/**
 * Resolves a favicon URL for a given domain.
 *
 * On Chrome MV3 with the `favicon` permission, returns a URL backed by the
 * browser's own favicon cache (no network, no third party). On Firefox and
 * other browsers where the API is unavailable, returns null so callers can
 * render a generic fallback icon.
 *
 * Never fetches from any external URL. No domain name leaves the browser.
 */
export function faviconUrl(domain: string): string | null {
  if (!domain) return null;
  if (typeof chrome === "undefined" || !chrome.runtime?.getURL) return null;

  try {
    const url = new URL(chrome.runtime.getURL("/_favicon/"));
    url.searchParams.set("pageUrl", `https://${domain}`);
    url.searchParams.set("size", "32");
    return url.toString();
  } catch {
    return null;
  }
}

/**
 * Derives 2-letter initials from a domain or title string. Used as the
 * favicon fallback label when no real favicon is available.
 */
export function titleInitials(source: string): string {
  const cleaned = source.replace(/^https?:\/\//, "").replace(/^www\./, "").trim();
  if (!cleaned) return "·";
  const words = cleaned.split(/[\s.\-_/]+/).filter(Boolean);
  if (words.length >= 2 && words[0]![0] && words[1]![0]) {
    return (words[0]![0]! + words[1]![0]!).toUpperCase();
  }
  const single = words[0] ?? cleaned;
  return single.slice(0, 2).toUpperCase();
}

/**
 * Hashes a string to a deterministic HSL background colour for an initials
 * chip. Fixed saturation + lightness keeps the palette muted and dark so
 * light initials read clearly on top.
 */
export function titleBackground(source: string): string {
  let hash = 0;
  for (let i = 0; i < source.length; i++) {
    hash = ((hash << 5) - hash + source.charCodeAt(i)) | 0;
  }
  const hue = ((hash % 360) + 360) % 360;
  return `hsl(${hue}, 32%, 26%)`;
}

// Renders an img element to an 8×8 canvas and returns its RGB pixels as a
// compact string. Used to fingerprint what Chrome returns for "no favicon".
function imgSig(img: HTMLImageElement): string {
  const c = document.createElement("canvas");
  c.width = c.height = 8;
  const ctx = c.getContext("2d")!;
  ctx.drawImage(img, 0, 0, 8, 8);
  const d = ctx.getImageData(0, 0, 8, 8).data;
  let s = "";
  for (let i = 0; i < d.length; i += 4) s += `${d[i]},${d[i + 1]},${d[i + 2]};`;
  return s;
}

// Cached promise so the globe reference image is only loaded once per page.
let _globeSig: Promise<string | null> | undefined;

/**
 * Returns the pixel signature of what Chrome's _favicon/ endpoint returns
 * when it has no stored favicon for a domain (the default globe icon).
 * Cached after the first call. Returns null when the favicon API is
 * unavailable (Firefox) or the canvas read is blocked.
 */
export function globeFaviconSig(): Promise<string | null> {
  if (_globeSig) return _globeSig;
  _globeSig = new Promise<string | null>((resolve) => {
    const src = faviconUrl("zz-peach-no-favicon-sentinel.nonexistent");
    if (!src) { resolve(null); return; }
    const img = new Image();
    img.onload = () => { try { resolve(imgSig(img)); } catch { resolve(null); } };
    img.onerror = () => resolve(null);
    img.src = src;
  });
  return _globeSig;
}

/**
 * Returns the 8×8 canvas pixel signature for a loaded img element.
 * Returns null if the canvas read is blocked (e.g. security error).
 */
export function faviconImgSig(img: HTMLImageElement): string | null {
  try { return imgSig(img); } catch { return null; }
}