Optimize #add_referenced

[pkmiec]

Summary

The #add_referenced kept track of existing object with a hash when the keys were the objects. This seemed to have been ok with Ruby 2.7, but became significantly slower in Ruby 3.2 (and possibly earlier).

Profiling showed that many of those objects are instances of Hash and Ruby uses #eql? method to compare Hash keys. It is not clear what actually changed in Ruby, but we can work around the issue by using #hash so that the key is an integer. We just need to recompute that integer if the object changes.

Performance

The benchmark was done with the following script,

require 'benchmark'
require 'combine_pdf'

puts "Ruby: #{RUBY_VERSION}"
puts "CombinePDF: #{CombinePDF::VERSION}"

files = []
68.times { |i| files << "/tmp/pdfs/#{i}.pdf" }
files = files * 10 # to exacerbate the effect

result_pdf = CombinePDF.new
files.each { |file| result_pdf << CombinePDF.parse(IO.read(file)) }
puts(Benchmark.measure do
    result_pdf.save('/tmp/combined.pdf')
end)

FYI ... we end up with 32053 pdf objects in @objects array.

Before

Ruby: 2.7.7
CombinePDF: 1.0.26
2.598427 0.011980 2.610407 ( 2.617881)

Ruby: 3.2.2
CombinePDF: 1.0.26
15.067833 0.026986 15.094819 ( 15.139298)

After

Ruby: 2.7.7
CombinePDF: 1.0.26 (with this PR)
2.768545 0.006937 2.775482 ( 2.786386)

Ruby: 3.2.2
CombinePDF: 1.0.26 (with this PR)
1.997242 0.016295 2.013537 ( 2.021782)