Allow downloads with a HTTP range header

lib/cellar/cellar_adapter.rb

@@ -47,8 +47,11 @@ module Cellar
         end
       end
 
-      def download(key)
+      def download(key, range: nil)
         request = Net::HTTP::Get.new("/#{key}")
+        if range.present?
+          add_range_header(request, range)
+        end
         @signer.sign(request, key)
         if block_given?
           @http.request(request) do |response|
@@ -106,6 +109,12 @@ module Cellar
 
       private
 
+      def add_range_header(request, range)
+        bytes_end = range.exclude_end? ? range.end - 1 : range.end
+
+        request['range'] = "bytes=#{range.begin}-#{bytes_end}"
+      end
+
       def parse_bucket_listing(bucket_listing_xml)
         doc = Nokogiri::XML(bucket_listing_xml)
         doc

spec/lib/cellar/cellar_adapter_spec.rb

@@ -4,6 +4,26 @@ describe 'CellarAdapter' do
   before { Timecop.freeze(Time.gm(2016, 10, 2)) }
   after { Timecop.return }
 
+  describe 'add_range_header' do
+    let(:request) { Net::HTTP::Get.new('/whatever') }
+
+    before { session.send(:add_range_header, request, range) }
+
+    subject { request['range'] }
+
+    context 'with end included' do
+      let(:range) { 100..500 }
+
+      it { is_expected.to eq('bytes=100-500') }
+    end
+
+    context 'with end excluded' do
+      let(:range) { 10...50 }
+
+      it { is_expected.to eq('bytes=10-49') }
+    end
+  end
+
   describe 'parse_bucket_listing' do
     let(:response) do
       <<~EOS