/**
 * Edge middleware for cran.rpkgs.com.
 *
 * Two jobs:
 *
 * 1. Resolve the bare `https://cran.rpkgs.com` form to a concrete
 *    `<arch>/<os>` slot from the User-Agent, or send the client to CRAN when
 *    the distro cannot be identified.
 * 2. Route `PACKAGES*` requests to the per-R-minor slot
 *    (`…/latest/src/contrib/<x.y>/`), so a stock `install.packages()` sees the
 *    packages that only exist there.
 *
 * Only index files are routed. Tarballs are deliberately left alone: R keeps
 * the contrib URL it asked for, not the one it was redirected to, so every
 * tarball URL is resolved against the flat directory and the union index steers
 * the per-minor ones with a `Path: <x.y>` field. Rewriting a tarball request
 * here would send flat-slot packages into a directory that does not hold them.
 *
 * Routing is gated on UNION_SLOTS. The raw per-minor index holds only the
 * ABI-sensitive subset of a slot; it is safe to route to it only once bincraft
 * has republished it as a union of the per-minor and flat slots.
 *
 * Deployed by OpenTofu from this file (`bunnynet_compute_script.rpkgs_router`).
 * Test with `just edge-test`.
 */
import * as BunnySDK from 'https://esm.sh/@bunny.net/edgescript-sdk@0.12';

const PUBLIC_CDN_ORIGIN = 'https://cran.rpkgs.com';
const CRAN_ORIGIN = 'https://cran.r-project.org';

/** Slots ("<arch>/<os>", comma separated) whose per-minor index is a union. */
const UNION_SLOTS = new Set(
  (Deno.env.get('UNION_SLOTS') ?? '')
    .split(',')
    .map((slot) => slot.trim())
    .filter((slot) => slot.length > 0),
);

/** `/<arch>/<os>/latest/src/contrib[/<rest>]` */
const SLOT_PATH_REGEX = /^\/(amd64|arm64)\/([a-z0-9._-]+)\/latest\/src\/contrib\/?(.*)$/;

/** A path that already sits in a per-minor slot, e.g. `4.5/PACKAGES.gz`. */
const MINOR_DIR_REGEX = /^\d+\.\d+\//;

/** The only files this script routes. */
const INDEX_FILE_REGEX = /^PACKAGES(\.gz|\.rds)?$/;

const SRC_CONTRIB_REGEX = /^\/src\/contrib\/(.+)$/;

const MACOS_BIN_REGEX =
  /^\/bin\/macosx\/(big-sur-arm64|big-sur-x86_64|monterey-arm64|monterey-x86_64)\/contrib\/([0-9.]+)\/(.+)$/;

const RHEL_REGEX = /(almalinux|rocky)[^\d]*(\d+)/i;

const UBUNTU_REGEX = /Ubuntu ([\d.]+)/i;
const UBUNTU_CODENAMES: Record<string, string> = {
  '24.04': 'noble',
  '22.04': 'jammy',
};

const ALPINE_REGEX = /(?:Alpine Linux(?:\s+VERSION_ID=)?|alpine-)\s*(\d+)\.(\d+)/i;

/**
 * R's own User-Agent is `R (4.5.3 x86_64-pc-linux-musl …)`; the Posit-style one
 * some sites configure is `R/4.5.3 R (…)`. Both carry the minor, which is why
 * per-minor routing works without the distro being identifiable.
 */
const R_MINOR_REGEXES = [/\bR\/(\d+)\.(\d+)/, /\bR \((\d+)\.(\d+)/];

function normalizePathname(pathname: string): string {
  return pathname.replace(/\/{2,}/g, '/');
}

function redirectTo(location: string, status = 302): Response {
  return new Response(null, {
    status,
    headers: {
      Location: location,
      // The target depends on the User-Agent, so the redirect itself must
      // never be cached; only its target is a cacheable, UA-independent URL.
      'Cache-Control': 'no-store',
      'X-Via': 'MyMiddleware',
      'X-Rewritten-By': 'rpkgs-edge-middleware',
    },
  });
}

function extractRMinor(userAgent: string): string | null {
  for (const regex of R_MINOR_REGEXES) {
    const match = userAgent.match(regex);
    if (match) {
      return `${match[1]}.${match[2]}`;
    }
  }
  return null;
}

function mapArch(arch: string): string {
  if (arch === 'aarch64') return 'arm64';
  if (arch === 'x86_64') return 'amd64';
  return arch;
}

function extractArch(userAgent: string): string {
  const match = userAgent.match(/(x86_64|aarch64|arm64|i386|i686)/);
  return match ? mapArch(match[1]) : '';
}

