Implement masking

Closes #33

Author: nhooyr

mask.go

@@ -1,15 +1,16 @@
 package websocket
 
-// Mask applies the WebSocket masking algorithm to p
+// mask applies the WebSocket masking algorithm to p
 // with the given key where the first 3 bits of pos
 // are the starting position in the key.
 // See https://tools.ietf.org/html/rfc6455#section-5.3
 //
-// It is highly optimized to mask per word with the usage
-// of unsafe.
-//
-// For targets that do not support unsafe, please report an issue.
-// There is a mask by byte function below that will be used for such targets.
+// The returned value is the position of the next byte
+// to be used for masking in the key.
 func mask(key [4]byte, pos int, p []byte) int {
-	panic("TODO")
+	for i := range p {
+		p[i] ^= key[pos&3]
+		pos++
+	}
+	return pos & 3
 }