Add ToExistingArray function

Author: TheHippo

roaring.go

@@ -421,18 +421,27 @@ func FromBitSet(bitset *bitset.BitSet) *Bitmap {
 // ToArray creates a new slice containing all of the integers stored in the Bitmap in sorted order
 func (rb *Bitmap) ToArray() []uint32 {
 	array := make([]uint32, rb.GetCardinality())
+	ar := rb.toArray(&array)
+	return *ar
+}
+
+func (rb *Bitmap) toArray(array *[]uint32) *[]uint32 {
 	pos := 0
 	pos2 := 0
 
 	for pos < rb.highlowcontainer.size() {
 		hs := uint32(rb.highlowcontainer.getKeyAtIndex(pos)) << 16
 		c := rb.highlowcontainer.getContainerAtIndex(pos)
 		pos++
-		pos2 = c.fillLeastSignificant16bits(array, pos2, hs)
+		pos2 = c.fillLeastSignificant16bits(*array, pos2, hs)
 	}
 	return array
 }
 
+func (rb *Bitmap) ToExistingArray(array *[]uint32) *[]uint32 {
+	return rb.toArray(array)
+}
+
 // GetSizeInBytes estimates the memory usage of the Bitmap. Note that this
 // might differ slightly from the amount of bytes required for persistent storage
 func (rb *Bitmap) GetSizeInBytes() uint64 {