Add test coverage; account for no artifacts on CI

Drop coverage a bit for JRuby and Rubinius because they
don't generate the same Coverage as CRuby

.simplecov

@@ -0,0 +1,88 @@
+# https://github.com/colszowka/simplecov#using-simplecov-for-centralized-config
+# see https://github.com/colszowka/simplecov/blob/master/lib/simplecov/defaults.rb
+# vim: set ft=ruby
+@minimum_coverage = ENV.fetch('COVERAGE_MINIMUM') {
+  case (defined?(RUBY_ENGINE) && RUBY_ENGINE) || "ruby"
+  when 'jruby', 'rbx'
+    96.0
+  else
+    98.3
+  end
+}.to_f.round(2)
+# rubocop:disable Style/DoubleNegation
+ENV['FULL_BUILD'] ||= ENV['CI']
+@running_ci       = !!(ENV['FULL_BUILD'] =~ /\Atrue\z/i)
+@generate_report  = @running_ci || !!(ENV['COVERAGE'] =~ /\Atrue\z/i)
+@output = STDOUT
+# rubocop:enable Style/DoubleNegation
+
+SimpleCov.pid = $$ # In case there's any forking
+
+SimpleCov.profiles.define 'app' do
+  coverage_dir 'coverage'
+  load_profile 'test_frameworks'
+
+  add_group 'Libraries', 'lib'
+
+  add_group 'Long files' do |src_file|
+    src_file.lines.count > 100
+  end
+  class MaxLinesFilter < SimpleCov::Filter
+    def matches?(source_file)
+      source_file.lines.count < filter_argument
+    end
+  end
+  add_group 'Short files', MaxLinesFilter.new(5)
+
+  # Exclude these paths from analysis
+  add_filter '/config/'
+  add_filter '/db/'
+  add_filter 'tasks'
+end
+
+## START TRACKING COVERAGE
+require 'coverage'
+Coverage.start
+
+# rubocop:disable Style/MultilineBlockChain
+AppCoverage = Class.new do
+  def initialize(&block)
+    @block = block
+  end
+
+  def start
+    @block.call
+  end
+end.new do
+  SimpleCov.start 'app'
+  if @generate_report
+    if @running_ci
+      @output.puts '[COVERAGE] Running with SimpleCov Simple Formatter'
+      formatters = [SimpleCov::Formatter::SimpleFormatter]
+    else
+      @output.puts '[COVERAGE] Running with SimpleCov HTML Formatter'
+      formatters = [SimpleCov::Formatter::HTMLFormatter]
+    end
+  else
+    formatters = []
+  end
+  SimpleCov.formatters = formatters
+end
+# rubocop:enable Style/MultilineBlockChain
+SimpleCov.at_exit do
+  header = "#{'*' * 20} SimpleCov Results #{'*' * 20}"
+  @output.puts
+  @output.puts header
+  @output.puts SimpleCov.result.format!
+  percent = Float(SimpleCov.result.covered_percent)
+  if percent < @minimum_coverage
+    @output.puts "Spec coverage was not high enough: "\
+    "#{percent.round(2)} is < #{@minimum_coverage}%\n"
+    exit 1 if @generate_report
+  else
+    @output.puts "Nice job! Spec coverage (#{percent.round(2)}) "\
+    "is still at or above #{@minimum_coverage}%\n"
+  end
+  @output.puts
+  @output.puts '*' * header.size
+end

Gemfile

@@ -3,17 +3,21 @@ source 'https://rubygems.org'
 # Specify your gem's dependencies in active_model_serializers.gemspec
 gemspec
 
-gem "minitest"
+gem 'minitest'
 
-version = ENV["RAILS_VERSION"] || "4.2"
+version = ENV['RAILS_VERSION'] || '4.2'
 
-if version == "master"
-  gem "rails", github: "rails/rails"
+if version == 'master'
+  gem 'rails', github: 'rails/rails'
 
   # ugh https://github.com/rails/rails/issues/16063#issuecomment-48090125
-  gem "arel", github: "rails/arel"
+  gem 'arel', github: 'rails/arel'
 else
-  gem "rails", "~> #{version}.0"
+  gem 'rails', "~> #{version}.0"
+end
+
+group :test, :development do
+  gem 'simplecov', '~> 0.10', require: false
 end
 
 # Windows does not include zoneinfo files, so bundle the tzinfo-data gem

Rakefile

@@ -1,4 +1,9 @@
-require "bundler/gem_tasks"
+begin
+  require 'simplecov'
+rescue LoadError
+end
+
+require 'bundler/gem_tasks'
 
 require 'rake/testtask'
 

test/support/simplecov.rb

@@ -0,0 +1,6 @@
+# https://github.com/colszowka/simplecov/pull/400
+# https://github.com/ruby/ruby/blob/trunk/lib/English.rb
+unless defined?(English)
+  # The exception object passed to +raise+.
+  alias $ERROR_INFO              $!
+end

test/test_helper.rb

@@ -1,5 +1,16 @@
 require 'bundler/setup'
 
+begin
+  require 'simplecov'
+  # HACK till https://github.com/colszowka/simplecov/pull/400 is merged and released.
+  # Otherwise you may get:
+  # simplecov-0.10.0/lib/simplecov/defaults.rb:50: warning: global variable `$ERROR_INFO' not initialized
+  require 'support/simplecov'
+  AppCoverage.start
+rescue LoadError
+  STDERR.puts 'Running without SimpleCov'
+end
+
 require 'timecop'
 require 'rails'
 require 'action_controller'