data)

        while tokens:
            yield popleft()

        while pos < data_size or isinstance(self._awaiting, _PeekBuffer):
            _awaiting = self._awaiting
            if isinstance(_awaiting, _Read1):
                self._awaiting = self._gen.send(data[pos : pos + 1])
                pos += 1

            elif isinstance(_awaiting, _PeekBuffer):
                self._awaiting = self._gen.send(data[pos:])

            elif isinstance(_awaiting, _Read):
                remaining = _awaiting.remaining
                chunk = data[pos : pos + remaining]
                chunk_size = len(chunk)
                pos += chunk_size
                _buffer.write(chunk)
                remaining -= chunk_size
                if remaining:
                    _awaiting.remaining = remaining
                else:
                    _awaiting = self._gen.send(_buffer.getvalue())
                    _buffer.seek(0)
                    _buffer.truncate()

            elif isinstance(_awaiting, _ReadUntil):
                chunk = data[pos:]
                _buffer.write(chunk)
                sep = _awaiting.sep
                sep_index = _buffer.getvalue().find(sep)

                if sep_index == -1:
                    pos += len(chunk)
                    if (
                        _awaiting.max_bytes is not None
                        and _buffer.tell() > _awaiting.max_bytes
                    ):
                        self._gen.throw(ParseError(f"expected {sep}"))
                else:
                    sep_index += len(sep)
                    if (
                        _awaiting.max_bytes is not None
                        and sep_index > _awaiting.max_bytes
                    ):
                        self._gen.throw(ParseError(f"expected {sep}"))
                    data = _buffer.getvalue()[sep_index:]
                    pos = 0
                    self._awaiting = self._gen.send(_buffer.getvalue()[:sep_index])
                    _buffer.seek(0)
                    _buffer.truncate()

            while tokens:
                yield popleft()

    def parse(self, on_token: Callable[[T], None]) -> Generator[Awaitable, str, None]:
        yield from ()


if __name__ == "__main__":
    data = "Where there is a Will there is a way!"

    class TestParser(Parser[str]):
        def parse(
            self, on_token: Callable[[str], None]
        ) -> Generator[Awaitable, str, None]:
            while True:
                data = yield self.read1()
                if not data:
                    break
                on_token(data)

    test_parser = TestParser()

    for n in range(0, len(data), 5):
        for token in test_parser.feed(data[n : n + 5]):
            print(token)
    for token in test_parser.feed(""):
        print(token)
