"""Cursor-paginated gallery queries.

Cursor format: opaque base64-encoded "<iso8601_created_at>:<image_id>".
Pagination key is (created_at DESC, id DESC) so we don't drift when new
imports arrive between page loads. Decoding rejects malformed cursors with
a ValueError; the API layer translates that to HTTP 400.
"""

import base64
from dataclasses import dataclass
from datetime import datetime

from sqlalchemy import and_, func, or_, select
from sqlalchemy.ext.asyncio import AsyncSession

from ..models import ImageRecord, Tag
from ..models.tag import image_tag

CURSOR_SEPARATOR = "|"


def encode_cursor(created_at: datetime, image_id: int) -> str:
    raw = f"{created_at.isoformat()}{CURSOR_SEPARATOR}{image_id}"
    return base64.urlsafe_b64encode(raw.encode()).decode()


def decode_cursor(cursor: str) -> tuple[datetime, int]:
    try:
        raw = base64.urlsafe_b64decode(cursor.encode()).decode()
        ts_part, id_part = raw.split(CURSOR_SEPARATOR, 1)
        return datetime.fromisoformat(ts_part), int(id_part)
    except Exception as exc:
        raise ValueError(f"invalid cursor: {cursor!r}") from exc


@dataclass(frozen=True)
class GalleryImage:
    id: int
    path: str
    sha256: str
    mime: str
    width: int | None
    height: int | None
    created_at: datetime
    thumbnail_url: str


@dataclass(frozen=True)
class GalleryPage:
    images: list[GalleryImage]
    next_cursor: str | None
    date_groups: list[tuple[int, int, list[int]]]  # (year, month, [image_id...])


@dataclass(frozen=True)
class TimelineBucket:
    year: int
    month: int
    count: int


def thumbnail_url(sha256_hex: str, mime: str) -> str:
    # Quart serves /images/* via the frontend blueprint (FC-1); thumbnails go
    # under /images/thumbs/. The MIME determines the extension.
    ext = ".png" if mime in ("image/png", "image/gif") else ".jpg"
    bucket = sha256_hex[:3]
    return f"/images/thumbs/{bucket}/{sha256_hex}{ext}"


class GalleryService:
    def __init__(self, session: AsyncSession):
        self.session = session

    async def scroll(
        self,
        cursor: str | None,
        limit: int = 50,
        tag_id: int | None = None,
    ) -> GalleryPage:
        if limit < 1 or limit > 200:
            raise ValueError("limit must be between 1 and 200")

        stmt = select(ImageRecord)
        if tag_id is not None:
            stmt = stmt.join(image_tag, image_tag.c.image_record_id == ImageRecord.id).where(
                image_tag.c.tag_id == tag_id
            )

        if cursor:
            cur_ts, cur_id = decode_cursor(cursor)
            stmt = stmt.where(
                or_(
                    ImageRecord.created_at < cur_ts,
                    and_(ImageRecord.created_at == cur_ts, ImageRecord.id < cur_id),
                )
            )

        stmt = stmt.order_by(ImageRecord.created_at.desc(), ImageRecord.id.desc()).limit(limit + 1)
        rows = (await self.session.execute(stmt)).scalars().all()

        next_cursor = None
        if len(rows) > limit:
            last = rows[limit - 1]
            next_cursor = encode_cursor(last.created_at, last.id)
            rows = rows[:limit]

        images = [
            GalleryImage(
                id=r.id,
                path=r.path,
                sha256=r.sha256,
                mime=r.mime,
                width=r.width,
                height=r.height,
                created_at=r.created_at,
                thumbnail_url=thumbnail_url(r.sha256, r.mime),
            )
            for r in rows
        ]
        return GalleryPage(
            images=images,
            next_cursor=next_cursor,
            date_groups=_group_by_year_month(images),
        )

