/**
 * Reads a link preview's metadata out of a page's HTML. {@link parsePage}
 * scans the page once for the tags that carry it (`<meta>`, `<link>`, `<img>`,
 * `<title>` and JSON-LD scripts) without building a document, and returns a
 * {@link Page} that the providers' `genericPreview` picks from. Every field is
 * undefined when the page doesn't say.
 *
 * @module
 */

import { hexOf, parseCssColor } from "../lib/css-color.ts";

/**
 * The named entities decoded besides numeric ones: the XML five, and those
 * page titles and descriptions actually use, Latin-1 letters and typography.
 */
const NAMED_ENTITIES: Record<string, string> = {
  amp: "&",
  lt: "<",
  gt: ">",
  quot: '"',
  apos: "'",
  nbsp: " ",
  aelig: "æ",
  AElig: "Æ",
  oelig: "œ",
  OElig: "Œ",
  szlig: "ß",
  oslash: "ø",
  Oslash: "Ø",
  eth: "ð",
  ETH: "Ð",
  thorn: "þ",
  THORN: "Þ",
  hellip: "…",
  lsquo: "‘",
  rsquo: "’",
  sbquo: "‚",
  ldquo: "“",
  rdquo: "”",
  bdquo: "„",
  laquo: "«",
  raquo: "»",
  lsaquo: "‹",
  rsaquo: "›",
  ndash: "–",
  mdash: "—",
  bull: "•",
  middot: "·",
  dagger: "†",
  Dagger: "‡",
  prime: "′",
  Prime: "″",
  euro: "€",
  pound: "£",
  yen: "¥",
  cent: "¢",
  copy: "©",
  reg: "®",
  trade: "™",
  deg: "°",
  plusmn: "±",
  times: "×",
  divide: "÷",
  micro: "µ",
  para: "¶",
  sect: "§",
  iexcl: "¡",
  iquest: "¿",
  ordf: "ª",
  ordm: "º",
  sup2: "²",
  sup3: "³",
  frac14: "¼",
  frac12: "½",
  frac34: "¾",
  shy: "­",
  ensp: " ",
  emsp: " ",
  thinsp: " ",
  zwnj: "‌",
  zwj: "‍",
};

// The accented letters, `&eacute;` and the like, composed from their letter
// and accent rather than listed one by one.
for (
  const [accent, mark, letters] of [
    ["grave", "̀", "aeiouAEIOU"],
    ["acute", "́", "aeiouyAEIOUY"],
    ["circ", "̂", "aeiouAEIOU"],
    ["tilde", "̃", "anoANO"],
    ["uml", "̈", "aeiouyAEIOUY"],
    ["ring", "̊", "aA"],
    ["cedil", "̧", "cC"],
  ]
) {
  for (const letter of letters) {
    NAMED_ENTITIES[letter + accent] = (letter + mark).normalize("NFC");
  }
}

/** Returns the character of code point `n`, or undefined when there is none. */
function fromCodePoint(n: number): string | undefined {
  if (n <= 0 || n > 0x10ffff || (n >= 0xd800 && n <= 0xdfff)) {
    return undefined;
  }
  return String.fromCodePoint(n);
}

/**
 * Decodes the entities in `str` in a single pass, so `&amp;quot;` reads
 * `&quot;`. An entity it doesn't know, or a code point that doesn't exist, is
 * left as written. Also used by `bandcamp-tralbum.ts` on the JSON a page
 * carries in an attribute.
 */
export function decodeHtmlEntities(str: string): string {
  return str.replace(
    /&(?:#(\d{1,8})|#[xX]([0-9a-fA-F]{1,6})|([a-zA-Z][a-zA-Z0-9]{1,31}));/g,
    (match, dec?: string, hex?: string, name?: string) => {
      if (dec) return fromCodePoint(Number(dec)) ?? match;
      if (hex) return fromCodePoint(parseInt(hex, 16)) ?? match;
      return NAMED_ENTITIES[name!] ?? NAMED_ENTITIES[name!.toLowerCase()] ??
        match;
    },
  );
}

/**
 * Returns `raw` resolved against `base` as an absolute http(s) URL; undefined
 * for a `data:` URI, another scheme or a URL that doesn't parse. Every URL a
 * page names goes through it, so `/og.png` and `//cdn/og.png` work.
 */
export function absoluteHttpUrl(
  raw: string | undefined,
  base: string,
): string | undefined {
  if (!raw?.trim()) return undefined;
  try {
    const url = new URL(raw.trim(), base);
    return url.protocol === "http:" || url.protocol === "https:"
      ? url.href
      : undefined;
  } catch {
    return undefined;
  }
}

/** A tag's attributes by lowercase name, values decoded; the first one wins. */
type Attrs = Map<string, string>;

