Fix too aggressive Map/MapOf shrinking on deletion leading to out of range panic

map.go

@@ -481,7 +481,7 @@ func (m *Map) resize(knownTable *mapTable, hint mapResizeHint) {
 		newTable = newMapTable(tableLen << 1)
 	case mapShrinkHint:
 		shrinkThreshold := int64((tableLen * entriesPerMapBucket) / mapShrinkFraction)
-		if table.sumSize() <= shrinkThreshold {
+		if tableLen > minMapTableLen && table.sumSize() <= shrinkThreshold {
 			// Shrink the table with factor of 2.
 			atomic.AddInt64(&m.totalShrinks, 1)
 			newTable = newMapTable(tableLen >> 1)

map_test.go

@@ -470,6 +470,37 @@ func TestMapStoreThenLoadAndDelete(t *testing.T) {
 	}
 }
 
+func TestMapStoreThenParallelDelete_DoesNotShrinkBelowMinTableLen(t *testing.T) {
+	const numEntries = 1000
+	m := NewMap()
+	for i := 0; i < numEntries; i++ {
+		m.Store(strconv.Itoa(i), i)
+	}
+
+	cdone := make(chan bool)
+	go func() {
+		for i := 0; i < numEntries; i++ {
+			m.Delete(strconv.Itoa(int(i)))
+		}
+		cdone <- true
+	}()
+	go func() {
+		for i := 0; i < numEntries; i++ {
+			m.Delete(strconv.Itoa(int(i)))
+		}
+		cdone <- true
+	}()
+
+	// Wait for the goroutines to finish.
+	<-cdone
+	<-cdone
+
+	stats := CollectMapStats(m)
+	if stats.RootBuckets < MinMapTableLen {
+		t.Fatalf("table was too small: %d", stats.RootBuckets)
+	}
+}
+
 func sizeBasedOnRange(m *Map) int {
 	size := 0
 	m.Range(func(key string, value interface{}) bool {

mapof.go

@@ -430,7 +430,7 @@ func (m *MapOf[K, V]) resize(knownTable *mapOfTable[K, V], hint mapResizeHint) {
 		newTable = newMapOfTable[K, V](tableLen << 1)
 	case mapShrinkHint:
 		shrinkThreshold := int64((tableLen * entriesPerMapBucket) / mapShrinkFraction)
-		if table.sumSize() <= shrinkThreshold {
+		if tableLen > minMapTableLen && table.sumSize() <= shrinkThreshold {
 			// Shrink the table with factor of 2.
 			atomic.AddInt64(&m.totalShrinks, 1)
 			newTable = newMapOfTable[K, V](tableLen >> 1)

mapof_test.go

@@ -515,6 +515,37 @@ func TestMapOfStructStoreThenLoadAndDelete(t *testing.T) {
 	}
 }
 
+func TestMapOfStoreThenParallelDelete_DoesNotShrinkBelowMinTableLen(t *testing.T) {
+	const numEntries = 1000
+	m := NewMapOf[int, int]()
+	for i := 0; i < numEntries; i++ {
+		m.Store(i, i)
+	}
+
+	cdone := make(chan bool)
+	go func() {
+		for i := 0; i < numEntries; i++ {
+			m.Delete(i)
+		}
+		cdone <- true
+	}()
+	go func() {
+		for i := 0; i < numEntries; i++ {
+			m.Delete(i)
+		}
+		cdone <- true
+	}()
+
+	// Wait for the goroutines to finish.
+	<-cdone
+	<-cdone
+
+	stats := CollectMapOfStats(m)
+	if stats.RootBuckets < MinMapTableLen {
+		t.Fatalf("table was too small: %d", stats.RootBuckets)
+	}
+}
+
 func sizeBasedOnTypedRange(m *MapOf[string, int]) int {
 	size := 0
 	m.Range(func(key string, value int) bool {