initial import: random background api

functions/api/list.js

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+import { getVariantPayload, loadManifest } from '../lib/background.js';
+
+export async function onRequestGet(context) {
+  const { request } = context;
+  const manifest = await loadManifest(request);
+
+  return Response.json(
+    {
+      generated_at: manifest?.generated_at || null,
+      variants: {
+        mobile: summarizeVariant(manifest, 'mobile'),
+        desktop: summarizeVariant(manifest, 'desktop'),
+      },
+    },
+    { headers: { 'Cache-Control': 'no-store, max-age=0', 'Access-Control-Allow-Origin': '*' } },
+  );
+}
+
+function summarizeVariant(manifest, variant) {
+  const { folder, images } = getVariantPayload(manifest, variant);
+  return { folder, count: images.length, images };
+}

functions/api/random.js

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+import { buildAssetUrl, detectVariant, getVariantPayload, loadManifest, pickRandom } from '../lib/background.js';
+
+export async function onRequestGet(context) {
+  const { request } = context;
+  const url = new URL(request.url);
+  const format = (url.searchParams.get('format') || 'redirect').toLowerCase();
+  const manifest = await loadManifest(request);
+  const variant = detectVariant(request);
+  const { folder, images } = getVariantPayload(manifest, variant);
+
+  if (!images.length) {
+    return Response.json(
+      { error: 'no images available', variant, folder },
+      { status: 404, headers: { 'Cache-Control': 'no-store, max-age=0', 'Access-Control-Allow-Origin': '*' } },
+    );
+  }
+
+  const filename = pickRandom(images);
+  const assetUrl = buildAssetUrl(request, folder, filename);
+
+  if (format === 'json') {
+    return Response.json(
+      { variant, folder, filename, url: assetUrl, count: images.length },
+      { headers: { 'Cache-Control': 'no-store, max-age=0', 'Access-Control-Allow-Origin': '*' } },
+    );
+  }
+
+  return new Response(null, {
+    status: 302,
+    headers: {
+      Location: assetUrl,
+      'Cache-Control': 'no-store, max-age=0',
+      'Access-Control-Allow-Origin': '*',
+    },
+  });
+}

functions/lib/background.js

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+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)];
+}