from typing import List, Optional

from ._models import ServerSentEvent


def _splitlines_sse(text: str) -> List[str]:
    """Split text on \\r\\n, \\r, or \\n only."""
    lines = []
    start = 0
    i = 0

    while i < len(text):
        # Check for \r\n first (2 chars)
        if i < len(text) - 1 and text[i : i + 2] == "\r\n":
            lines.append(text[start:i])
            start = i + 2
            i += 2
        # Then check for \r or \n (1 char each)
        elif text[i] in "\r\n":
            lines.append(text[start:i])
            start = i + 1
            i += 1
        else:
            i += 1

    # Add remaining text if any
    if start < len(text):
        lines.append(text[start:])

    return lines


class SSELineDecoder:
    """
    Handles incrementally reading lines from text.

    Mostly a copy of httpx._decoders.LineDecoder, but as per SSE spec, only \r\n, \r,
    and \n are treated as newlines, which differs from the behavior of splitlines()
    used by httpx._decoders.LineDecoder.
    """

    def __init__(self) -> None:
        self.buffer: list[str] = []
        self.trailing_cr: bool = False

    def decode(self, text: str) -> list[str]:
        # We always push a trailing `\r` into the next decode iteration.
        if self.trailing_cr:
            text = "\r" + text
            self.trailing_cr = False
        if text.endswith("\r"):
            self.trailing_cr = True
            text = text[:-1]

        if not text:
            # NOTE: the edge case input of empty text doesn't occur in practice,
            # because other httpx internals filter out this value
            return []  # pragma: no cover

        trailing_newline = text[-1] in "\n\r"
        lines = _splitlines_sse(text)

        if len(lines) == 1 and not trailing_newline:
            # No new lines, buffer the input and continue.
            self.buffer.append(lines[0])
            return []

        if self.buffer:
            # Include any existing buffer in the first portion of the
            # splitlines result.
            lines = ["".join(self.buffer) + lines[0]] + lines[1:]
            self.buffer = []

        if not trailing_newline:
            # If the last segment of splitlines is not newline terminated,
            # then drop it from our output and start a new buffer.
            self.buffer = [lines.pop()]

        return lines

    def flush(self) -> list[str]:
        if not self.buffer and not self.trailing_cr:
            return []

        lines = ["".join(self.buffer)]
        self.buffer = []
        self.trailing_cr = False
        return lines


class SSEDecoder:
    def __init__(self) -> None:
        self._event = ""
        self._data: List[str] = []
        self._last_event_id = ""
        self._retry: Optional[int] = None

    def decode(self, line: str) -> Optional[ServerSentEvent]:
        # See: https://html.spec.whatwg.org/multipage/server-sent-events.html#event-stream-interpretation  # noqa: E501

        if not line:
            if (
                not self._event
                and not self._data
                and not self._last_event_id
                and self._retry is None
            ):
                return None

            sse = ServerSentEvent(
                event=self._event,
                data="\n".join(self._data),
                id=self._last_event_id,
                retry=self._retry,
            )

            # NOTE: as per the SSE spec, do not reset last_event_id.
            self._event = ""
            self._data = []
            self._retry = None

            return sse

        if line.startswith(":"):
            return None

        fieldname, _, value = line.partition(":")

        if value.startswith(" "):
            value = value[1:]

        if fieldname == "event":
            self._event = value
        elif fieldname == "data":
            self._data.append(value)
        elif fieldname == "id":
            if "\0" in value:
                pass
            else:
                self._last_event_id = value
        elif fieldname == "retry":
            try:
                self._retry = int(value)
            except (TypeError, ValueError):
                pass
        else:
            pass  # Field is ignored.

        return None