stream_input: avoid allocation for common #read case

When entering StreamInput#read, it is common for @rbuf to
be drained and empty, meaning a simple String#clear can
be used in place of a more complex String#slice! String#replace
combo on an empty @rbuf.

While we're at it, we'll avoid an extra instructions for calling
`@rbuf.size` by storing the size in a local.

1 files changed, 3 insertions, 2 deletions

diff --git a/lib/yahns/stream_input.rb b/lib/yahns/stream_input.rb
index 496faa2..a6190fc 100644
--- a/lib/yahns/stream_input.rb
+++ b/lib/yahns/stream_input.rb
@@ -43,8 +43,9 @@ class Yahns::StreamInput # :nodoc:
         length < 0 and raise ArgumentError, "negative length #{length} given"
         rv.replace(@rbuf.slice!(0, length))
       else
-        to_read = length - @rbuf.size
-        rv.replace(@rbuf.slice!(0, @rbuf.size))
+        cur = @rbuf.size
+        to_read = length - cur
+        cur == 0 ? rv.clear : rv.replace(@rbuf.slice!(0, cur))
         until to_read == 0 || eof? || (rv.size > 0 && @chunked)
           @client.yahns_read(to_read, @buf) or eof!
           filter_body(@rbuf, @buf)