How can I index duplicate items in an array?

Question

Starting with the following array (of hashes):

[
  {:name=>"site a", :url=>"http://example.org/site/1/"}, 
  {:name=>"site b", :url=>"http://example.org/site/2/"}, 
  {:name=>"site c", :url=>"http://example.org/site/3/"}, 
  {:name=>"site d", :url=>"http://example.org/site/1/"}, 
  {:name=>"site e", :url=>"http://example.org/site/2/"}, 
  {:name=>"site f", :url=>"http://example.org/site/6/"},
  {:name=>"site g", :url=>"http://example.org/site/1/"}
]

How can I add an index of the duplicate urls like so:

[
  {:name=>"site a", :url=>"http://example.org/site/1/", :index => 1}, 
  {:name=>"site b", :url=>"http://example.org/site/2/", :index => 1}, 
  {:name=>"site c", :url=>"http://example.org/site/3/", :index => 1}, 
  {:name=>"site d", :url=>"http://example.org/site/1/", :index => 2}, 
  {:name=>"site e", :url=>"http://example.org/site/2/", :index => 2}, 
  {:name=>"site f", :url=>"http://example.org/site/6/", :index => 1},
  {:name=>"site g", :url=>"http://example.org/site/1/", :index => 3}
]

Answer 1

I would use a hash for keeping track of the indices. Scanning the previous entries again and again seems inefficient

counts = Hash.new(0)
array.each { | hash | 
  hash[:index] = counts[hash[:url]] = counts[hash[:url]] + 1
}

or a bit cleaner

array.each_with_object(Hash.new(0)) { | hash, counts | 
  hash[:index] = counts[hash[:url]] = counts[hash[:url]] + 1
}

Answer 2

array = [
  {:name=>"site a", :url=>"http://example.org/site/1/"}, 
  {:name=>"site b", :url=>"http://example.org/site/2/"}, 
  {:name=>"site c", :url=>"http://example.org/site/3/"}, 
  {:name=>"site d", :url=>"http://example.org/site/1/"}, 
  {:name=>"site e", :url=>"http://example.org/site/2/"}, 
  {:name=>"site f", :url=>"http://example.org/site/6/"},
  {:name=>"site g", :url=>"http://example.org/site/1/"}
]

array.inject([]) { |ar, it| 
    count_so_far = ar.count{|i| i[:url] == it[:url]}
    it[:index] = count_so_far+1
    ar << it
}
#=>
[
  {:name=>"site a", :url=>"http://example.org/site/1/", :index=>1}, 
  {:name=>"site b", :url=>"http://example.org/site/2/", :index=>1}, 
  {:name=>"site c", :url=>"http://example.org/site/3/", :index=>1}, 
  {:name=>"site d", :url=>"http://example.org/site/1/", :index=>2}, 
  {:name=>"site e", :url=>"http://example.org/site/2/", :index=>2}, 
  {:name=>"site f", :url=>"http://example.org/site/6/", :index=>1}, 
  {:name=>"site g", :url=>"http://example.org/site/1/", :index=>3}
]