    async def timeline(self, tag_id: int | None = None) -> list[TimelineBucket]:
        year_col = func.date_part("year", ImageRecord.created_at).label("yr")
        month_col = func.date_part("month", ImageRecord.created_at).label("mo")
        stmt = select(
            year_col, month_col, func.count(ImageRecord.id).label("cnt")
        )
        if tag_id is not None:
            stmt = stmt.join(image_tag, image_tag.c.image_record_id == ImageRecord.id).where(
                image_tag.c.tag_id == tag_id
            )
        stmt = stmt.group_by(year_col, month_col).order_by(year_col.desc(), month_col.desc())
        rows = (await self.session.execute(stmt)).all()
        return [TimelineBucket(year=int(r.yr), month=int(r.mo), count=int(r.cnt)) for r in rows]

    async def jump_cursor(
        self, year: int, month: int, tag_id: int | None = None
    ) -> str | None:
        """Returns a cursor that, when passed to scroll(), positions at the
        first image of the given year-month. None if the bucket is empty.
        """
        from sqlalchemy import extract

        stmt = select(ImageRecord).where(
            extract("year", ImageRecord.created_at) == year,
            extract("month", ImageRecord.created_at) == month,
        )
        if tag_id is not None:
            stmt = stmt.join(image_tag, image_tag.c.image_record_id == ImageRecord.id).where(
                image_tag.c.tag_id == tag_id
            )
        stmt = stmt.order_by(ImageRecord.created_at.desc(), ImageRecord.id.desc()).limit(1)
        first = (await self.session.execute(stmt)).scalar_one_or_none()
        if first is None:
            return None
        # Cursor is exclusive; we encode a cursor with id+1 so the row itself
        # is the first result in the next scroll().
        return encode_cursor(first.created_at, first.id + 1)

    async def get_image_with_tags(self, image_id: int) -> dict | None:
        record = await self.session.get(ImageRecord, image_id)
        if record is None:
            return None
        tag_stmt = (
            select(Tag)
            .join(image_tag, image_tag.c.tag_id == Tag.id)
            .where(image_tag.c.image_record_id == image_id)
            .order_by(Tag.kind.asc(), Tag.name.asc())
        )
        tags = (await self.session.execute(tag_stmt)).scalars().all()
        neighbors = await self._neighbors(record)
        return {
            "id": record.id,
            "path": record.path,
            "sha256": record.sha256,
            "mime": record.mime,
            "width": record.width,
            "height": record.height,
            "size_bytes": record.size_bytes,
            "created_at": record.created_at.isoformat(),
            "thumbnail_url": thumbnail_url(record.sha256, record.mime),
            "image_url": f"/images/{record.path.split('/images/', 1)[-1]}",
            "tags": [
                {
                    "id": t.id,
                    "name": t.name,
                    "kind": t.kind.value if hasattr(t.kind, "value") else t.kind,
                    "fandom_id": t.fandom_id,
                }
                for t in tags
            ],
            "neighbors": neighbors,
        }

    async def _neighbors(self, record: ImageRecord) -> dict:
        prev_stmt = (
            select(ImageRecord.id)
            .where(
                or_(
                    ImageRecord.created_at > record.created_at,
                    and_(
                        ImageRecord.created_at == record.created_at,
                        ImageRecord.id > record.id,
                    ),
                )
            )
            .order_by(ImageRecord.created_at.asc(), ImageRecord.id.asc())
            .limit(1)
        )
        next_stmt = (
            select(ImageRecord.id)
            .where(
                or_(
                    ImageRecord.created_at < record.created_at,
                    and_(
                        ImageRecord.created_at == record.created_at,
                        ImageRecord.id < record.id,
                    ),
                )
            )
            .order_by(ImageRecord.created_at.desc(), ImageRecord.id.desc())
            .limit(1)
        )
        prev_id = (await self.session.execute(prev_stmt)).scalar_one_or_none()
        next_id = (await self.session.execute(next_stmt)).scalar_one_or_none()
        return {"prev_id": prev_id, "next_id": next_id}


def _group_by_year_month(
    images: list[GalleryImage],
) -> list[tuple[int, int, list[int]]]:
    groups: list[tuple[int, int, list[int]]] = []
    for img in images:
        y, m = img.created_at.year, img.created_at.month
        if groups and groups[-1][0] == y and groups[-1][1] == m:
            groups[-1][2].append(img.id)
        else:
            groups.append((y, m, [img.id]))
    return groups