Replace `String#byteslice` by custom `IO._bufread`.

`byteslice` creates 2 string objects. `_bufread` creates one, and
modifies the original buffer string, that is more efficient.

mrbgems/mruby-io/mrblib/io.rb

@@ -208,8 +208,7 @@ class IO
 
       if length
         consume = (length <= @buf.bytesize) ? length : @buf.bytesize
-        array.push @buf.byteslice(0, consume)
-        @buf = @buf.byteslice(consume, @buf.bytesize - consume)
+        array.push IO._bufread(@buf, consume)
         length -= consume
         break if length == 0
       else
@@ -255,13 +254,11 @@ class IO
       end
 
       if limit && limit <= @buf.bytesize
-        array.push @buf.byteslice(0, limit)
-        @buf = @buf.byteslice(limit, @buf.bytesize - limit)
+        array.push IO._bufread(@buf, limit)
         break
       elsif idx = @buf.index(rs)
         len = idx + rs.bytesize
-        array.push @buf.byteslice(0, len)
-        @buf = @buf.byteslice(len, @buf.bytesize - len)
+        array.push IO._bufread(@buf, len)
         break
       else
         array.push @buf
@@ -284,9 +281,7 @@ class IO
 
   def readchar
     _read_buf
-    c = @buf.byteslice(0,1)
-    @buf = @buf.byteslice(1, @buf.bytesize)
-    c
+    IO._bufread(@buf, 1)
   end
 
   def getc

mrbgems/mruby-io/src/io.c

@@ -1294,6 +1294,27 @@ mrb_io_sync(mrb_state *mrb, mrb_value self)
   return mrb_bool_value(fptr->sync);
 }
 
+static mrb_value
+io_bufread(mrb_state *mrb, mrb_value self)
+{
+  mrb_value str, str2;
+  mrb_int len, newlen;
+  struct RString *s;
+  char *p;
+
+  mrb_get_args(mrb, "Si", &str, &len);
+  s = RSTRING(str);
+  mrb_str_modify(mrb, s);
+  p = RSTR_PTR(s);
+  str2 = mrb_str_new(mrb, p, len);
+  newlen = RSTR_LEN(s)-len;
+  memmove(p, p+len, newlen);
+  p[newlen] = '\0';
+  RSTR_SET_LEN(s, newlen);
+
+  return str2;
+}
+
 void
 mrb_init_io(mrb_state *mrb)
 {
@@ -1328,4 +1349,6 @@ mrb_init_io(mrb_state *mrb)
   mrb_define_method(mrb, io, "closed?",    mrb_io_closed,     MRB_ARGS_NONE());   /* 15.2.20.5.2 */
   mrb_define_method(mrb, io, "pid",        mrb_io_pid,        MRB_ARGS_NONE());   /* 15.2.20.5.2 */
   mrb_define_method(mrb, io, "fileno",     mrb_io_fileno,     MRB_ARGS_NONE());
+
+  mrb_define_class_method(mrb, io, "_bufread",   io_bufread,        MRB_ARGS_REQ(2));
 }

mrbgems/mruby-io/test/io.rb

@@ -129,10 +129,11 @@ end
 
 assert "IO#read(n) with n > IO::BUF_SIZE" do
   skip "pipe is not supported on this platform" if MRubyIOTestUtil.win?
-  r,w = IO.pipe
+  IO.pipe do |r,w|
     n = IO::BUF_SIZE+1
     w.write 'a'*n
-  assert_equal r.read(n), 'a'*n
+    assert_equal 'a'*n, r.read(n)
+  end
 end
 
 assert('IO#readchar', '15.2.20.5.15') do