initial import: random background api

functions/lib/background.js

@@ -0,0 +1,52 @@
+const FALLBACK_MANIFEST = {
+  variants: {
+    mobile: { folder: 'mobile-image', images: [] },
+    desktop: { folder: 'desketop-image', images: [] },
+  },
+};
+
+export async function loadManifest(request) {
+  try {
+    const response = await fetch(new URL('/manifest.json', request.url));
+    if (!response.ok) return FALLBACK_MANIFEST;
+    return await response.json();
+  } catch {
+    return FALLBACK_MANIFEST;
+  }
+}
+
+export function detectVariant(request) {
+  const url = new URL(request.url);
+  const params = url.searchParams;
+  const mode = (params.get('mode') || 'auto').toLowerCase();
+  const orientation = (params.get('orientation') || '').toLowerCase();
+  const device = (params.get('device') || '').toLowerCase();
+  const ua = (request.headers.get('user-agent') || '').toLowerCase();
+  const chMobile = request.headers.get('sec-ch-ua-mobile');
+
+  if (mode === 'mobile' || mode === 'desktop') return mode;
+  if (orientation === 'portrait' || orientation === 'vertical') return 'mobile';
+  if (orientation === 'landscape' || orientation === 'horizontal') return 'desktop';
+  if (device === 'mobile' || device === 'desktop') return device;
+  if (chMobile === '?1') return 'mobile';
+  if (chMobile === '?0') return 'desktop';
+  if (['mobi', 'android', 'iphone', 'ipad', 'ipod', 'phone'].some((token) => ua.includes(token))) return 'mobile';
+  return 'desktop';
+}
+
+export function getVariantPayload(manifest, variant) {
+  const data = manifest?.variants?.[variant] || {};
+  const folder = data.folder || (variant === 'mobile' ? 'mobile-image' : 'desketop-image');
+  const images = Array.isArray(data.images)
+    ? data.images.filter((name) => typeof name === 'string' && name.toLowerCase().endsWith('.webp'))
+    : [];
+  return { folder, images };
+}
+
+export function buildAssetUrl(request, folder, filename) {
+  return new URL(`/${folder}/${filename}`, request.url).toString();
+}
+
+export function pickRandom(items) {
+  return items[Math.floor(Math.random() * items.length)];
+}