#!/usr/bin/env bun
// Generates src/apps/frontend/index.gen.html from index.html, substituting __PUBLIC_URL__ with
// PUBLIC_URL from .env.
//
// Favicon and OpenGraph hrefs have to be absolute. Bun's HTML bundler treats a root-relative href as
// an asset to resolve on disk and fails the build ("Could not resolve: /favicon.ico"), and crawlers
// fetch OG tags standalone so they cannot resolve a relative URL either. Hardcoding the domain would
// force every deployment to carry its own edit of index.html, and Bun's HTML import offers no
// substitution hook — so the swap happens here and the server imports the generated file.
//
// index.gen.html is gitignored; index.html is the tracked template and is identical on every instance.

import { existsSync } from "node:fs";
import { dirname, join, resolve } from "node:path";
import { fileURLToPath } from "node:url";

const root = resolve(dirname(fileURLToPath(import.meta.url)), "..");
const template = join(root, "src/apps/frontend/index.html");
const output = join(root, "src/apps/frontend/index.gen.html");

// Where the URL comes from, most specific first:
//
//   1. the first argument   `bun gen:index https://example.com`
//   2. PUBLIC_URL in this project's .env
//   3. PUBLIC_URL in the environment
//
// Note the order: .env BEATS the ambient environment, which is the reverse of the usual convention.
// A host that exports its own PUBLIC_URL into every process it spawns would otherwise stamp its
// domain onto this app's favicon hrefs. A value in this project's own .env is a statement about this
// project; an inherited one is a statement about whatever launched it.
const argUrl = process.argv[2]?.trim();

let dotenvUrl: string | undefined;
const envPath = join(root, ".env");
if (!argUrl && existsSync(envPath)) {
  for (const line of (await Bun.file(envPath).text()).split("\n")) {
    const match = line.match(/^\s*PUBLIC_URL\s*=\s*(.*)$/);
    if (match) dotenvUrl = match[1]!.trim().replace(/^["']|["']$/g, "");
  }
}

const publicUrl = (argUrl || dotenvUrl || process.env.PUBLIC_URL || "").replace(/\/+$/, "");
if (!publicUrl) {
  console.error("[gen-index] no public URL. Pass one — `bun gen:index http://localhost:3001` —");
  console.error("[gen-index] or set PUBLIC_URL in .env.");
  process.exit(1);
}

// Caught here rather than left to a crawler: a relative or scheme-less value substitutes without
// complaint and produces tags nothing can resolve, which stays invisible until someone shares a link.
try {
  const parsed = new URL(publicUrl);
  if (parsed.protocol !== "http:" && parsed.protocol !== "https:") throw new Error("not http(s)");
} catch {
  console.error(`[gen-index] "${publicUrl}" is not an absolute http(s) URL.`);
  process.exit(1);
}

const html = await Bun.file(template).text();
const generated = html.replaceAll("__PUBLIC_URL__", publicUrl);

if (generated.includes("__PUBLIC_URL__")) {
  console.error("[gen-index] substitution left placeholders behind");
  process.exit(1);
}

// Only write when the content actually changes, so `bun --hot` does not restart in a loop.
if (existsSync(output) && (await Bun.file(output).text()) === generated) {
  console.log(`[gen-index] up to date (${publicUrl})`);
} else {
  await Bun.write(output, generated);
  console.log(`[gen-index] wrote index.gen.html (${publicUrl})`);
}