Fix ByteBuffer#index for read-write boundary match

The previous implementation searched the buffers individually, which
will only work correctly for search substrings of length 1.

This instead implements the continued search by folding the write buffer
into the read buffer unless a match was found in the initial segment.

An alternative that doesn't fold the two buffers could naturally be
implemented, but that would be much more complicated.

In principle, the old implementation could be retained for one character
substrings, but I did not think it would be a common enough case to be
worth the extra effort.

lib/ione/byte_buffer.rb

@@ -179,10 +179,12 @@ def index(substring, start_index=0)
         swap_buffers
       end
       read_buffer_length = @read_buffer.bytesize
-      if start_index < read_buffer_length - @offset && (index = @read_buffer.index(substring, @offset + start_index))
+      if start_index + substring.bytesize <= read_buffer_length - @offset && (index = @read_buffer.index(substring, @offset + start_index))
         index - @offset
-      elsif (index = @write_buffer.index(substring, start_index - read_buffer_length + @offset))
-        index + read_buffer_length - @offset
+      elsif start_index + substring.bytesize <= read_buffer_length - @offset + @write_buffer.bytesize
+        merge_read_buffer
+        start_index = read_buffer_length - substring.bytesize if read_buffer_length - substring.bytesize > start_index
+        @read_buffer.index(substring, start_index)
       else
         nil
       end
@@ -298,5 +300,11 @@ def swap_buffers
       @read_buffer = @write_buffer
       @write_buffer = ''
     end
+
+    def merge_read_buffer
+      @read_buffer = @read_buffer[@offset, @read_buffer.length - @offset] << @write_buffer
+      @write_buffer = ''
+      @offset = 0
+    end
   end
 end

spec/ione/byte_buffer_spec.rb

@@ -354,6 +354,20 @@ module Ione
         buffer.append(' baz baz')
         buffer.index('baz', 8).should == 11
       end
+
+      it 'returns the first index when the matching substring spans the read and write buffer' do
+        buffer.append('foo bar')
+        buffer.read(1)
+        buffer.append('bar barbar')
+        buffer.index('barbar', 0).should == 3
+      end
+
+      it 'returns nil when the substring does not fit in the search space' do
+        buffer.append('foo')
+        buffer.read(1)
+        buffer.append('bar')
+        buffer.index('bar', 3).should be_nil
+      end
     end
 
     context 'when reading and appending' do