Fix bug with memo after insertion

README.md

@@ -6,7 +6,7 @@
 original location in the source file through a source map.
 
 You may already be familiar with the [`source-map`][source-map] package's `SourceMapConsumer`. This
-provides the same `originalPositionFor` API, without requires WASM.
+provides the same `originalPositionFor` and `generatedPositionFor` API, without requires WASM.
 
 ## Installation
 
@@ -17,7 +17,7 @@ npm install @jridgewell/trace-mapping
 ## Usage
 
 ```typescript
-import { TraceMap, originalPositionFor } from '@jridgewell/trace-mapping';
+import { TraceMap, originalPositionFor, generatedPositionFor } from '@jridgewell/trace-mapping';
 
 const tracer = new TraceMap({
   version: 3,
@@ -34,6 +34,12 @@ assert.deepEqual(traced, {
   column: 4,
   name: 'foo',
 });
+
+const generated = generatedPositionFor(tracer, { source: 'input.js', line: 42, column: 4 });
+assert.deepEqual(generated, {
+  line: 1,
+  column: 5,
+});
 ```
 
 We also provide a lower level API to get the actual segment that matches our line and column. Unlike

src/by-source.ts

@@ -40,7 +40,7 @@ export default function buildBySources(
         memoizedBinarySearch(originalLine, sourceColumn, memo, sourceLine),
       );
 
-      insert(originalLine, index + 1, [sourceColumn, i, seg[COLUMN]]);
+      insert(originalLine, (memo.lastIndex = index + 1), [sourceColumn, i, seg[COLUMN]]);
     }
   }
 

test/trace-mapping.test.ts

@@ -223,6 +223,11 @@ describe('TraceMap', () => {
       test('generatedPositionFor', (t) => {
         const tracer = new TraceMap(map);
 
+        t.deepEqual(generatedPositionFor(tracer, { source: 'input.js', line: 4, column: 3 }), {
+          line: 5,
+          column: 3,
+        });
+
         t.deepEqual(generatedPositionFor(tracer, { source: 'input.js', line: 1, column: 0 }), {
           line: 1,
           column: 0,