import hashlib
import os
import time
import urllib.request
import urllib.parse
import mimetypes
from pathlib import Path
from typing import Iterable, List, Tuple
from django.conf import settings
from django.core.mail import EmailMultiAlternatives

def _ensure_cache_dir() -> Path:
    """
    Ensure that the cache directory for attachments exists.
    Creates it recursively if it doesn't.
    """
    cache_dir = Path(settings.MAIL_ATTACHMENT_CACHE_DIR)
    cache_dir.mkdir(parents=True, exist_ok=True)
    return cache_dir


def _cached_filename_for(url: str) -> str:
    """
    Generate a unique cache filename for the given URL (hash + original suffix if present).
    """
    h = hashlib.sha1(url.encode('utf-8')).hexdigest()[:16]
    parsed = urllib.parse.urlparse(url)
    # path part only (without query/fragment)
    name = Path(parsed.path).name  # e.g. 'foo.pdf'
    suffix = Path(name).suffix  # e.g. '.pdf'
    return f'{h}{suffix}' if suffix else h


def _is_fresh(path: Path, ttl_seconds: int) -> bool:
    """
    Check if the cached file exists and is still fresh within TTL.
    """
    try:
        age = time.time() - path.stat().st_mtime
        return age < ttl_seconds
    except FileNotFoundError:
        return False

def download_with_cache(url: str, *, timeout: float = 10.0, size_cap_bytes: int = 8 * 1024 * 1024) -> Path:
    """
    Download the file from the given URL into the cache directory, or return the cached
    file if it's still fresh. Uses a temporary '.part' file and atomic replace.
    A simple size cap protects against unexpectedly large downloads.
    """
    cache_dir = _ensure_cache_dir()
    ttl = int(getattr(settings, 'MAIL_ATTACHMENT_TTL_SECONDS', 86400))
    filename = _cached_filename_for(url)
    path = cache_dir / filename

    if _is_fresh(path, ttl):
        return path

    tmp_path = path.with_suffix(path.suffix + '.part')
    try:
        with urllib.request.urlopen(url, timeout=timeout) as resp, open(tmp_path, 'wb') as f:
            # Read in chunks up to size_cap_bytes
            remaining = size_cap_bytes
            chunk_size = 64 * 1024
            while True:
                chunk = resp.read(min(chunk_size, remaining))
                if not chunk:
                    break
                f.write(chunk)
                remaining -= len(chunk)
                if remaining <= 0:
                    break
        os.replace(tmp_path, path)
        return path
    except Exception:
        # Best-effort cleanup of partial file
        try:
            if tmp_path.exists():
                tmp_path.unlink(missing_ok=True)
        except Exception:
            pass
        # Re-raise to let caller decide
        raise

def _filename_from_url(url: str) -> str:
    """
    Derive a display filename from URL path as a fallback when none provided in settings.
    """
    parsed = urllib.parse.urlparse(url)
    name = Path(parsed.path).name or 'attachment'
    return name


def collect_attachment_paths(kind: str, choice: str) -> List[Tuple[Path, str]]:
    """
    Return a list of (path, filename) for attachments based on settings.MAIL_ATTACHMENT_URLS.
    Supports both 'url' strings and (url, filename) tuples.
    """
    cfg = getattr(settings, 'MAIL_ATTACHMENT_URLS', {})
    channel = cfg.get(kind, {})
    urls: list = []
    urls.extend(channel.get('ALL', []))
    urls.extend(channel.get(choice, []))

    result: List[Tuple[Path, str]] = []
    for item in urls:
        if isinstance(item, tuple):
            url, filename = item
        else:
            url, filename = item, _filename_from_url(item)

        path = download_with_cache(url)
        # Only append if the file exists (download_with_cache raises on error by default)
        result.append((path, filename))
    return result


def attach_files(message: EmailMultiAlternatives, files: Iterable[Tuple[Path, str]]) -> None:
    """
    Attach files to the EmailMultiAlternatives message.
    MIME type is guessed from filename; falls back to application/octet-stream.
    """
    for path, filename in files:
        # Guess MIME type from final filename first; fallback to path suffix
        ctype, _ = mimetypes.guess_type(filename)
        if not ctype:
            ctype, _ = mimetypes.guess_type(str(path))
        ctype = ctype or 'application/octet-stream'

        with open(path, 'rb') as f:
            message.attach(filename, f.read(), ctype)