Handle empty document -- root node with 0 children

Author: Andy Hanson

src/harness/unittests/versionCache.ts

@@ -10,6 +10,12 @@ namespace ts {
         return lineIndex.absolutePositionOfStartOfLine(line) + (col - 1);
     }
 
+    function foo(lineIndex: server.LineIndex) {
+        lineIndex.load([]);
+        lineIndex.positionToLineOffset(0);
+    }
+    foo;
+
     function validateEdit(lineIndex: server.LineIndex, sourceText: string, position: number, deleteLength: number, insertString: string): void {
         const checkText = editFlat(position, deleteLength, insertString, sourceText);
         const snapshot = lineIndex.edit(position, deleteLength, insertString);
@@ -49,6 +55,12 @@ var q:Point=<Point>p;`;
             validateEditAtLineCharIndex = undefined;
         });
 
+        it("handles empty lines array", () => {
+            const lineIndex = new server.LineIndex();
+            lineIndex.load([]);
+            assert.equal(lineIndex.positionToLineOffset(0), { line: 0, offset: 0 });
+        });
+
         it(`change 9 1 0 1 {"y"}`, () => {
             validateEditAtLineCharIndex(9, 1, 0, "y");
         });

src/server/scriptVersionCache.ts

@@ -660,7 +660,10 @@ namespace ts.server {
         // Input position is relative to the start of this node.
         // Output line number is absolute.
         charOffsetToLineInfo(lineNumberAccumulator: number, relativePosition: number): { oneBasedLine: number, zeroBasedColumn: number, lineText: string | undefined } {
-            Debug.assert(this.children.length !== 0);
+            if (this.children.length === 0) {
+                // Root node might have no children if this is an empty document.
+                return { oneBasedLine: lineNumberAccumulator, zeroBasedColumn: relativePosition, lineText: undefined };
+            }
 
             for (const child of this.children) {
                 if (child.charCount() > relativePosition) {