Allow construction of empty filters.

gcs/builder/builder.go

@@ -372,14 +372,9 @@ func BuildExtFilter(block *wire.MsgBlock) (*gcs.Filter, error) {
 
 // GetFilterHash returns the double-SHA256 of the filter.
 func GetFilterHash(filter *gcs.Filter) (chainhash.Hash, error) {
-	var zero chainhash.Hash
-	if filter == nil {
-		return zero, nil
-	}
-
 	filterData, err := filter.NBytes()
 	if err != nil {
-		return zero, err
+		return chainhash.Hash{}, err
 	}
 
 	return chainhash.DoubleHashH(filterData), nil

gcs/gcs.go

@@ -25,9 +25,6 @@ var (
 	// ErrPTooBig signifies that the filter can't handle `1/2**P`
 	// collision probability.
 	ErrPTooBig = fmt.Errorf("P is too big to fit in uint32")
-
-	// ErrNoData signifies that an empty slice was passed.
-	ErrNoData = fmt.Errorf("No data provided")
 )
 
 const (
@@ -98,9 +95,6 @@ func BuildGCSFilter(P uint8, key [KeySize]byte, data [][]byte) (*Filter, error)
 	// Some initial parameter checks: make sure we have data from which to
 	// build the filter, and make sure our parameters will fit the hash
 	// function we're using.
-	if len(data) == 0 {
-		return nil, ErrNoData
-	}
 	if uint64(len(data)) >= (1 << 32) {
 		return nil, ErrNTooBig
 	}
@@ -115,6 +109,11 @@ func BuildGCSFilter(P uint8, key [KeySize]byte, data [][]byte) (*Filter, error)
 	}
 	f.modulusNP = uint64(f.n) << P
 
+	// Shortcut if the filter is empty.
+	if f.n == 0 {
+		return &f, nil
+	}
+
 	// Build the filter.
 	values := make(uint64Slice, 0, len(data))
 	b := bstream.NewBStreamWriter(0)
@@ -358,7 +357,7 @@ func (f *Filter) MatchAny(key [KeySize]byte, data [][]byte) (bool, error) {
 
 	// Basic sanity check.
 	if len(data) == 0 {
-		return false, ErrNoData
+		return false, nil
 	}
 
 	// Create a filter bitstream.