arrayDiff filter duplicates

packages/noya-component/src/__tests__/arrayDiff.test.ts

@@ -137,3 +137,13 @@ test('swaps multiple pairs', () => {
   expect(applied).toEqual(b);
   expect(diff).toEqual([moved(0, 1), moved(4, 5)]);
 });
+
+test('filter duplicates', () => {
+  let a = ['a'];
+  let b = ['a', 'a', 'b'];
+
+  const diff = computeArrayDiff(a, b);
+  const applied = applyArrayDiff(a, diff);
+
+  expect(applied).toEqual(['a', 'b']);
+});

packages/noya-component/src/arrayDiff.ts

@@ -37,6 +37,15 @@ function computeFirstArrayMove<T>(
   }
 }
 
+function filterDuplicates<T>(
+  a: T[],
+  identity: (item: T) => unknown = (item) => item,
+): T[] {
+  const map = new Map(a.map((item, index) => [identity(item), index]));
+
+  return [...map.values()].map((index) => a[index]);
+}
+
 export function computeArrayDiff<T>(a: T[], b: T[]): ArrayDiffItem<T>[];
 export function computeArrayDiff<T, K>(
   a: T[],
@@ -52,6 +61,14 @@ export function computeArrayDiff<T>(
   const aMap = new Map(a.map((item, index) => [identity(item), index]));
   const bMap = new Map(b.map((item, index) => [identity(item), index]));
 
+  // Detect duplicates by comparing array length vs map size.
+  if (aMap.size !== a.length || bMap.size !== b.length) {
+    return computeArrayDiff(
+      filterDuplicates(a, identity),
+      filterDuplicates(b, identity),
+    );
+  }
+
   // Check for removed items.
   let removalOffset = 0;
   for (let i = 0; i < a.length; i++) {