snappy: Use dedicated function for scoring (#614)

* snappy: Use dedicated function for scoring

Saves a byte here and there, nothing significant. A bit faster

s2/encode_best.go

@@ -370,7 +370,7 @@ func encodeBlockBestSnappy(dst, src []byte) (d int) {
 				}
 				offset := m.s - m.offset
 
-				return score - emitCopySize(offset, m.length)
+				return score - emitCopyNoRepeatSize(offset, m.length)
 			}
 
 			matchAt := func(offset, s int, first uint32) match {
@@ -581,6 +581,28 @@ func emitCopySize(offset, length int) int {
 	return 2
 }
 
+// emitCopyNoRepeatSize returns the size to encode the offset+length
+//
+// It assumes that:
+//	1 <= offset && offset <= math.MaxUint32
+//	4 <= length && length <= 1 << 24
+func emitCopyNoRepeatSize(offset, length int) int {
+	if offset >= 65536 {
+		return 5 + 5*(length/64)
+	}
+
+	// Offset no more than 2 bytes.
+	if length > 64 {
+		// Emit remaining as repeats, at least 4 bytes remain.
+		return 3 + 3*(length/60)
+	}
+	if length >= 12 || offset >= 2048 {
+		return 3
+	}
+	// Emit the remaining copy, encoded as 2 bytes.
+	return 2
+}
+
 // emitRepeatSize returns the number of bytes required to encode a repeat.
 // Length must be at least 4 and < 1<<24
 func emitRepeatSize(offset, length int) int {