/**
 * Identify the `<arch>/<os>` slot from the User-Agent, or null.
 *
 * A stock R User-Agent carries only `linux-gnu` / `linux-musl`, which are not
 * slot names: returning them produced redirects into slots that do not exist
 * (`/amd64/linux-musl/latest/…`, a guaranteed 404). An unidentifiable distro
 * is reported as such so the caller can fall back to CRAN.
 */
function parseSlot(userAgent: string): string | null {
  const arch = extractArch(userAgent);
  if (!arch) {
    return null;
  }

  const rhel = userAgent.match(RHEL_REGEX);
  if (rhel) {
    return `${arch}/rhel${rhel[2]}`;
  }

  const ubuntu = userAgent.match(UBUNTU_REGEX);
  if (ubuntu) {
    const codename = UBUNTU_CODENAMES[ubuntu[1]];
    if (codename) {
      return `${arch}/${codename}`;
    }
  }

  const alpine = userAgent.match(ALPINE_REGEX);
  if (alpine) {
    return `${arch}/alpine${alpine[1]}${alpine[2]}`;
  }

  return null;
}

function parseMacUserAgent(userAgent: string): { os: string; arch: string; rver: string } | null {
  const rverMatch = userAgent.match(/R \((\d+)\.(\d+)/);
  const archMatch = userAgent.match(/(aarch64|arm64|x86_64)/);
  const osMatch = userAgent.match(/darwin(\d+)/);

  if (!rverMatch || !archMatch || !osMatch) {
    return null;
  }

  const arch = archMatch[1] === 'aarch64' ? 'arm64' : archMatch[1];
  const darwinVer = parseInt(osMatch[1], 10);
  const os = darwinVer >= 21 && darwinVer < 22 ? `monterey-${arch}` : `big-sur-${arch}`;

  return { os, arch, rver: `${rverMatch[1]}.${rverMatch[2]}` };
}

/**
 * The contrib path a request should be served from, relative to the slot.
 *
 * Returns the per-minor path for an index file when the slot is known to carry
 * a union index and the client's R minor is known; otherwise the flat path,
 * which is what every client sees today.
 */
function contribPath(slot: string, rest: string, userAgent: string): string {
  const flat = rest ? `/${slot}/latest/src/contrib/${rest}` : `/${slot}/latest/src/contrib`;

  if (!INDEX_FILE_REGEX.test(rest) || !UNION_SLOTS.has(slot)) {
    return flat;
  }

  const rMinor = extractRMinor(userAgent);
  return rMinor ? `/${slot}/latest/src/contrib/${rMinor}/${rest}` : flat;
}

BunnySDK.net.http
  .servePullZone({ url: 'https://cran.rpkgs.com/' })
  .onOriginRequest((ctx) => {
    const url = new URL(ctx.request.url);
    const path = normalizePathname(url.pathname);
    const userAgent = ctx.request.headers.get('User-Agent') || '';

    // macOS clients are served from CRAN's own binary tree.
    const srcContrib = path.match(SRC_CONTRIB_REGEX);
    if (srcContrib && /darwin/.test(userAgent)) {
      const mac = parseMacUserAgent(userAgent);
      if (mac) {
        return Promise.resolve(
          redirectTo(`${PUBLIC_CDN_ORIGIN}/bin/macosx/${mac.os}/contrib/${mac.rver}/${srcContrib[1]}`),
        );
      }
    }

    if (MACOS_BIN_REGEX.test(path)) {
      return Promise.resolve(redirectTo(`${CRAN_ORIGIN}${path}`));
    }

    // Already-qualified slot URLs: what the runtime images have baked in.
    const slotPath = path.match(SLOT_PATH_REGEX);
    if (slotPath) {
      const slot = `${slotPath[1]}/${slotPath[2]}`;
      const rest = slotPath[3];

      // Never rewrite a request that is already in a per-minor slot, or the
      // redirect would chase its own tail.
      if (MINOR_DIR_REGEX.test(rest)) {
        return Promise.resolve(ctx.request);
      }

      const target = contribPath(slot, rest, userAgent);
      if (target === path) {
        return Promise.resolve(ctx.request);
      }
      return Promise.resolve(redirectTo(`${PUBLIC_CDN_ORIGIN}${target}`));
    }

    // The bare `https://cran.rpkgs.com` form, resolved from the User-Agent.
    if (path === '/' || path === '/src/contrib' || path.startsWith('/src/contrib/')) {
      const slot = parseSlot(userAgent);
      if (!slot) {
        return Promise.resolve(redirectTo(`${CRAN_ORIGIN}${path}`));
      }

      const rest = srcContrib ? srcContrib[1] : '';
      return Promise.resolve(redirectTo(`${PUBLIC_CDN_ORIGIN}${contribPath(slot, rest, userAgent)}`));
    }

    return Promise.resolve(ctx.request);
  })
  .onOriginResponse((ctx) => {
    ctx.response.headers.append('X-Via', 'MyMiddleware');
    return Promise.resolve(ctx.response);
  });