"""Best-effort retrieval of an Instagram post caption without login.

Instagram has no login-free API for captions any more (the official oEmbed endpoint needs a
Meta app and no longer returns post content). We make a single, honest request to the
public embed page of the post and read the caption from it. Instagram frequently serves an
empty app shell to servers instead – then the caller falls back to "paste the caption" or
"import a screenshot". No login, no cookies, no evasion of bot detection.
"""

import re
from html.parser import HTMLParser
from urllib.parse import urlsplit

from .fetch import FetchError, fetch
from .schema_org import parse_page

HOSTS = {"instagram.com", "www.instagram.com", "m.instagram.com", "instagr.am", "www.instagr.am"}
_PATH_RE = re.compile(r"^/(?:[\w.]+/)?(?:p|reel|reels|tv)/([A-Za-z0-9_-]{5,40})(?:/|$)")
_OG_CAPTION_RE = re.compile(r':\s*["“](.+)["”]\s*\.?\s*$', re.DOTALL)


class InstagramUnavailable(Exception):
    pass


def shortcode(url):
    """Shortcode of a post/reel URL, or None if the URL is not an Instagram post."""
    try:
        parts = urlsplit(url.strip())
    except ValueError:
        return None
    if (parts.hostname or "").lower() not in HOSTS:
        return None
    match = _PATH_RE.match(parts.path)
    return match.group(1) if match else None


def is_instagram(url):
    return (urlsplit(url.strip()).hostname or "").lower() in HOSTS


class _EmbedCaptionParser(HTMLParser):
    """Reads <div class="Caption"> of the embed page, without username and comment links."""

    def __init__(self):
        super().__init__(convert_charrefs=True)
        self.depth = 0  # >0 while inside the caption div
        self.skip_depth = 0
        self.parts = []
        self.image = ""

    def handle_starttag(self, tag, attrs):
        attrs = dict(attrs)
        classes = (attrs.get("class") or "").split()
        if tag == "img" and "EmbeddedMediaImage" in classes and not self.image:
            self.image = attrs.get("src", "")
        if self.depth:
            if tag == "br":
                self.parts.append("\n")
                return
            if self.skip_depth or "CaptionUsername" in classes or "CaptionComments" in classes:
                self.skip_depth += 1
            if tag == "div":
                self.depth += 1
        elif tag == "div" and "Caption" in classes:
            self.depth = 1

    def handle_endtag(self, tag):
        if not self.depth:
            return
        if self.skip_depth:
            self.skip_depth -= 1
        if tag == "div":
            self.depth -= 1

    def handle_data(self, data):
        if self.depth and not self.skip_depth:
            self.parts.append(data)

    def caption(self):
        text = "".join(self.parts)
        text = re.sub(r"[ \t]+", " ", text)
        return re.sub(r"\n{3,}", "\n\n", text).strip()


def parse_embed(page_html):
    """Return (caption, image_url) from an embed page; caption may be empty."""
    parser = _EmbedCaptionParser()
    try:
        parser.feed(page_html)
    except Exception:
        return "", ""
    caption = parser.caption()
    image = parser.image
    if not caption:
        meta = parse_page(page_html).meta
        match = _OG_CAPTION_RE.search(meta.get("og:description", "") or meta.get("og:title", ""))
        caption = match.group(1).strip() if match else ""
        image = image or meta.get("og:image", "")
    return caption, image


def fetch_caption(code):
    """Try to read the caption of a post. Raises InstagramUnavailable if Instagram doesn't deliver it."""
    try:
        result = fetch(f"https://www.instagram.com/p/{code}/embed/captioned/")
    except FetchError as error:
        raise InstagramUnavailable(error.user_message) from None
    if result.status != 200:
        raise InstagramUnavailable(f"Instagram antwortete mit Status {result.status}.")
    caption, image = parse_embed(result.text)
    if len(caption) < 20:
        raise InstagramUnavailable("Instagram hat die Beschreibung nicht herausgegeben.")
    return caption, image