Improve Windows drive letter URI handling

packages/langium/src/utils/uri-utils.ts

@@ -16,15 +16,32 @@ export namespace UriUtils {
     export const joinPath = Utils.joinPath;
     export const resolvePath = Utils.resolvePath;
 
+    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]:$/;
+            if (fromParts[0] && upperCaseDriveLetter.test(fromParts[0])) {
+                fromParts[0] = fromParts[0].toLowerCase();
+            }
+            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;
         for (; i < fromParts.length; i++) {
             if (fromParts[i] !== toParts[i]) {

packages/langium/test/utils/uri-utils.test.ts

@@ -27,6 +27,24 @@ describe('URI Utils', () => {
         expect(UriUtils.relative(from, to)).toBe('../d.txt');
     });
 
+    test.skipIf(process.platform !== 'win32')('relative path in parent directory win32, uppercase drive letters', () => {
+        const from = URI.file('C:\\a\\b');
+        const to = URI.file('C:\\a\\d.txt');
+        expect(UriUtils.relative(from, to)).toBe('../d.txt');
+    });
+
+    test.skipIf(process.platform !== 'win32')('relative path in parent directory win32, mixed drive letter cases 1', () => {
+        const from = URI.file('C:\\a\\b');
+        const to = URI.file('c:\\a\\d.txt');
+        expect(UriUtils.relative(from, to)).toBe('../d.txt');
+    });
+
+    test.skipIf(process.platform !== 'win32')('relative path in parent directory win32, mixed drive letter cases 2', () => {
+        const from = URI.file('c:\\a\\b');
+        const to = URI.file('C:\\a\\d.txt');
+        expect(UriUtils.relative(from, to)).toBe('../d.txt');
+    });
+
     test('relative path in sub directory', () => {
         const from = URI.file('/a');
         const to = URI.file('/a/b/c.txt');
@@ -51,6 +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';