/**
 * Matches the start of a tag the scan reads, or of a comment. Quoted values
 * may hold `>`, but no tag spans a `<`, so each attempt stops at the next one
 * and the scan stays linear in the page's size.
 */
const TAG_RE =
  /<(meta|link|img|title|script)\b((?:[^<>"']|"[^"<]*"|'[^'<]*')*)>|<!--/gi;

/** Matches one attribute: unquoted, single- or double-quoted, or bare. */
const ATTR_RE =
  /([^\s"'<>\/=]+)(?:\s*=\s*(?:"([^"]*)"|'([^']*)'|([^\s"'=<>`]+)))?/g;

function parseAttrs(source: string): Attrs {
  const attrs: Attrs = new Map();
  for (const m of source.matchAll(ATTR_RE)) {
    const name = m[1].toLowerCase();
    if (attrs.has(name)) continue;
    attrs.set(name, decodeHtmlEntities(m[2] ?? m[3] ?? m[4] ?? ""));
  }
  return attrs;
}

/** What {@link scanTags} found, in page order. */
interface Scan {
  metas: Attrs[];
  links: Attrs[];
  imgs: Attrs[];
  /** The raw text of the first `<title>`. */
  title?: string;
  /** The bodies of the `application/ld+json` scripts. */
  jsonLd: string[];
}

/**
 * Collects the tags {@link parsePage} reads in one pass. Comments and the
 * bodies of scripts and the title are skipped over, so a tag written inside
 * them isn't read; one left open runs to the end of the page, as in a browser.
 */
function scanTags(html: string): Scan {
  const scan: Scan = { metas: [], links: [], imgs: [], jsonLd: [] };
  const re = new RegExp(TAG_RE);
  let m: RegExpExecArray | null;
  while ((m = re.exec(html)) !== null) {
    const tag = m[1]?.toLowerCase();
    if (!tag) {
      const end = html.indexOf("-->", re.lastIndex);
      if (end < 0) break;
      re.lastIndex = end + 3;
      continue;
    }
    const attrs = parseAttrs(m[2]);
    if (tag === "meta") scan.metas.push(attrs);
    else if (tag === "link") scan.links.push(attrs);
    else if (tag === "img") scan.imgs.push(attrs);
    else {
      const close = new RegExp(`</${tag}\\s*>`, "gi");
      close.lastIndex = re.lastIndex;
      const end = close.exec(html);
      if (!end) break;
      const body = html.slice(re.lastIndex, end.index);
      if (tag === "title") scan.title ??= body;
      else if (
        attrs.get("type")?.trim().toLowerCase() === "application/ld+json"
      ) {
        scan.jsonLd.push(body);
      }
      re.lastIndex = close.lastIndex;
    }
  }
  return scan;
}

/** Returns `value` trimmed, or undefined when nothing is left. */
function nonBlank(value: string | undefined): string | undefined {
  const trimmed = value?.trim();
  return trimmed ? trimmed : undefined;
}

/**
 * Returns the page's `theme-color`, as written: pass it to
 * {@link normalizeCssColor}. Sites often ship one per color scheme with the
 * dark one first, so the unscoped tag wins, then the light one, then any.
 */
function themeColorOf(metas: Attrs[]): string | undefined {
  let unscoped: string | undefined;
  let light: string | undefined;
  let any: string | undefined;
  for (const meta of metas) {
    if (meta.get("name")?.toLowerCase() !== "theme-color") continue;
    const content = nonBlank(meta.get("content"));
    if (!content) continue;
    const media = meta.get("media");
    if (!media) unscoped ??= content;
    else if (/light/i.test(media)) light ??= content;
    any ??= content;
  }
  return unscoped ?? light ?? any;
}

/** Returns the lowercase tokens of a `<link>`'s `rel`. */
function relTokens(link: Attrs): string[] {
  return (link.get("rel") ?? "").toLowerCase().split(/\s+/);
}

/**
 * Returns the `color` of `<link rel="mask-icon">`, Safari's pinned-tab icon,
 * as written. The generic preview's last resort for a brand color.
 */
function maskIconColorOf(links: Attrs[]): string | undefined {
  for (const link of links) {
    if (!relTokens(link).includes("mask-icon")) continue;
    const color = nonBlank(link.get("color"));
    if (color) return color;
  }
  return undefined;
}

