Added tests with Rspec, slight cleanup

.rspec

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+--require spec_helper

lib/file_nav.rb

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+# frozen_string_literal: true
+# TODO: add recursion support to directories
+class DirTermine
+
+  attr_accessor :save_dir, :source_dir
+
+  def initialize
+     @save_dir = save_dir
+     @source_dir = source_dir
+  end
+
+  def default_dir
+    File.expand_path('~/Pictures/stickerized')
+  end
+
+  def create_dir(dir)
+    Dir.mkdir(File.expand_path(dir)) unless File.directory?(File.expand_path(dir))
+  end
+
+  def destination_dir
+    puts 'Please enter desired save directory (Leave blank for ~/Pictures/stickerized)'
+    dir = gets.chomp
+    dir = default_dir if dir.empty? || dir.nil?
+    create_dir(dir)
+    File.expand_path(dir)
+  end
+
+  def origin_dir
+    puts 'Please enter directory containing pictures to stickerize (Leave blank for current directory)'
+    dir = gets.chomp
+    dir = Dir.getwd if dir.empty?
+    File.expand_path(dir)
+  end
+
+end

lib/main.rb

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+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require_relative 'stickerizer'
+
+sticker_machine = Stickerizer.new
+sticker_machine.convert_images

lib/stickerizer.rb

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+# frozen_string_literal: true
+
+require 'mini_magick'
+require_relative 'file_nav'
+
+class Stickerizer
+  Extension = Struct.new(:extension)
+
+  def file_ext(filename)
+    Extension.new(filename[-4..-1])
+  end
+
+  ImageFormat = Struct.new(:size, :format)
+
+  def telegram_format
+    ImageFormat.new('512x512', 'PNG')
+  end
+
+  def initialize
+    @convert = DirTermine.new
+  end
+
+  def stickerize(pic_file, save_dir)
+    sticker_format = telegram_format
+    image = MiniMagick::Image.open(pic_file)
+    image.resize sticker_format.size
+    image.format sticker_format.format
+    image.write(save_dir + '/' + stickername(pic_file))
+  end
+
+  def stickername(filename)
+    file_only = filename.sub %r{/.+/}, ''
+    "#{file_only[0..-5]}_sticker.png"
+  end
+
+  def origin_dir
+    @convert.origin_dir
+  end
+
+  def dest_dir
+    @convert.destination_dir
+  end
+
+  def valid?(file)
+    # TODO: Change this to check file types instead of relying on filenames
+    valid = false
+    file_info = file_ext(file)
+    valid = true if file_info.extension.downcase == '.png'
+    valid = true if file_info.extension.downcase == '.jpg'
+    valid = true if file_info.extension.downcase == '.gif'
+    puts "File #{file}: valid: #{valid}"
+    valid
+  end
+
+  def convert_images
+    origin = origin_dir
+    destination = dest_dir
+    Dir.children(origin).each do |unstickerized|
+      next unless valid?(unstickerized)
+
+      stickerize("#{origin}/#{unstickerized}", destination)
+    end
+  end
+end

