"""The envelope (§3.3): a background block after the user's text.

Assembled when the turn starts, never when the message is queued: the
time and what changed in the vault since the last envelope (added lines
for the ``full`` files, names and counts for the rest). Ceilings keep it
a signal, not a document; the injects that ride along are bundled below
it by the queue, with their own header.
"""

from __future__ import annotations

from dataclasses import dataclass
from datetime import UTC, datetime
from typing import TYPE_CHECKING
from zoneinfo import ZoneInfo

if TYPE_CHECKING:
    from collections.abc import Sequence

    from beaver_gateway.core.watch import Change, VaultWatch

__all__ = ["Envelope", "render"]

HEADER = (
    "[конверт - фоновый сигнал, не обращение; "
    "реагируй, только если относится к вопросу]"
)


@dataclass(slots=True)
class Envelope:
    watch: VaultWatch | None = None
    tz: str = "UTC"
    max_lines: int = 120
    per_file: int = 30
    names_only_within: float = 600.0
    last_at: datetime | None = None

    def build(self, *, now: datetime | None = None) -> str:
        now = now or datetime.now(UTC)
        changes = self.watch.take() if self.watch is not None else []
        names_only = (
            self.last_at is not None
            and (now - self.last_at).total_seconds() < self.names_only_within
        )
        text = render(
            now=now,
            tz=self.tz,
            changes=changes,
            since=self.last_at,
            names_only=names_only,
            max_lines=self.max_lines,
            per_file=self.per_file,
        )
        self.last_at = now
        return text


def render(
    *,
    now: datetime,
    tz: str,
    changes: Sequence[Change],
    since: datetime | None,
    names_only: bool,
    max_lines: int = 120,
    per_file: int = 30,
) -> str:
    zone = ZoneInfo(tz)
    stamp = now.astimezone(zone)
    lines = [HEADER, f"время: {stamp:%Y-%m-%d %H:%M} ({_zone_label(tz)})"]
    ordered = sorted(changes, key=lambda c: (not c.full, c.path))
    since_label = (
        f"с {since.astimezone(zone):%H:%M}" if since is not None else "со старта"  # noqa: RUF001
    )
    if ordered:
        names = ", ".join(f"{c.path} (+{c.added_count})" for c in ordered)
        lines.append(f"vault, изменено {since_label}: {names}")
    if not names_only:
        _append_diffs(lines, ordered, max_lines=max_lines, per_file=per_file)
    return "\n".join(lines[:max_lines])


def _append_diffs(
    lines: list[str], changes: Sequence[Change], *, max_lines: int, per_file: int
) -> None:
    budget = max_lines - len(lines) - 1
    for change in changes:
        if not change.full or not change.added:
            continue
        if budget < 3:
            lines.append("… (потолок конверта)")
            return
        shown = change.added[: min(per_file, budget - 2)]
        lines.append(f"--- {change.path}, только добавленное ---")
        lines.extend(f"+ {line}" for line in shown)
        budget -= 1 + len(shown)
        if len(change.added) > len(shown):
            lines.append(f"+ … ещё {len(change.added) - len(shown)}")
            budget -= 1


def _zone_label(tz: str) -> str:
    return tz.rsplit("/", 1)[-1].replace("_", " ")