/**
 * Returns the absolute URL of the page's largest icon, among its `icon` and
 * `apple-touch-icon` links, by declared `sizes`; the first one when none
 * declares a size. A `mask-icon` is no candidate: it is a one-color shape for
 * Safari to tint, black when drawn as is.
 *
 * An SVG icon wins over any size: it declares none, yet scales to any.
 */
function bestIconOf(links: Attrs[], baseUrl: string): string | undefined {
  let best: { href: string; area: number } | undefined;
  for (const link of links) {
    const rel = relTokens(link);
    if (
      !rel.includes("icon") && !rel.includes("apple-touch-icon") &&
      !rel.includes("apple-touch-icon-precomposed")
    ) continue;
    const href = absoluteHttpUrl(link.get("href"), baseUrl);
    if (!href) continue;
    const sizes = (link.get("sizes") ?? "").toLowerCase();
    const isSvg = /\.svg(\?|$)/i.test(href) ||
      link.get("type")?.toLowerCase() === "image/svg+xml" ||
      sizes === "any";
    const area = isSvg ? Number.MAX_SAFE_INTEGER : Math.max(
      0,
      ...[...sizes.matchAll(/(\d+)x(\d+)/g)].map(([, w, h]) => +w * +h),
    );
    if (!best || area > best.area) best = { href, area };
  }
  return best?.href;
}

/**
 * Returned by the JSON-LD reading of {@link parsePage}, which the generic
 * preview ranks below the page's OG and Twitter tags.
 */
export interface JsonLdResult {
  /** From `name`, or else `headline`. */
  title?: string;
  description?: string;
  /** From `image`, or else `thumbnailUrl` or `thumbnail`. */
  thumbnailUrl?: string;
}

/**
 * The JSON-LD types that describe the site around a page rather than the
 * page itself. Their name is the site's or the company's, so a node of one of
 * them is read only when no other node has a title or an image.
 */
const SITE_LD_TYPES = new Set([
  "WebSite",
  "Organization",
  "Corporation",
  "NewsMediaOrganization",
  "Person",
  "BreadcrumbList",
  "SearchAction",
  "ImageObject",
  "SiteNavigationElement",
  "WPHeader",
  "WPFooter",
]);

function ldString(v: unknown): string | undefined {
  if (typeof v === "string") return nonBlank(v);
  if (Array.isArray(v) && typeof v[0] === "string") return nonBlank(v[0]);
  return undefined;
}

function ldImage(v: unknown, baseUrl: string): string | undefined {
  if (typeof v === "string") return absoluteHttpUrl(v, baseUrl);
  if (Array.isArray(v)) return ldImage(v[0], baseUrl);
  if (v && typeof v === "object") {
    const o = v as Record<string, unknown>;
    return ldImage(o.url ?? o.contentUrl, baseUrl);
  }
  return undefined;
}

/** Lists every node of `data`, through arrays and `@graph`, in order. */
function ldNodes(data: unknown): Record<string, unknown>[] {
  if (Array.isArray(data)) return data.flatMap(ldNodes);
  if (!data || typeof data !== "object") return [];
  const o = data as Record<string, unknown>;
  return o["@graph"] ? ldNodes(o["@graph"]) : [o];
}

/**
 * Returns the first JSON-LD node about the page that has a title or an image,
 * else the first about its site ({@link SITE_LD_TYPES}); empty when there is
 * none. An image may be a URL, a list of them or an `ImageObject`. Blocks that
 * aren't valid JSON are skipped.
 */
function jsonLdOf(blocks: string[], baseUrl: string): JsonLdResult {
  let fallback: JsonLdResult | undefined;
  for (const block of blocks) {
    let data: unknown;
    try {
      data = JSON.parse(block);
    } catch {
      continue;
    }
    for (const node of ldNodes(data)) {
      const result: JsonLdResult = {
        title: ldString(node.name ?? node.headline),
        description: ldString(node.description),
        thumbnailUrl: ldImage(
          node.image ?? node.thumbnailUrl ?? node.thumbnail,
          baseUrl,
        ),
      };
      if (!result.title && !result.thumbnailUrl) continue;
      const types = [node["@type"]].flat();
      if (!types.some((t) => typeof t === "string" && SITE_LD_TYPES.has(t))) {
        return result;
      }
      fallback ??= result;
    }
  }
  return fallback ?? {};
}

/**
 * Returns the source of an `<img>`: its `src`, or for a lazy-loaded image
 * whose `src` is a `data:` placeholder, its `data-src` or `data-lazy-src`.
 */
function imageSource(img: Attrs, baseUrl: string): string | undefined {
  const src = img.get("src")?.trim();
  return src && !src.startsWith("data:")
    ? absoluteHttpUrl(src, baseUrl)
    : absoluteHttpUrl(img.get("data-src") ?? img.get("data-lazy-src"), baseUrl);
}

/** Returns an `<img>`'s declared `name` dimension in pixels; 0 when none. */
function declaredSize(img: Attrs, name: "width" | "height"): number {
  return parseInt(img.get(name) ?? "") || 0;
}

