map Concurrent::Hash to ThreadSafe::Cache in Ruby 1.8.7

Gemfile

@@ -4,25 +4,30 @@ source 'https://rubygems.org'
 gemspec
 
 group :development do
-  if (ruby_version = Gem::Version.new RUBY_VERSION) < (Gem::Version.new '2.1.0')
-    if ruby_version < (Gem::Version.new '2.0.0')
-      gem 'haml', '~> 4.0.0'
-      if ruby_version < (Gem::Version.new '1.9.3')
-        gem 'cucumber', '~> 1.3.0'
-        gem 'nokogiri', '~> 1.5.0'
-        gem 'slim', '~> 2.1.0'
-        gem 'tilt', '2.0.7'
+  ruby_version = Gem::Version.new RUBY_VERSION
+  gem 'concurrent-ruby', '~> 1.0.0' unless ruby_version < (Gem::Version.new '1.9.3')
+  if ruby_version < (Gem::Version.new '2.2.0')
+    if ruby_version < (Gem::Version.new '2.1.0')
+      if ruby_version < (Gem::Version.new '2.0.0')
+        gem 'haml', '~> 4.0.0'
+        if ruby_version < (Gem::Version.new '1.9.3')
+          gem 'cucumber', '~> 1.3.0'
+          gem 'nokogiri', '~> 1.5.0'
+          gem 'slim', '~> 2.1.0'
+          gem 'thread_safe', '0.3.6'
+          gem 'tilt', '2.0.7'
+        else
+          gem 'nokogiri', '~> 1.6.0'
+          gem 'slim', '<= 3.0.7'
+        end
       else
         gem 'nokogiri', '~> 1.6.0'
-        gem 'slim', '<= 3.0.7'
       end
     else
-      gem 'nokogiri', '~> 1.6.0'
+      gem 'nokogiri', '~> 1.7.0' if Gem::Platform.local =~ 'x86-mingw32' || Gem::Platform.local =~ 'x64-mingw32'
+      gem 'racc', '~> 1.4.0' if RUBY_VERSION == '2.1.0' && RUBY_ENGINE == 'rbx'
     end
-  elsif ruby_version < (Gem::Version.new '2.2.0')
-    gem 'nokogiri', '~> 1.7.0' if Gem::Platform.local =~ 'x86-mingw32' || Gem::Platform.local =~ 'x64-mingw32'
   end
-  gem 'racc', '~> 1.4.0' if RUBY_VERSION == '2.1.0' && RUBY_ENGINE == 'rbx'
 end
 
 group :doc do

asciidoctor.gemspec

@@ -51,7 +51,8 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rspec-expectations', '~> 2.14.0'
   # slim is needed for testing custom templates
   s.add_development_dependency 'slim', '~> 3.0.0'
-  s.add_development_dependency 'concurrent-ruby', '~> 1.0.5'
+  # concurrent-ruby is defined in Gemfile due to enforcement of minimum required Ruby version
+  #s.add_development_dependency 'concurrent-ruby', '~> 1.0.0'
   # tilt is needed for testing custom templates
   s.add_development_dependency 'tilt', '~> 2.0.0'
   s.add_development_dependency 'minitest', '~> 5.3.0'

lib/asciidoctor/converter/template.rb

@@ -1,5 +1,3 @@
-autoload :Concurrent, 'concurrent'
-
 # encoding: UTF-8
 module Asciidoctor
   # A {Converter} implementation that uses templates composed in template
@@ -36,7 +34,13 @@ class Converter::TemplateConverter < Converter::Base
     }
 
     begin
-      # triggers autoload of concurrent
+      unless defined? ::Concurrent::Hash
+        if ::RUBY_MIN_VERSION_1_9
+          require 'concurrent/hash'
+        else
+          require 'asciidoctor/core_ext/1.8.7/concurrent/hash'
+        end
+      end
       @caches = { :scans => ::Concurrent::Hash.new, :templates => ::Concurrent::Hash.new }
     rescue ::LoadError
       @caches = { :scans => {}, :templates => {} }

lib/asciidoctor/core_ext/1.8.7/concurrent/hash.rb

@@ -0,0 +1,5 @@
+autoload :ThreadSafe, 'thread_safe'
+
+module Concurrent
+  Hash = ::ThreadSafe::Cache
+end