"""Debug sessions (E16): Start → reproduce → Stop, and the evidence a bug report is built from.

While a session is open its user's browser names it in `X-Debug-Session` on every request. The auth
dependency accepts the header only for the caller's OWN open session (anything else is ignored, never
an error — a stale tab must not break the app) and then logs that request at the session's level with
every line tagged by its id; the browser tags its own events the same way. The issue is later built
from exactly the rows carrying the tag, so what the review list shows is what gets sent.

Starting a session IS the consent for its level, for its duration: the start dialog carries the
consent text. It needs no change to the user's own diagnostics level, and none is made — when the
session ends the level in force is simply the user's own again.

A session is capped at `MAX_MINUTES`, set by the server at Start. An open one past its cap is over
whether or not anything has closed it yet: it reads as over at once, a new Start closes it, and the
hourly log job sweeps the rest. Sessions go with the logs they point at, after `log_retention_days`.

Core layer: imports `models`, `diagnostics`.
"""

import secrets
from datetime import datetime, timedelta, timezone
from enum import StrEnum
from typing import Literal, get_args

from sqlalchemy import and_, delete, func, not_, select, update
from sqlalchemy.orm import Session, defer

from app import diagnostics
from app.models import DebugScreenshot, DebugSession, LogEvent, User

HEADER = "x-debug-session"
MAX_MINUTES = diagnostics.SESSION_MINUTES
SCREENSHOT_MAX_BYTES = 10 * 1024 * 1024
SCREENSHOTS_MAX = 8
# Rows an issue can carry. A 60-minute session at Teljes is a few thousand rows; past this the
# bundle says how many it left out rather than growing without bound.
BUNDLE_MAX = 5_000

SessionLevel = Literal["DEBUG", "TRACE"]
SESSION_LEVELS: tuple[str, ...] = get_args(SessionLevel)
# Detail order of the diagnostics levels: a session raises the level in force, never lowers it.
_DETAIL = {"INFO": 0, "DEBUG": 1, "TRACE": 2}
_ID_LEN = 16


class EndReason(StrEnum):
    STOPPED = "stopped"
    EXPIRED = "expired"


def _is_live(session: DebugSession, now: datetime) -> bool:
    return session.ended_at is None and session.until > now


def open_session(db: Session, user: User) -> DebugSession | None:
    """The user's live session, or None. Read-only: one past its cap is over and reads as None;
    `start` and the hourly sweep are what close it."""
    session = _unclosed(db, user)
    if session is None or not _is_live(session, datetime.now(timezone.utc)):
        return None
    return session


def _unclosed(db: Session, user: User) -> DebugSession | None:
    return db.scalar(
        select(DebugSession).where(DebugSession.user_id == user.id, DebugSession.ended_at.is_(None))
    )


def start(db: Session, user: User, level: SessionLevel) -> DebugSession:
    """Open a session, or return the live one — a second tab pressing Start joins the recording
    rather than ending it. One past its cap is closed first: the one-open-per-user index would
    refuse the new row beside it. Flushes; the caller commits. Two concurrent Starts can still both
    get here: the index refuses the second, and the caller retries once."""
    now = datetime.now(timezone.utc)
    current = _unclosed(db, user)
    if current is not None:
        if _is_live(current, now):
            return current
        end(current, EndReason.EXPIRED)
        db.flush()
    session = DebugSession(
        id=secrets.token_hex(_ID_LEN // 2),
        user_id=user.id,
        level=level,
        started_at=now,
        until=now + timedelta(minutes=MAX_MINUTES),
    )
    db.add(session)
    db.flush()
    return session


def end(session: DebugSession, reason: EndReason) -> None:
    """Close a session. An expired one is dated at its cap, not at whenever it was noticed."""
    if session.ended_at is not None:
        return
    session.ended_at = (
        session.until
        if reason is EndReason.EXPIRED
        else min(datetime.now(timezone.utc), session.until)
    )
    session.ended_reason = reason


def from_header(db: Session, user: User, value: str | None) -> DebugSession | None:
    """The session a request names, if it is the caller's own and still live; else None."""
    if not value or len(value) != _ID_LEN or not all(c in "0123456789abcdef" for c in value):
        return None
    session = db.get(DebugSession, value)
    if (
        session is None
        or session.user_id != user.id
        or not _is_live(session, datetime.now(timezone.utc))
    ):
        return None
    return session


def level_for(user: User, session: DebugSession | None) -> tuple[str, datetime | None]:
    """The diagnostics level (and its expiry) a request is logged at: the user's own, raised to the
    session's while one is named — whichever records more."""
    own = diagnostics.effective_level(user)
    until = user.diagnostics_until if own != diagnostics.BASE else None
    if session is None or _DETAIL[session.level] <= _DETAIL[own]:
        return own, until
    return session.level, session.until


def save_draft(session: DebugSession, draft: dict) -> None:
    """Keep the composer's draft (the caller validated and commits). Shape: `title`, `body` (the
    rendered markdown an export or an issue carries), `excluded` (log row ids taken out of the
    evidence) and `form` (the composer's own fields, to reopen it as it was)."""
    session.draft = draft


def serialize(session: DebugSession | None, screenshots: int = 0) -> dict | None:
    if session is None:
        return None
    return {
        "id": session.id,
        "level": session.level,
        "started_at": session.started_at.isoformat(),
        "until": session.until.isoformat(),
        "ended_at": session.ended_at.isoformat() if session.ended_at else None,
        "ended_reason": session.ended_reason,
        "issue_url": session.issue_url,
        "screenshots": screenshots,
        "draft": session.draft,
    }


def screenshot_count(db: Session, session: DebugSession) -> int:
    return (
        db.scalar(
            select(func.count())
            .select_from(DebugScreenshot)
            .where(DebugScreenshot.session_id == session.id)
        )
        or 0
    )


