StringIO: Add spec for printf and polish a bit the existing ones

Author: andrykonchin

spec/ruby/library/stringio/open_spec.rb

@@ -167,10 +167,14 @@
     io.should equal(ret)
   end
 
-  it "sets the mode to read-write" do
+  it "sets the mode to read-write (r+)" do
     io = StringIO.open("example")
     io.closed_read?.should be_false
     io.closed_write?.should be_false
+
+    io = StringIO.new("example")
+    io.printf("%d", 123)
+    io.string.should == "123mple"
   end
 
   it "tries to convert the passed Object to a String using #to_str" do
@@ -195,10 +199,14 @@
     io.should equal(ret)
   end
 
-  it "sets the mode to read-write" do
+  it "sets the mode to read-write (r+)" do
     io = StringIO.open
     io.closed_read?.should be_false
     io.closed_write?.should be_false
+
+    io = StringIO.new("example")
+    io.printf("%d", 123)
+    io.string.should == "123mple"
   end
 
   it "uses an empty String as the StringIO backend" do

spec/ruby/library/stringio/printf_spec.rb

@@ -4,17 +4,17 @@
 
 describe "StringIO#printf" do
   before :each do
-    @io = StringIO.new('example')
+    @io = StringIO.new()
   end
 
   it "returns nil" do
     @io.printf("%d %04x", 123, 123).should be_nil
   end
 
   it "pads self with \\000 when the current position is after the end" do
-    @io.pos = 10
+    @io.pos = 3
     @io.printf("%d", 123)
-    @io.string.should == "example\000\000\000123"
+    @io.string.should == "\000\000\000123"
   end
 
   it "performs format conversion" do
@@ -39,6 +39,27 @@
   end
 end
 
+describe "StringIO#printf when in read-write mode" do
+  before :each do
+    @io = StringIO.new("example", "r+")
+  end
+
+  it "starts from the beginning" do
+    @io.printf("%s", "abcdefghijk")
+    @io.string.should == "abcdefghijk"
+  end
+
+  it "does not truncate existing string" do
+    @io.printf("%s", "abc")
+    @io.string.should == "abcmple"
+  end
+
+  it "correctly updates self's position" do
+    @io.printf("%s", "abc")
+    @io.pos.should eql(3)
+  end
+end
+
 describe "StringIO#printf when in append mode" do
   before :each do
     @io = StringIO.new("example", "a")