shd.build/src/routes/builds/[slug]/preview.png/+server.ts

import { createHash } from 'node:crypto';
import { error } from '@sveltejs/kit';
import {
  renderBuildSocialCardPng,
  type BuildSocialCardFact,
  type BuildSocialCardInput,
} from '$lib/server/build-social-card';
import { publicOrigin } from '$lib/server/public-origin';
import { loadSharedBuildProjection } from '$lib/server/shared-build';
import type { RequestHandler } from './$types';

const number = new Intl.NumberFormat('en-US', { maximumFractionDigits: 2 });
const MAX_CACHE_BYTES = 16 * 1024 * 1024;
const MAX_ACTIVE_REQUESTS = 4;
const MAX_QUEUED_REQUESTS = 32;
type CachedImage = { body: Buffer; etag: string };
const imageCache = new Map<string, CachedImage>();
const pendingImages = new Map<string, Promise<CachedImage>>();
const requestQueue: Array<(release: () => void) => void> = [];
let cacheBytes = 0;
let activeRequests = 0;

function unitValue(value: number, unit: string | null | undefined) {
  const rendered = number.format(value);
  return unit === 'Shots' ? `${rendered} ${unit}` : `${rendered}${unit ?? ''}`;
}

export function _socialCardInput(
  projection: NonNullable<
    Awaited<ReturnType<typeof loadSharedBuildProjection>>
  >,
  directUrl: string,
): BuildSocialCardInput {
  const attribute = (id: string) =>
    projection.attributes.find((item) => String(item.id) === id);
  const statByName = (name: string) =>
    projection.summary.stats.find(
      (stat) => String(attribute(stat.id)?.name) === name,
    );
  const statValue = (name: string) => {
    const stat = statByName(name);
    return stat ? unitValue(Number(stat.value), attribute(stat.id)?.unit) : '0';
  };
  const weaponName = (index: number) => {
    const selected = projection.loadout.weapons[index];
    return (
      projection.weapons.find((item) => String(item.id) === selected?.itemId)
        ?.name ?? 'Unassigned'
    );
  };
  const coreNames = new Set(['Firepower', 'Toughness', 'Engineering']);
  const supportingAttributes: BuildSocialCardFact[] = projection.summary.stats
    .filter((stat) => !coreNames.has(String(attribute(stat.id)?.name)))
    .slice(0, 3)
    .map((stat) => ({
      label: String(attribute(stat.id)?.name ?? 'Attribute'),
      value: unitValue(Number(stat.value), attribute(stat.id)?.unit),
    }));

  return {
    title: projection.build.title,
    author: projection.build.author,
    primaryWeapon: String(weaponName(0)),
    secondaryWeapon: String(weaponName(1)),
    firepower: statValue('Firepower'),
    toughness: statValue('Toughness'),
    engineering: statValue('Engineering'),
    supportingAttributes,
    visibility: projection.build.visibility as 'public' | 'unlisted',
    directUrl,
  };
}

function matchesEtag(request: Request, etag: string) {
  return request.headers
    .get('if-none-match')
    ?.split(',')
    .some((value) => value.trim() === etag);
}

function releaseRequest() {
  const next = requestQueue.shift();
  if (next) {
    next(releaseRequest);
    return;
  }
  activeRequests -= 1;
}

function acquireRequest(): Promise<(() => void) | null> {
  if (activeRequests < MAX_ACTIVE_REQUESTS) {
    activeRequests += 1;
    return Promise.resolve(releaseRequest);
  }
  if (requestQueue.length >= MAX_QUEUED_REQUESTS) return Promise.resolve(null);
  return new Promise((resolve) => requestQueue.push(resolve));
}

function storeImage(key: string, image: CachedImage) {
  imageCache.set(key, image);
  cacheBytes += image.body.byteLength;
  while (cacheBytes > MAX_CACHE_BYTES && imageCache.size > 1) {
    const oldestKey = imageCache.keys().next().value;
    if (!oldestKey) break;
    const oldest = imageCache.get(oldestKey);
    imageCache.delete(oldestKey);
    cacheBytes -= oldest?.body.byteLength ?? 0;
  }
}

async function loadImage(input: BuildSocialCardInput, inputKey: string) {
  const cached = imageCache.get(inputKey);
  if (cached) {
    imageCache.delete(inputKey);
    imageCache.set(inputKey, cached);
    return cached;
  }
  const pending = pendingImages.get(inputKey);
  if (pending) return pending;

  const request = renderBuildSocialCardPng(input).then((body) => {
    const image = {
      body,
      etag: `"${createHash('sha256').update(body).digest('base64url')}"`,
    };
    storeImage(inputKey, image);
    return image;
  });
  pendingImages.set(inputKey, request);
  try {
    return await request;
  } finally {
    pendingImages.delete(inputKey);
  }
}

export const GET: RequestHandler = async ({ params, request, url }) => {
  const release = await acquireRequest();
  if (!release) error(503, 'Build preview capacity exceeded');
  try {
    const projection = await loadSharedBuildProjection(params.slug, {
      kind: 'public-preview',
    });
    if (!projection) error(404, 'Build preview not found');

    const origin = publicOrigin(url.origin);
    const directUrl = new URL(
      `/builds/${encodeURIComponent(projection.build.slug)}`,
      origin,
    ).toString();
    const input = _socialCardInput(projection, directUrl);
    const inputKey = createHash('sha256')
      .update(JSON.stringify({ version: 1, input }))
      .digest('base64url');
    const image = await loadImage(input, inputKey);
    const cacheControl = 'public, max-age=0, must-revalidate';
    if (matchesEtag(request, image.etag)) {
      return new Response(null, {
        status: 304,
        headers: { 'cache-control': cacheControl, etag: image.etag },
      });
    }

    return new Response(new Uint8Array(image.body), {
      headers: {
        'cache-control': cacheControl,
        'content-length': String(image.body.byteLength),
        'content-type': 'image/png',
        'cross-origin-resource-policy': 'same-origin',
        etag: image.etag,
        'x-content-type-options': 'nosniff',
        ...(projection.build.visibility === 'unlisted'
          ? { 'x-robots-tag': 'noindex' }
          : {}),
      },
    });
  } finally {
    release();
  }
};