spec/spec_helper.rb

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+# This file was generated by the `rspec --init` command. Conventionally, all
+# specs live under a `spec` directory, which RSpec adds to the `$LOAD_PATH`.
+# The generated `.rspec` file contains `--require spec_helper` which will cause
+# this file to always be loaded, without a need to explicitly require it in any
+# files.
+#
+# Given that it is always loaded, you are encouraged to keep this file as
+# light-weight as possible. Requiring heavyweight dependencies from this file
+# will add to the boot time of your test suite on EVERY test run, even for an
+# individual file that may not need all of that loaded. Instead, consider making
+# a separate helper file that requires the additional dependencies and performs
+# the additional setup, and require it from the spec files that actually need
+# it.
+#
+# See http://rubydoc.info/gems/rspec-core/RSpec/Core/Configuration
+RSpec.configure do |config|
+  # rspec-expectations config goes here. You can use an alternate
+  # assertion/expectation library such as wrong or the stdlib/minitest
+  # assertions if you prefer.
+  config.expect_with :rspec do |expectations|
+    # This option will default to `true` in RSpec 4. It makes the `description`
+    # and `failure_message` of custom matchers include text for helper methods
+    # defined using `chain`, e.g.:
+    #     be_bigger_than(2).and_smaller_than(4).description
+    #     # => "be bigger than 2 and smaller than 4"
+    # ...rather than:
+    #     # => "be bigger than 2"
+    expectations.include_chain_clauses_in_custom_matcher_descriptions = true
+  end
+
+  # rspec-mocks config goes here. You can use an alternate test double
+  # library (such as bogus or mocha) by changing the `mock_with` option here.
+  config.mock_with :rspec do |mocks|
+    # Prevents you from mocking or stubbing a method that does not exist on
+    # a real object. This is generally recommended, and will default to
+    # `true` in RSpec 4.
+    mocks.verify_partial_doubles = true
+  end
+
+  # This option will default to `:apply_to_host_groups` in RSpec 4 (and will
+  # have no way to turn it off -- the option exists only for backwards
+  # compatibility in RSpec 3). It causes shared context metadata to be
+  # inherited by the metadata hash of host groups and examples, rather than
+  # triggering implicit auto-inclusion in groups with matching metadata.
+  config.shared_context_metadata_behavior = :apply_to_host_groups
+
+# The settings below are suggested to provide a good initial experience
+# with RSpec, but feel free to customize to your heart's content.
+=begin
+  # This allows you to limit a spec run to individual examples or groups
+  # you care about by tagging them with `:focus` metadata. When nothing
+  # is tagged with `:focus`, all examples get run. RSpec also provides
+  # aliases for `it`, `describe`, and `context` that include `:focus`
+  # metadata: `fit`, `fdescribe` and `fcontext`, respectively.
+  config.filter_run_when_matching :focus
+
+  # Allows RSpec to persist some state between runs in order to support
+  # the `--only-failures` and `--next-failure` CLI options. We recommend
+  # you configure your source control system to ignore this file.
+  config.example_status_persistence_file_path = "spec/examples.txt"
+
+  # Limits the available syntax to the non-monkey patched syntax that is
+  # recommended. For more details, see:
+  #   - http://rspec.info/blog/2012/06/rspecs-new-expectation-syntax/
+  #   - http://www.teaisaweso.me/blog/2013/05/27/rspecs-new-message-expectation-syntax/
+  #   - http://rspec.info/blog/2014/05/notable-changes-in-rspec-3/#zero-monkey-patching-mode
+  config.disable_monkey_patching!
+
+  # This setting enables warnings. It's recommended, but in some cases may
+  # be too noisy due to issues in dependencies.
+  config.warnings = true
+
+  # Many RSpec users commonly either run the entire suite or an individual
+  # file, and it's useful to allow more verbose output when running an
+  # individual spec file.
+  if config.files_to_run.one?
+    # Use the documentation formatter for detailed output,
+    # unless a formatter has already been configured
+    # (e.g. via a command-line flag).
+    config.default_formatter = "doc"
+  end
+
+  # Print the 10 slowest examples and example groups at the
+  # end of the spec run, to help surface which specs are running
+  # particularly slow.
+  config.profile_examples = 10
+
+  # Run specs in random order to surface order dependencies. If you find an
+  # order dependency and want to debug it, you can fix the order by providing
+  # the seed, which is printed after each run.
+  #     --seed 1234
+  config.order = :random
+
+  # Seed global randomization in this process using the `--seed` CLI option.
+  # Setting this allows you to use `--seed` to deterministically reproduce
+  # test failures related to randomization by passing the same `--seed` value
+  # as the one that triggered the failure.
+  Kernel.srand config.seed
+=end
+end

spec/stickerizer_spec.rb

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+require './lib/stickerizer'
+
+RSpec.describe Stickerizer do
+  describe '#file_ext' do
+
+    it 'returns the last four characters of a string' do
+      stickerizer = Stickerizer.new
+      expect(stickerizer.file_ext('bananas.png').extension).to eql('.png')
+    end
+  end
+  describe '#valid?' do
+    it 'returns false when given an unsupported file extension' do
+      stickerizer = Stickerizer.new
+      expect(stickerizer.valid?('pork.txt')).to eql(false)
+    end
+
+    it 'returns true when given a supported image format' do
+      stickerizer = Stickerizer.new
+      expect(stickerizer.valid?('buenosdias.png')).to eql(true)
+    end
+  end
+
+end