config: break mergeURLPath into a separate module

packages/config/DittoConfig.ts

@@ -6,6 +6,7 @@ import { encodeBase64Url } from '@std/encoding/base64url';
 
 import { getEcdsaPublicKey } from './utils/crypto.ts';
 import { optionalBooleanSchema, optionalNumberSchema } from './utils/schema.ts';
+import { mergeURLPath } from './utils/url.ts';
 
 /** Ditto application-wide configuration. */
 export class DittoConfig {
@@ -288,7 +289,7 @@ export class DittoConfig {
 
   /** Merges the path with the localDomain. */
   local(path: string): string {
-    return mergePaths(this.localDomain, path);
+    return mergeURLPath(this.localDomain, path);
   }
 
   /** URL to send Sentry errors to. */
@@ -461,18 +462,3 @@ export class DittoConfig {
     return Number(this.env.get('STREAK_WINDOW') || 129600);
   }
 }
-
-function mergePaths(base: string, path: string) {
-  const url = new URL(
-    path.startsWith('/') ? path : new URL(path).pathname,
-    base,
-  );
-
-  if (!path.startsWith('/')) {
-    // Copy query parameters from the original URL to the new URL
-    const originalUrl = new URL(path);
-    url.search = originalUrl.search;
-  }
-
-  return url.toString();
-}

packages/config/utils/url.test.ts

@@ -0,0 +1,9 @@
+import { assertEquals } from '@std/assert';
+
+import { mergeURLPath } from './url.ts';
+
+Deno.test('mergeURLPath', () => {
+  assertEquals(mergeURLPath('https://mario.com', '/path'), 'https://mario.com/path');
+  assertEquals(mergeURLPath('https://mario.com', 'https://luigi.com/path'), 'https://mario.com/path');
+  assertEquals(mergeURLPath('https://mario.com', 'https://luigi.com/path?q=1'), 'https://mario.com/path?q=1');
+});

packages/config/utils/url.ts

@@ -0,0 +1,23 @@
+/**
+ * Produce a URL whose origin is guaranteed to be the same as the base URL.
+ * The path is either an absolute path (starting with `/`), or a full URL. In either case, only its path is used.
+ */
+export function mergeURLPath(
+  /** Base URL. Result is guaranteed to use this URL's origin. */
+  base: string,
+  /** Either an absolute path (starting with `/`), or a full URL. If a full URL, its path */
+  path: string,
+): string {
+  const url = new URL(
+    path.startsWith('/') ? path : new URL(path).pathname,
+    base,
+  );
+
+  if (!path.startsWith('/')) {
+    // Copy query parameters from the original URL to the new URL
+    const originalUrl = new URL(path);
+    url.search = originalUrl.search;
+  }
+
+  return url.toString();
+}