resolve gem dependencies

read_deps_from_gemspec.rb

require 'rubygems'
# https://github.com/rubygems/rubygems/blob/master/lib/rubygems/specification.rb#L981
gemspec = 'foo.gemspec'
spec = Gem::Specification.load(gemspec) # from_yaml
spec.dependencies # type (runtime), name (activesupport), requirements (">= 0")

resolve_deps.rb

require 'bundler'
# https://github.com/bundler/bundler/blob/master/lib/bundler/source/rubygems.rb
# https://github.com/bundler/bundler/blob/master/lib/bundler/resolver.rb
# https://github.com/rubygems/rubygems/blob/master/lib/rubygems/dependency_resolver.rb
gems = %w(metric_fu).map do |name|
  Gem::Dependency.new(name, ['>= 0'], :runtime)
end.select{|gem| gem.type == :runtime}

def resolve(gems)
  gem_deps = gems.map do |gem|
    Bundler::Dependency.new(gem.name, :version => gem.requirement)
  end
  platforms = %w(ruby)
  deps = platforms.flat_map do |platform|
    gem_deps.map do |dep|
      Bundler::DepProxy.new(dep, platform)
    end
  end
  index = Bundler::Source::Rubygems.new.specs #  Bundler.rubygems.all_specs
  Bundler::Resolver.resolve(deps, index)
end
resolved_deps = resolve(gems)
resolved_deps.map{|gem| 
  [gem.name, gem.version.to_s]    # dependencies, source, platform, full_name
}