Stub out extension build on JRuby

JRuby currently ships its own internal bigdecimal extension as
part of the core libraries. In order for users to be able to add
bigdecimal to their Gemfile or gem dependencies, we need to stub
out the C extension and just load the extension shipped with
JRuby.

In the future we will try to move our BigDecimal implementation
into the gem, but for now this is the simplest way to make it
installable on JRuby.

See ruby#169

Gemfile

@@ -3,7 +3,7 @@ source 'https://rubygems.org'
 gemspec
 
 gem "benchmark_driver"
-gem "fiddle"
+gem "fiddle", platform: :ruby
 gem "rake", ">= 12.3.3"
 gem "rake-compiler", ">= 0.9"
 gem "minitest", "< 5.0.0"

Rakefile

@@ -6,7 +6,12 @@ require "rake/extensiontask"
 require "rake/testtask"
 
 spec = eval(File.read('bigdecimal.gemspec'))
-Rake::ExtensionTask.new('bigdecimal', spec)
+if RUBY_ENGINE == 'jruby'
+  # JRuby's extension is included with JRuby currently
+  task :compile do; end
+else
+  Rake::ExtensionTask.new('bigdecimal', spec)
+end
 
 Rake::TestTask.new do |t|
   t.libs << 'test/lib'

bigdecimal.gemspec

@@ -12,17 +12,8 @@ Gem::Specification.new do |s|
   s.licenses       = ["Ruby", "bsd-2-clause"]
 
   s.require_paths = %w[lib]
-  s.extensions    = %w[ext/bigdecimal/extconf.rb]
   s.files         = %w[
     bigdecimal.gemspec
-    ext/bigdecimal/bigdecimal.c
-    ext/bigdecimal/bigdecimal.h
-    ext/bigdecimal/bits.h
-    ext/bigdecimal/feature.h
-    ext/bigdecimal/missing.c
-    ext/bigdecimal/missing.h
-    ext/bigdecimal/missing/dtoa.c
-    ext/bigdecimal/static_assert.h
     lib/bigdecimal.rb
     lib/bigdecimal/jacobian.rb
     lib/bigdecimal/ludcmp.rb
@@ -33,6 +24,21 @@ Gem::Specification.new do |s|
     sample/nlsolve.rb
     sample/pi.rb
   ]
+  if Gem::Platform === s.platform and s.platform =~ 'java' or RUBY_ENGINE == 'jruby'
+    s.platform      = 'java'
+  else
+    s.extensions    = %w[ext/bigdecimal/extconf.rb]
+    s.files.append %w[
+      ext/bigdecimal/bigdecimal.c
+      ext/bigdecimal/bigdecimal.h
+      ext/bigdecimal/bits.h
+      ext/bigdecimal/feature.h
+      ext/bigdecimal/missing.c
+      ext/bigdecimal/missing.h
+      ext/bigdecimal/missing/dtoa.c
+      ext/bigdecimal/static_assert.h
+    ]
+  end
 
   s.required_ruby_version = Gem::Requirement.new(">= 2.5.0")
 end

lib/bigdecimal.rb

@@ -1 +1,5 @@
-require 'bigdecimal.so'
+if RUBY_ENGINE == 'jruby'
+  JRuby::Util.load_ext("org.jruby.ext.bigdecimal.BigDecimalLibrary")
+else
+  require 'bigdecimal.so'
+end