Add `File::count_entries`.

This method provides a short cut to finding out how many entries are in
an archive by reading this number directly from the central directory,
and not iterating through the entire set of entries.

lib/zip/file.rb

@@ -171,6 +171,20 @@ module Zip
           zip_file.each(&block)
         end
       end
+
+      # Count the entries in a zip archive without reading the whole set of
+      # entry data into memory.
+      def count_entries(path_or_io)
+        cdir = ::Zip::CentralDirectory.new
+
+        if path_or_io.kind_of?(String)
+          ::File.open(path_or_io, 'rb') do |f|
+            cdir.count_entries(f)
+          end
+        else
+          cdir.count_entries(path_or_io)
+        end
+      end
     end
 
     # Returns an input stream to the specified entry. If a block is passed

test/file_test.rb

@@ -186,6 +186,24 @@ class ZipFileTest < MiniTest::Test
     Zip::File.open('test/data/max_length_file_comment.zip')
   end
 
+  def test_count_entries
+    [
+      ['test/data/osx-archive.zip', 4],
+      ['test/data/zip64-sample.zip', 2],
+      ['test/data/max_length_file_comment.zip', 1]
+    ].each do |filename, num_entries|
+      assert_equal(num_entries, ::Zip::File.count_entries(filename))
+
+      ::File.open(filename, 'rb') do |f|
+        assert_equal(num_entries, ::Zip::File.count_entries(f))
+
+        f.seek(0)
+        s = StringIO.new(f.read)
+        assert_equal(num_entries, ::Zip::File.count_entries(s))
+      end
+    end
+  end
+
   def test_cleans_up_tempfiles_after_close
     zf = ::Zip::File.new(EMPTY_FILENAME, create: true)
     zf.get_output_stream('myFile') do |os|