A class developed via TDD

1_spec.rb

# Year 2014

# encoding: UTF-8
require 'export'
require 'export/secure_name'
require 'active_support/core_ext/object/blank'

describe Export::SecureName do
  subject { described_class.new('name', '.ext') }

  it '#initialize' do
    expect(subject.name).to eql('name')
    expect(subject.ext).to eql('.ext')

    instance = described_class.new('name')
    expect(instance.ext).to eql('')
  end

  it 'supports reading and writing a name' do
    subject.name = 'foo'
    expect(subject.name).to eql('foo')
  end

  it 'supports reading and writing an ext' do
    expect { subject.ext = nil }.to raise_error(ArgumentError, "ext can't be nil")

    subject.ext = 'txt'
    expect(subject.ext).to eql('.txt')

    subject.ext = '.txt'
    expect(subject.ext).to eql('.txt')

    subject.ext = 'tar.gz'
    expect(subject.ext).to eql('.tar.gz')

    subject.ext = ''
    expect(subject.ext).to eql('')
  end

  context '#make' do
    it 'sanitizes output name' do
      subject.name = '/foobar/abc \/:*?"<>|.rb'; subject.ext = '.txt'
      expect(subject.make).to eql('_foobar_abc _________.rb.txt')

      subject.name = '...'; subject.ext = ''
      expect(subject.make).to eql('_...')

      subject.name = ''; subject.ext = ''
      expect(subject.make).to eql('unnamed')
    end

    it 'limits length of output name' do
      too_long_name = 'l' * 300
      good_enough_name = 'l' * 255
      good_enough_name_with_ext = 'l' * 251 + '.txt'

      subject.name = too_long_name; subject.ext = ''
      expect(subject.make).to eql(good_enough_name)

      subject.name = too_long_name; subject.ext = '.txt'
      expect(subject.make).to eql(good_enough_name_with_ext)

      # http://en.wikipedia.org/wiki/UTF-8#Codepage_layout
      # "\u0580\u3042" => "րあ", "րあ".bytesize => 5
      too_many_bytes_within_name = "\u0580\u3042" * 60
      enough_bytes_within_name = "\u0580\u3042" * 51

      subject.name = too_many_bytes_within_name; subject.ext = ''
      expect(subject.make).to eql(enough_bytes_within_name)

      too_long_cyrillic_name = 'д' * 150
      good_enough_cyrillic_name = 'д' * 127

      subject.name = too_long_cyrillic_name; subject.ext = ''
      expect(subject.make).to eql(good_enough_cyrillic_name)
    end

    it 'sets right extension to output name' do
      subject.name = 'File Name.txt'; subject.ext = ''
      expect(subject.make).to eql('File Name.txt')

      subject.name = '1.1 File Name.rb'; subject.ext = '.txt'
      expect(subject.make).to eql('1.1 File Name.rb.txt')

      subject.name = '1.1 File Name.rb.txt'; subject.ext = '.txt'
      expect(subject.make).to eql('1.1 File Name.rb.txt')
    end
  end
end

2_code.rb

class Export::SecureName
  MAX_BYTE_NAME_LENGTH = 255

  attr_accessor :name, :ext

  def initialize(name, ext = '')
    self.name = name
    self.ext = ext
  end

  def ext=(value)
    raise(ArgumentError, "ext can't be nil") if value.nil?
    @ext = (value.empty? || value.start_with?('.')) ? value : ".#{value}"
  end

  def make
    name = sanitize(self.name)
    name = File.basename(name, ext)
    name = name.byteslice(0, MAX_BYTE_NAME_LENGTH - ext.length)
    name = name.encode('UTF-16', :invalid => :replace, :replace => '').encode('UTF-8')
    name + ext
  end

  private
    def sanitize(name)
      name = name.gsub(sanitize_regexp, '_')
      name = "_#{name}" if name =~ /\A\.+\z/
      name = 'unnamed' if name.size == 0
      name
    end

    def sanitize_regexp
      /[\\\/:"\*\?<>\|]/ # Invalid filesystem characters
    end
end