Fix weakMapMemoize maxSize counting argument nodes instead of results

src/weakMapMemoize.ts

@@ -236,7 +236,7 @@ export function weakMapMemoize<Func extends AnyFunction>(
   const { resultEqualityCheck, maxSize } = options
 
   // Generational bounding for `maxSize`: `prevNode` holds the demoted cache
-  // tree, `insertionCount` counts primitive-Map insertions into the current
+  // tree, `insertionCount` counts results cached or promoted into the current
   // tree. Reaching `maxSize` flips generations at the end of that call.
   const useGenerations = maxSize !== undefined
   if (useGenerations && (!Number.isInteger(maxSize) || maxSize < 1)) {
@@ -257,6 +257,7 @@ export function weakMapMemoize<Func extends AnyFunction>(
   // a walk, so a flip can never happen while pointers into the tree being
   // demoted are still live. The hit path never reaches this.
   function maybeFlipGenerations() {
+    insertionCount++
     if (insertionCount >= (maxSize as number)) {
       prevNode = fnNode
       fnNode = createCacheNode()
@@ -295,7 +296,6 @@ export function weakMapMemoize<Func extends AnyFunction>(
         if (primitiveNode === undefined) {
           cacheNode = createCacheNode()
           primitiveCache.set(arg, cacheNode)
-          insertionCount++
 
           if (process.env.NODE_ENV !== 'production') {
             // A single primitive `Map` growing past the threshold means this

test/weakMapMemoize.maxSize.spec.ts

@@ -1,4 +1,8 @@
-import { setGlobalDevModeChecks, weakMapMemoize } from 'reselect'
+import {
+  createSelector,
+  setGlobalDevModeChecks,
+  weakMapMemoize
+} from 'reselect'
 
 // `maxSize` bounds cache growth for primitive arguments with a generational
 // scheme: after `maxSize` results are cached, the whole cache is demoted to a
@@ -23,6 +27,63 @@ describe('weakMapMemoize maxSize option', () => {
     expect(() => weakMapMemoize((x: number) => x, { maxSize: 1 })).not.toThrow()
   })
 
+  test.each([2, 4])(
+    'maxSize: %i caches that many results with multiple primitive arguments',
+    maxSize => {
+      const spy = vi.fn((price: number, qty: number) => ({
+        total: price * qty
+      }))
+      const fn = weakMapMemoize(spy, { maxSize })
+      const args = Array.from(
+        { length: maxSize },
+        (_, i) => [10 + i, i + 1] as const
+      )
+      const results = [...args, ...args, ...args].map(args => fn(...args))
+      expect(spy).toHaveBeenCalledTimes(maxSize)
+      results.forEach((result, i) => expect(result).toBe(results[i % maxSize]))
+    }
+  )
+
+  test('createSelector reuses results for multiple primitive inputs with maxSize', () => {
+    const selector = createSelector(
+      [
+        (state: { price: number; qty: number }) => state.price,
+        state => state.qty
+      ],
+      (price, qty) => ({ total: price * qty }),
+      { memoizeOptions: { maxSize: 2 } }
+    )
+    const states = [
+      { price: 10, qty: 1 },
+      { price: 11, qty: 2 }
+    ]
+    const results = states.map(state => selector(state))
+
+    for (let pass = 0; pass < 2; pass++) {
+      states.forEach((state, i) =>
+        expect(selector({ ...state })).toBe(results[i])
+      )
+    }
+    expect(selector.recomputations()).toBe(2)
+    expect(selector.dependencyRecomputations()).toBe(6)
+  })
+
+  test('createSelector caches multiple primitive arguments with argsMemoize maxSize', () => {
+    const selector = createSelector(
+      [(price: number, qty: number) => price, (price, qty) => qty],
+      (price, qty) => ({ total: price * qty }),
+      { argsMemoizeOptions: { maxSize: 2 } }
+    )
+    const first = selector(10, 1)
+    const second = selector(11, 2)
+
+    for (let pass = 0; pass < 2; pass++) {
+      expect(selector(10, 1)).toBe(first)
+      expect(selector(11, 2)).toBe(second)
+    }
+    expect(selector.dependencyRecomputations()).toBe(2)
+  })
+
   test('bounds primitive cache growth to roughly 2x maxSize', () => {
     const spy = vi.fn((x: number) => ({ value: x }))
     const fn = weakMapMemoize(spy, { maxSize: 10 })
@@ -57,9 +118,7 @@ describe('weakMapMemoize maxSize option', () => {
     expect(fn(1)).toBe(first)
     expect(spy).toHaveBeenCalledTimes(5)
 
-    // Note: promotion does NOT increment insertionCount (the node was created
-    // by the walk before the previous-generation probe ran... it was created
-    // and inserted, so insertionCount DID increment). current = {5, 1}.
+    // Promoting a result also counts toward maxSize. current = {5, 1}.
     fn(6)
     fn(7) // current = {5,1,6,7} -> next call flips
     fn(8) // flip: prev = {5,1,6,7}, current = {8}