Made trailer ID deterministic

lib/pdf/core/renderer.rb

@@ -1,5 +1,5 @@
 require 'stringio'
-require 'securerandom'
+require 'digest'
 
 module PDF
   module Core
@@ -163,9 +163,9 @@ def render(output = StringIO.new)
         finalize_all_page_contents
 
         render_header(output)
-        render_body(output)
+        body_hash = render_body(output)
         render_xref(output)
-        render_trailer(output)
+        render_trailer(output, body_hash)
         if output.instance_of?(StringIO)
           str = output.string
           str.force_encoding(::Encoding::ASCII_8BIT)
@@ -198,7 +198,13 @@ def render_header(output)
       # Write out the PDF Body, as per spec 3.4.2
       #
       def render_body(output)
+        body_start = output.tell
         state.render_body(output)
+        body_end = output.tell
+        output.seek body_start
+        body_hash = Digest::MD5.digest output.read(body_end - body_start)
+        output.seek body_end
+        body_hash
       end
 
       # Write out the PDF Cross Reference Table, as per spec 3.4.3
@@ -216,8 +222,8 @@ def render_xref(output)
 
       # Write out the PDF Trailer, as per spec 3.4.4
       #
-      def render_trailer(output)
-        trailer_id = PDF::Core::ByteString.new(SecureRandom.random_bytes(16))
+      def render_trailer(output, body_hash)
+        trailer_id = PDF::Core::ByteString.new(body_hash)
         trailer_hash = {
           Size: state.store.size + 1,
           Root: state.store.root,