"""Download INSPIRE Index Polygons from HM Land Registry.

Downloads GML files for all local authorities from the INSPIRE download page.
Each ZIP contains a GML file with title extent polygons for that authority.

Source: https://use-land-property-data.service.gov.uk/datasets/inspire/download
License: INSPIRE End User Licence
"""

import argparse
from concurrent.futures import ThreadPoolExecutor, as_completed
from html.parser import HTMLParser
from pathlib import Path
import time
import zipfile
from urllib.parse import urljoin, urlparse

import httpx
from tqdm import tqdm

BASE_URL = "https://use-land-property-data.service.gov.uk"
INDEX_URL = f"{BASE_URL}/datasets/inspire/download"
HEADERS = {
    "User-Agent": "Mozilla/5.0 (compatible; perfect-postcode-data-pipeline/1.0)"
}
CHUNK_SIZE = 1024 * 1024
MAX_ATTEMPTS = 5
BACKOFF_BASE = 2.0


class ZipLinkParser(HTMLParser):
    """Collect links to Land Registry INSPIRE ZIP downloads."""

    def __init__(self, base_url: str) -> None:
        super().__init__()
        self.base_url = base_url
        self.base_netloc = urlparse(base_url).netloc
        self.urls: set[str] = set()

    def handle_starttag(
        self, tag: str, attrs: list[tuple[str, str | None]]
    ) -> None:
        if tag != "a":
            return

        href = dict(attrs).get("href")
        if not href:
            return

        url = urljoin(self.base_url, href)
        parsed = urlparse(url)
        if (
            parsed.scheme in {"http", "https"}
            and parsed.netloc == self.base_netloc
            and parsed.path.startswith("/datasets/inspire/download/")
            and parsed.path.endswith(".zip")
        ):
            self.urls.add(parsed._replace(query="", fragment="").geturl())


def parse_zip_urls(html: str, base_url: str = BASE_URL) -> list[str]:
    """Parse the INSPIRE download page for all council ZIP URLs."""
    parser = ZipLinkParser(base_url)
    parser.feed(html)
    return sorted(parser.urls)


def get_zip_urls() -> list[str]:
    """Scrape the INSPIRE download page for all .zip hrefs."""
    # The site requires a cookie jar to avoid redirect loops.
    with httpx.Client(
        follow_redirects=True,
        timeout=httpx.Timeout(30.0, read=60),
        headers={**HEADERS, "Accept": "text/html"},
    ) as client:
        resp = client.get(INDEX_URL)
        resp.raise_for_status()
        html = resp.text

    urls = parse_zip_urls(html)
    if not urls:
        raise RuntimeError(f"No INSPIRE ZIP links found at {INDEX_URL}")
    return urls


def _is_valid_zip(path: Path) -> bool:
    return path.exists() and zipfile.is_zipfile(path)


def _stream_download(url: str, output_path: Path, *, timeout: float) -> None:
    with httpx.stream(
        "GET",
        url,
        follow_redirects=True,
        timeout=httpx.Timeout(30.0, read=timeout),
        headers=HEADERS,
    ) as response:
        response.raise_for_status()
        with output_path.open("wb") as out:
            for chunk in response.iter_bytes(chunk_size=CHUNK_SIZE):
                out.write(chunk)


def download_one(
    url: str,
    output_dir: Path,
    *,
    force: bool = False,
    timeout: float = 600,
) -> str:
    """Download a single ZIP file. Returns the filename."""
    name = Path(urlparse(url).path).name
    if not name.endswith(".zip"):
        raise ValueError(f"Expected a ZIP download URL, got {url}")

    output_dir.mkdir(parents=True, exist_ok=True)
    dest = output_dir / name
    if not force and _is_valid_zip(dest):
        return f"{name} (skipped, valid ZIP exists)"

    tmp = dest.with_suffix(dest.suffix + ".tmp")
    last_exc: Exception | None = None
    try:
        for attempt in range(1, MAX_ATTEMPTS + 1):
            tmp.unlink(missing_ok=True)
            try:
                _stream_download(url, tmp, timeout=timeout)
                if not _is_valid_zip(tmp):
                    raise RuntimeError(
                        f"{name} did not download as a valid ZIP"
                    )
                tmp.replace(dest)
                return name
            except (httpx.HTTPError, OSError) as exc:
                last_exc = exc
                if attempt < MAX_ATTEMPTS:
                    time.sleep(BACKOFF_BASE ** (attempt - 1))
    finally:
        tmp.unlink(missing_ok=True)

    raise RuntimeError(
        f"{name} failed after {MAX_ATTEMPTS} attempts"
    ) from last_exc


def main() -> None:
    parser = argparse.ArgumentParser(
        description="Download INSPIRE Index Polygon GML files"
    )
    parser.add_argument(
        "--output",
        type=Path,
        required=True,
        help="Output directory for downloaded ZIPs",
    )
    parser.add_argument(
        "--workers",
        type=int,
        default=8,
        help="Number of parallel downloads (default: 8)",
    )
    parser.add_argument(
        "--force",
        action="store_true",
        help="Re-download files even when a valid ZIP already exists",
    )
    parser.add_argument(
        "--timeout",
        type=float,
        default=600,
        help="Per-file read timeout in seconds (default: 600)",
    )
    args = parser.parse_args()

    if args.workers < 1:
        raise SystemExit("--workers must be at least 1")

    args.output.mkdir(parents=True, exist_ok=True)

    print("Fetching download index...")
    urls = get_zip_urls()
    print(f"Found {len(urls)} files to download")

    failures: list[tuple[str, Exception]] = []
    with tqdm(total=len(urls), unit="file") as pbar:
        with ThreadPoolExecutor(max_workers=args.workers) as pool:
            futures = {
                pool.submit(
                    download_one,
                    url,
                    args.output,
                    force=args.force,
                    timeout=args.timeout,
                ): url
                for url in urls
            }
            for future in as_completed(futures):
                try:
                    result = future.result()
                    pbar.set_postfix_str(result[:40])
                except Exception as exc:  # noqa: BLE001
                    failures.append((futures[future], exc))
                    pbar.set_postfix_str("FAILED")
                pbar.update(1)

    succeeded = len(urls) - len(failures)
    print(f"Done. {succeeded}/{len(urls)} files in {args.output}")
    if failures:
        print(f"{len(failures)} file(s) failed:")
        for url, exc in failures:
            name = Path(urlparse(url).path).name
            print(f"  - {name}: {exc}")
        raise SystemExit(
            f"{len(failures)} INSPIRE download(s) failed; "
            "re-run to retry only the missing files"
        )


if __name__ == "__main__":
    main()