# --- screenshots ---------------------------------------------------------------------------
# By content, not by the declared type: what is stored is served back to a browser as an image.
_SIGNATURES = (
    (b"\x89PNG\r\n\x1a\n", "image/png"),
    (b"\xff\xd8\xff", "image/jpeg"),
)


def sniff(data: bytes) -> str | None:
    """The image type `data` really is — PNG, JPEG or WebP — or None."""
    for magic, mime in _SIGNATURES:
        if data.startswith(magic):
            return mime
    if len(data) >= 12 and data[:4] == b"RIFF" and data[8:12] == b"WEBP":
        return "image/webp"
    return None


class ScreenshotRefused(Exception):
    """A screenshot that cannot be kept; `code` is the refusal's machine word."""

    def __init__(self, code: str) -> None:
        super().__init__(code)
        self.code = code


def add_screenshot(db: Session, session: DebugSession, data: bytes) -> DebugScreenshot:
    """Keep one screenshot. Flushes; the caller commits. Refused once the issue was sent (it can no
    longer carry it), past the per-session count, or when it is not an image we can show."""
    if session.issue_url is not None:
        raise ScreenshotRefused("already_sent")
    if not data:
        raise ScreenshotRefused("empty")
    mime = sniff(data)
    if mime is None:
        raise ScreenshotRefused("not_an_image")
    if screenshot_count(db, session) >= SCREENSHOTS_MAX:
        raise ScreenshotRefused("too_many")
    shot = DebugScreenshot(
        session_id=session.id,
        created_at=datetime.now(timezone.utc),
        mime=mime,
        size=len(data),
        data=data,
    )
    db.add(shot)
    db.flush()
    return shot


def screenshots(db: Session, session: DebugSession) -> list[DebugScreenshot]:
    """The session's screenshots, oldest first, WITHOUT their bytes loaded."""
    return list(
        db.scalars(
            select(DebugScreenshot)
            .where(DebugScreenshot.session_id == session.id)
            .options(defer(DebugScreenshot.data))
            .order_by(DebugScreenshot.id)
        )
    )


# --- the evidence --------------------------------------------------------------------------
# The browser's own log upload (`POST` here, every few seconds while recording) is how the evidence
# arrives, not part of it: its server lines would fill the bundle with the recording's plumbing. The
# browser leaves its copies out for the same reason (frontend lib/diagnostics `isOwnRequest`).
INGEST_PATH = "/api/diagnostics/events"


def _evidence(session: DebugSession):
    upload_line = and_(
        LogEvent.event == "http.request", LogEvent.attrs["path"].astext == INGEST_PATH
    )
    # `IS TRUE`, not the bare condition: for a row with no event or no attrs it is NULL, and
    # `NOT NULL` would drop exactly the rows that are evidence.
    return (LogEvent.debug_session_id == session.id, not_(upload_line.is_(True)))


def bundle(
    db: Session, session: DebugSession, *, drop_excluded: bool = False
) -> tuple[list[LogEvent], int]:
    """The session's log rows, browser and server interleaved in time, capped at BUNDLE_MAX — and
    how many rows it has in all, so a cut bundle can say so. `drop_excluded` leaves out the rows
    the user took out in the composer: what an export or an issue carries."""
    where = list(_evidence(session))
    excluded = (session.draft or {}).get("excluded") or [] if drop_excluded else []
    if excluded:
        where.append(LogEvent.id.not_in(excluded))
    rows = list(
        db.scalars(
            select(LogEvent).where(*where).order_by(LogEvent.ts, LogEvent.id).limit(BUNDLE_MAX)
        )
    )
    total = len(rows)
    if total == BUNDLE_MAX:
        total = db.scalar(select(func.count()).select_from(LogEvent).where(*where)) or total
    return rows, total


# --- housekeeping (the hourly log job) -----------------------------------------------------
def sweep(db: Session) -> int:
    """Stage closing every open session past its cap; returns how many."""
    now = datetime.now(timezone.utc)
    result = db.execute(
        update(DebugSession)
        .where(DebugSession.ended_at.is_(None), DebugSession.until <= now)
        .values(ended_at=DebugSession.until, ended_reason=EndReason.EXPIRED.value)
        .execution_options(synchronize_session=False)
    )
    return result.rowcount or 0


def gc(db: Session, retention_days: int) -> int:
    """Stage deleting sessions (and by cascade their screenshots) started before the log retention
    window — the rows they would bundle are gone by then too. Returns how many."""
    cutoff = datetime.now(timezone.utc) - timedelta(days=max(1, retention_days))
    result = db.execute(delete(DebugSession).where(DebugSession.started_at < cutoff))
    return result.rowcount or 0