Implement StringIO#set_encoding_by_bom

lib/stringio.rb

@@ -372,6 +372,27 @@ def set_encoding(external_encoding, _ = nil, **_options)
     self
   end
 
+  def set_encoding_by_bom
+    return nil if closed_read?
+    raise FrozenError, "can't modify frozen #{self.class}" if frozen?
+    if @string.byteslice(0, 4) == "\x00\x00\xFE\xFF".b
+      set_encoding(Encoding::UTF_32BE)
+    elsif @string.byteslice(0, 4) == "\xFF\xFE\x00\x00".b
+      set_encoding(Encoding::UTF_32LE)
+    elsif @string.byteslice(0, 2) == "\xFE\xFF".b
+      set_encoding(Encoding::UTF_16BE)
+    elsif @string.byteslice(0, 2) == "\xFF\xFE".b
+      set_encoding(Encoding::UTF_16LE)
+    elsif @string.byteslice(0, "\u{FEFF}".bytesize) == "\u{FEFF}".b
+      set_encoding(Encoding::UTF_8)
+    else
+      @index = 0
+      return nil
+    end
+    @index = 1
+    external_encoding
+  end
+
   def size
     @string.size
   end

spec/library/stringio/set_encoding_by_bom_spec.rb

@@ -0,0 +1,237 @@
+require 'stringio'
+require_relative '../../spec_helper'
+
+# Should be synced with specs for IO#set_encoding_by_bom
+describe "StringIO#set_encoding_by_bom" do
+  it "returns nil if not readable" do
+    io = StringIO.new("".b, "wb")
+
+    io.set_encoding_by_bom.should be_nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+  end
+
+  it "returns the result encoding if found BOM UTF-8 sequence" do
+    io = StringIO.new("\u{FEFF}".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_8
+    io.external_encoding.should == Encoding::UTF_8
+    io.read.b.should == "".b
+
+    io = StringIO.new("\u{FEFF}abc".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_8
+    io.external_encoding.should == Encoding::UTF_8
+    io.read.b.should == "abc".b
+  end
+
+  it "returns the result encoding if found BOM UTF_16LE sequence" do
+    io = StringIO.new("\xFF\xFE".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_16LE
+    io.external_encoding.should == Encoding::UTF_16LE
+    io.read.b.should == "".b
+
+    io = StringIO.new("\xFF\xFEabc".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_16LE
+    io.external_encoding.should == Encoding::UTF_16LE
+    io.read.b.should == "abc".b
+  end
+
+  it "returns the result encoding if found BOM UTF_16BE sequence" do
+    io = StringIO.new("\xFE\xFF".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_16BE
+    io.external_encoding.should == Encoding::UTF_16BE
+    io.read.b.should == "".b
+
+    io = StringIO.new("\xFE\xFFabc".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_16BE
+    io.external_encoding.should == Encoding::UTF_16BE
+    io.read.b.should == "abc".b
+  end
+
+  it "returns the result encoding if found BOM UTF_32LE sequence" do
+    io = StringIO.new("\xFF\xFE\x00\x00".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_32LE
+    io.external_encoding.should == Encoding::UTF_32LE
+    io.read.b.should == "".b
+
+    io = StringIO.new("\xFF\xFE\x00\x00abc".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_32LE
+    io.external_encoding.should == Encoding::UTF_32LE
+    io.read.b.should == "abc".b
+  end
+
+  it "returns the result encoding if found BOM UTF_32BE sequence" do
+    io = StringIO.new("\x00\x00\xFE\xFF".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_32BE
+    io.external_encoding.should == Encoding::UTF_32BE
+    io.read.b.should == "".b
+
+    io = StringIO.new("\x00\x00\xFE\xFFabc".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_32BE
+    io.external_encoding.should == Encoding::UTF_32BE
+    io.read.b.should == "abc".b
+  end
+
+  it "returns nil if io is empty" do
+    io = StringIO.new("".b, "rb")
+    io.set_encoding_by_bom.should be_nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+  end
+
+  it "returns nil if UTF-8 BOM sequence is incomplete" do
+    io = StringIO.new("\xEF".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\xEF".b
+
+    io = StringIO.new("\xEFa".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\xEFa".b
+
+    io = StringIO.new("\xEF\xBB".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\xEF\xBB".b
+
+    io = StringIO.new("\xEF\xBBa".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\xEF\xBBa".b
+  end
+
+  it "returns nil if UTF-16BE BOM sequence is incomplete" do
+    io = StringIO.new("\xFE".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\xFE".b
+
+    io = StringIO.new("\xFEa".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\xFEa".b
+  end
+
+  it "returns nil if UTF-16LE/UTF-32LE BOM sequence is incomplete" do
+    io = StringIO.new("\xFF".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\xFF".b
+
+    io = StringIO.new("\xFFa".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\xFFa".b
+  end
+
+  it "returns UTF-16LE if UTF-32LE BOM sequence is incomplete" do
+    io = StringIO.new("\xFF\xFE".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_16LE
+    io.external_encoding.should == Encoding::UTF_16LE
+    io.read.b.should == "".b
+
+    io = StringIO.new("\xFF\xFE\x00".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_16LE
+    io.external_encoding.should == Encoding::UTF_16LE
+    io.read.b.should == "\x00".b
+
+    io = StringIO.new("\xFF\xFE\x00a".b, "rb")
+
+    io.set_encoding_by_bom.should == Encoding::UTF_16LE
+    io.external_encoding.should == Encoding::UTF_16LE
+    io.read.b.should == "\x00a".b
+  end
+
+  it "returns nil if UTF-32BE BOM sequence is incomplete" do
+    io = StringIO.new("\x00".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\x00".b
+
+    io = StringIO.new("\x00a".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\x00a".b
+
+    io = StringIO.new("\x00\x00".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\x00\x00".b
+
+    io = StringIO.new("\x00\x00a".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\x00\x00a".b
+
+    io = StringIO.new("\x00\x00\xFE".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\x00\x00\xFE".b
+
+    io = StringIO.new("\x00\x00\xFEa".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read.b.should == "\x00\x00\xFEa".b
+  end
+
+  it "returns nil if found BOM sequence not provided" do
+    io = StringIO.new("abc".b, "rb")
+
+    io.set_encoding_by_bom.should == nil
+    io.external_encoding.should == Encoding::ASCII_8BIT
+    io.read(3).should == "abc".b
+  end
+
+  it "does not raise exception if io not in binary mode" do
+    io = StringIO.new("", 'r')
+    io.set_encoding_by_bom.should == nil
+  end
+
+  it "does not raise exception if encoding already set" do
+    io = StringIO.new("".b, "rb")
+    io.set_encoding("utf-8")
+    io.set_encoding_by_bom.should == nil
+  end
+
+  it "does not raise exception if encoding conversion is already set" do
+    io = StringIO.new("".b, "rb")
+    io.set_encoding(Encoding::UTF_8, Encoding::UTF_16BE)
+
+    io.set_encoding_by_bom.should == nil
+  end
+
+  it "raises FrozenError when io is frozen" do
+    io = StringIO.new()
+    io.freeze
+    -> { io.set_encoding_by_bom }.should raise_error(FrozenError)
+  end
+
+  it "does not raise FrozenError when initial string is frozen" do
+    io = StringIO.new("".freeze)
+    io.set_encoding_by_bom.should == nil
+  end
+end