From 189a9376a0a95094d9e8bb3ceb7e2df0e6dca990 Mon Sep 17 00:00:00 2001
From: Christian Schneider <christian.schneider@typefox.io>
Date: Fri, 21 Feb 2025 15:29:11 +0100
Subject: [PATCH] more supplements

---
 packages/langium/src/utils/uri-utils.ts       | 12 ++++++++----
 packages/langium/test/utils/uri-utils.test.ts |  6 ++++++
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/packages/langium/src/utils/uri-utils.ts b/packages/langium/src/utils/uri-utils.ts
index 11d37e155..8fa081e1b 100644
--- a/packages/langium/src/utils/uri-utils.ts
+++ b/packages/langium/src/utils/uri-utils.ts
@@ -16,17 +16,17 @@ export namespace UriUtils {
     export const joinPath = Utils.joinPath;
     export const resolvePath = Utils.resolvePath;
 
-    const isWindows = process?.platform === 'win32';
+    const isWindows = typeof process === 'object' && process?.platform === 'win32';
 
     export function equals(a?: URI | string, b?: URI | string): boolean {
         return a?.toString() === b?.toString();
     }
 
     export function relative(from: URI | string, to: URI | string): string {
-        const fromPath = typeof from === 'string' ? from : from.path;
-        const toPath = typeof to === 'string' ? to : to.path;
+        const fromPath = typeof from === 'string' ? URI.parse(from).path : from.path;
+        const toPath   = typeof   to === 'string' ? URI.parse(to).path   : to.path;
         const fromParts = fromPath.split('/').filter(e => e.length > 0);
-        const toParts = toPath.split('/').filter(e => e.length > 0);
+        const toParts   =   toPath.split('/').filter(e => e.length > 0);
 
         if (isWindows) {
             const upperCaseDriveLetter = /^[A-Z]:$/;
@@ -36,6 +36,10 @@ export namespace UriUtils {
             if (toParts[0] && upperCaseDriveLetter.test(toParts[0])) {
                 toParts[0] = toParts[0].toLowerCase();
             }
+            if (fromParts[0] !== toParts[0]) {
+                // in case of different drive letters, we cannot compute a relative path, so...
+                return toPath.substring(1); // fall back to full 'to' path, drop the leading '/', keep everything else as is for good comparability
+            }
         }
 
         let i = 0;
diff --git a/packages/langium/test/utils/uri-utils.test.ts b/packages/langium/test/utils/uri-utils.test.ts
index 8427db58e..845f6fc48 100644
--- a/packages/langium/test/utils/uri-utils.test.ts
+++ b/packages/langium/test/utils/uri-utils.test.ts
@@ -69,6 +69,12 @@ describe('URI Utils', () => {
         expect(UriUtils.relative(from, to)).toBe('../c/d.txt');
     });
 
+    test.skipIf(process.platform !== 'win32')('different win32 drive letters', () => {
+        const from = URI.file('c:\\a\\b');
+        const to = URI.file('D:\\a\\c\\d.txt');
+        expect(UriUtils.relative(from, to)).toBe('d:/a/c/d.txt');
+    });
+
     test('Equal uris are equal', () => {
         const uri1 = 'file:///a/b';
         const uri2 = 'file:///a/b';