Add tool to write multi-module tests in a single file

tools/misc/explode-bundle.mjs

@@ -0,0 +1,85 @@
+import fs from "node:fs";
+import { resolve, basename, join as joinPaths } from "node:path";
+import { fileURLToPath } from "node:url";
+
+async function* walk(dir) {
+	for await (const d of fs.opendirSync(dir)) {
+		const entry = joinPaths(dir, d.name);
+		if (d.isDirectory()) yield* walk(entry);
+		else if (d.isFile()) yield entry;
+	}
+}
+
+const testsDir = resolve(fileURLToPath(import.meta.url), "../../../test");
+
+for await (const bundleFilename of walk(testsDir)) {
+	if (/\.multi(?:\.md)$/.test(bundleFilename)) {
+		const folder = bundleFilename.replace(/\.multi(?:\.md)$/, "__generated");
+
+		const bundle = fs.readFileSync(bundleFilename, "utf8");
+		const allFiles = parse(bundleFilename, bundle);
+
+		const fixtureRE = /_FIXTURE\.\w+$/;
+		if (allFiles.filter((f) => !fixtureRE.test(f.filename)).length !== 1) {
+			throw new Error(
+				`Exactly one file must not end in _FIXTURE (in ${bundleFilename})`
+			);
+		}
+
+		fs.rmSync(folder, { force: true, recursive: true });
+		fs.mkdirSync(folder);
+		for (const { filename, code } of allFiles) {
+			fs.writeFileSync(joinPaths(folder, filename), code);
+		}
+	}
+}
+
+/**
+ * @param {string} bundleFilename
+ * @param {string} bundle
+ */
+function parse(bundleFilename, bundle) {
+	let index = 0;
+
+	const files = [];
+	let prefix = null;
+
+	while (true) {
+		let nextTitle = bundle.indexOf("##", index);
+		let nextCodeStart = bundle.indexOf("```", index);
+		let nextCodeEnd = bundle.indexOf("```", nextCodeStart + 3);
+		if (nextCodeStart === -1 && nextTitle === -1) break;
+
+		if (nextTitle === -1 || nextTitle > nextCodeStart) {
+			throw new Error(
+				`Code blocks must be preceded by a filename, with ## (in ${bundleFilename})`
+			);
+		}
+		if (nextCodeEnd === -1) {
+			throw new Error(`Unclosed code block (in ${bundleFilename})`);
+		}
+
+		if (prefix === null) {
+			prefix = bundle.slice(index, nextTitle).trim();
+			if (prefix !== "") prefix = prefix.replaceAll(/^/gm, "// ") + "\n\n";
+			prefix =
+				"// THIS FILE IS AUTOGENERATED. DO NOT EDIT DIRECTLY.\n" +
+				`// TO MAKE CHANGES, EDIT ../${basename(bundleFilename)}\n\n` +
+				prefix;
+		}
+		const filename = bundle.slice(nextTitle + 2, nextCodeStart).trim();
+		const code =
+			(filename.endsWith(".js") ? prefix : "") +
+			bundle
+				.slice(nextCodeStart + 3, nextCodeEnd)
+				.replace(/^\w+/, "")
+				.trim() +
+			"\n";
+
+		files.push({ filename, code });
+
+		index = nextCodeEnd + 3;
+	}
+
+	return files;
+}