find_duplicates.rb

require 'set'
require 'digest/md5'

# Let original be a collection with the names of the files in the
# original directory. Let target be the ones in the target directory.
#
# We want to find which files in original are equal to some in target
# except perhaps for the filename.

osizes, tsizes = classify_by_size(original, target)
osizes.each do |size, os_fnames|
  next unless tsizes.key?(size)

  omd5s, tmd5s = classify_by_md5(os_fnames, tsizes[size])
  omd5s.each do |md5, om_fnames|
    next unless tmd5s.key?(md5)

    om_fnames.each do |om_fname|
      tmd5s[md5].each do |tm_fname|
        puts "#{tm_fname} is the same as #{om_fname}" if File.compare(om_fname, tm_fname)
      end
    end
  end
end

def classify_by_size(original, target)
  [original, target].map do |set|
    set.classify { |fname| File.size(fname) }
  end
end

def classify_by_md5(original, target)
  [original, target].map do |set|
    set.classify { |fname| Digest::MD5.file(fname).hexdigest }
  end
end