Merge pull request #515 from anacrolix/bsi-clearvalues-serial

BSI: fix ClearValues being slower than a serial loop

Author: lemire

roaring64/bsi64.go

@@ -971,23 +971,18 @@ func ClearBits(foundSet, target *Bitmap) {
 	target.AndNot(foundSet)
 }
 
-// ClearValues removes the values found in foundSet
+// ClearValues removes from the BSI all values whose column IDs are in
+// foundSet, modifying the BSI in place.
+//
+// The implementation is intentionally serial. A previous goroutine-per-bit-plane
+// approach was slower in practice: goroutine creation overhead dominated for
+// typical BSI sizes, and the cost compounds when ClearValues is called in a
+// tight loop (e.g. once per term across an entire index during a deletion pass).
 func (b *BSI) ClearValues(foundSet *Bitmap) {
-
-	var wg sync.WaitGroup
-	wg.Add(1)
-	go func() {
-		defer wg.Done()
-		b.eBM.AndNot(foundSet)
-	}()
-	for i := 0; i < b.BitCount(); i++ {
-		wg.Add(1)
-		go func(j int) {
-			defer wg.Done()
-			b.bA[j].AndNot(foundSet)
-		}(i)
+	b.eBM.AndNot(foundSet)
+	for i := range b.bA {
+		b.bA[i].AndNot(foundSet)
 	}
-	wg.Wait()
 }
 
 // NewBSIRetainSet - Construct a new BSI from a clone of existing BSI, retain only values contained in foundSet