From 990674f7e36bdf7153ed33f3a35b3fdbbd0671f2 Mon Sep 17 00:00:00 2001
From: OrcaWin <alpha-eng@stably.ai>
Date: Mon, 7 Sep 2026 22:21:52 -0700
Subject: [PATCH] perf: sum omitted workspace sizes without intermediate
 objects (#19491)

Co-authored-by: m4air <m4air@m4airs-MacBook-Air.local>
---
 src/shared/workspace-space-compaction.test.ts | 50 +++++++++++++++++++
 src/shared/workspace-space-compaction.ts      | 25 ++++------
 2 files changed, 61 insertions(+), 14 deletions(-)
 create mode 100644 src/shared/workspace-space-compaction.test.ts

diff --git a/src/shared/workspace-space-compaction.test.ts b/src/shared/workspace-space-compaction.test.ts
new file mode 100644
index 00000000000..bf774645fbb
--- /dev/null
+++ b/src/shared/workspace-space-compaction.test.ts
@@ -0,0 +1,50 @@
+import { expect, it, vi } from 'vitest'
+import { compactWorkspaceSpaceItems } from './workspace-space-compaction'
+import type { WorkspaceSpaceItem } from './workspace-space-types'
+
+it('sums omitted sizes without constructing a replacement object per omitted item', () => {
+  const items: WorkspaceSpaceItem[] = Array.from({ length: 10000 }, (_, index) => ({
+    name: String(index),
+    path: String(index),
+    kind: 'file',
+    sizeBytes: index
+  }))
+  const original = Array.prototype.reduce
+  let objectAccumulators = 0
+  const spy = vi.spyOn(Array.prototype, 'reduce').mockImplementation(function (
+    this: unknown[],
+    callback,
+    initial: unknown
+  ) {
+    if (initial && typeof initial === 'object' && 'name' in initial && initial.name === 'Other') {
+      objectAccumulators += this.length
+    }
+    return Reflect.apply(original, this, [callback, initial])
+  })
+  let result: ReturnType<typeof compactWorkspaceSpaceItems>
+  try {
+    result = compactWorkspaceSpaceItems(items)
+  } finally {
+    spy.mockRestore()
+  }
+  expect(objectAccumulators).toBe(0)
+  expect(result!.topLevelItems).toHaveLength(48)
+  expect(result!.omittedTopLevelItemCount).toBe(9953)
+  expect(result!.omittedTopLevelSizeBytes).toBe((9952 * 9953) / 2)
+  expect(result!.topLevelItems[0]).toBe(items[9999])
+  expect(items[0].sizeBytes).toBe(0)
+})
+
+it('preserves small-list size ties and empty results', () => {
+  expect(compactWorkspaceSpaceItems([]).topLevelItems).toEqual([])
+  const items: WorkspaceSpaceItem[] = ['b', 'a'].map((name) => ({
+    name,
+    path: name,
+    kind: 'file',
+    sizeBytes: 1
+  }))
+  expect(compactWorkspaceSpaceItems(items).topLevelItems.map((item) => item.name)).toEqual([
+    'a',
+    'b'
+  ])
+})
diff --git a/src/shared/workspace-space-compaction.ts b/src/shared/workspace-space-compaction.ts
index 9b3ab739e92..b2b6bf99fb2 100644
--- a/src/shared/workspace-space-compaction.ts
+++ b/src/shared/workspace-space-compaction.ts
@@ -19,23 +19,20 @@ export function compactWorkspaceSpaceItems(items: WorkspaceSpaceItem[]): {
   }
 
   const visible = sorted.slice(0, WORKSPACE_SPACE_MAX_TOP_LEVEL_ITEMS - 1)
-  const omitted = sorted.slice(WORKSPACE_SPACE_MAX_TOP_LEVEL_ITEMS - 1)
-  const other = omitted.reduce<WorkspaceSpaceItem>(
-    (acc, item) => ({
-      ...acc,
-      sizeBytes: acc.sizeBytes + item.sizeBytes
-    }),
-    {
-      name: 'Other',
-      path: '',
-      kind: 'other',
-      sizeBytes: 0
-    }
-  )
+  let omittedSizeBytes = 0
+  for (let index = visible.length; index < sorted.length; index += 1) {
+    omittedSizeBytes += sorted[index].sizeBytes
+  }
+  const other: WorkspaceSpaceItem = {
+    name: 'Other',
+    path: '',
+    kind: 'other',
+    sizeBytes: omittedSizeBytes
+  }
 
   return {
     topLevelItems: [...visible, other],
-    omittedTopLevelItemCount: omitted.length,
+    omittedTopLevelItemCount: sorted.length - visible.length,
     omittedTopLevelSizeBytes: other.sizeBytes
   }
 }