/**
 * Returns the absolute URL of the first `<img>` whose declared width and height
 * are both at least `minSize` pixels; an image without them doesn't count.
 */
function largeImageOf(
  imgs: Attrs[],
  baseUrl: string,
  minSize = 200,
): string | undefined {
  for (const img of imgs) {
    if (
      declaredSize(img, "width") < minSize ||
      declaredSize(img, "height") < minSize
    ) continue;
    const src = imageSource(img, baseUrl);
    if (src) return src;
  }
  return undefined;
}

/** Filename words that mark an image as page furniture rather than content. */
const UI_IMAGE_KEYWORDS = new Set([
  "logo",
  "icon",
  "sprite",
  "favicon",
  "avatar",
  "banner",
  "header",
  "nav",
  "menu",
  "cart",
  "search",
  "tracking",
  "pixel",
  "bg",
  "background",
  "placeholder",
  "spacer",
  "blank",
  "lazy",
  "loading",
]);

/**
 * Checks whether `src` looks like page furniture: an SVG, or a filename with
 * one of the {@link UI_IMAGE_KEYWORDS} between its `-`, `_` or `.`.
 */
function isUiImage(src: string): boolean {
  if (/\.svg(\?|$)/i.test(src)) return true;
  const filename = src.split("?")[0].split("/").pop() ?? "";
  const baseName = filename.replace(/\.[^.]+$/, "");
  return baseName.toLowerCase().split(/[-_.]/).some((t) =>
    UI_IMAGE_KEYWORDS.has(t)
  );
}

/**
 * Returns the absolute URL of the first `<img>` that looks like content rather
 * than a logo, icon, tracking pixel or other page furniture: one declared
 * smaller than 50 pixels either way doesn't count. The generic preview's last
 * resort for a thumbnail.
 */
function firstContentImageOf(
  imgs: Attrs[],
  baseUrl: string,
): string | undefined {
  for (const img of imgs) {
    const w = declaredSize(img, "width");
    const h = declaredSize(img, "height");
    if ((w && w < 50) || (h && h < 50)) continue;
    const src = imageSource(img, baseUrl);
    if (src && !isUiImage(src)) return src;
  }
  return undefined;
}

/**
 * Returned by {@link parsePage}: what a page says about itself, entities
 * decoded, values trimmed and URLs absolute.
 */
export interface Page {
  /**
   * Returns the `content` of the first `<meta>` whose `property` or `name` is
   * `key`, in any case: sites write `og:` tags under either attribute, and
   * Twitter's too.
   */
  meta(key: string): string | undefined;
  /** The text of the `<title>` element. */
  title?: string;
  jsonLd: JsonLdResult;
  /** The `theme-color`, as written: pass it to {@link normalizeCssColor}. */
  themeColor?: string;
  /** The `color` of the Safari pinned-tab icon, as written. */
  maskIconColor?: string;
  /** The largest icon the page links, which the thumbnail placeholder draws. */
  bestIcon?: string;
  /** The first image declared at least 200 pixels both ways. */
  largeImage?: string;
  /** The first image that doesn't look like page furniture. */
  firstContentImage?: string;
}

/**
 * Reads `html`, served from `pageUrl`, in one pass. The generic preview and
 * the providers built on it read a page through it.
 */
export function parsePage(html: string, pageUrl: string): Page {
  const scan = scanTags(html);

  const metas = new Map<string, string>();
  for (const meta of scan.metas) {
    const content = nonBlank(meta.get("content"));
    if (!content) continue;
    for (const attr of ["property", "name"]) {
      const key = meta.get(attr)?.trim().toLowerCase();
      if (key && !metas.has(key)) metas.set(key, content);
    }
  }

  return {
    meta: (key) => metas.get(key.toLowerCase()),
    title: nonBlank(scan.title && decodeHtmlEntities(scan.title)),
    jsonLd: jsonLdOf(scan.jsonLd, pageUrl),
    themeColor: themeColorOf(scan.metas),
    maskIconColor: maskIconColorOf(scan.links),
    bestIcon: bestIconOf(scan.links, pageUrl),
    largeImage: largeImageOf(scan.imgs, pageUrl),
    firstContentImage: firstContentImageOf(scan.imgs, pageUrl),
  };
}

/**
 * Returns `raw` as a lowercase `#rrggbb`, from a hex, `rgb()`, `rgba()` or
 * common named color; undefined for any other form. The generic preview
 * stores it as the preview's `accentColor`, a tint over the app's own surface,
 * so alpha is dropped, and a fully transparent one is no color at all.
 */
export function normalizeCssColor(raw: string | undefined): string | undefined {
  const color = raw ? parseCssColor(raw) : undefined;
  return color && color.a > 0 ? hexOf(color) : undefined;
}