Your challenge, should you accept it, is to write a Ruby library that will modify an existing program to output the number of times a specific method is called.
You solution library should be required at the top of the host program, or via ruby's -r flag (i.e. ruby -r ./solution.rb host_program.rb).
Your solution library should read the environment variable COUNT_CALLS_TO to determine the method it should count. Valid method signatures are Array#map!, ActiveRecord::Base#find, Base64.encode64, etc.
Your solution library should count calls to that method, and print the method signature and the number of times it was called when the program exits.
Also, your solution should have a minimal impact on the program's running time. set_trace_func is a no-go...
As an example, here's a valid solution being called with a one line program to count String#size calls:
COUNT_CALLS_TO='String#size' ruby -r ./solution.rb -e '(1..100).each{|i| i.to_s.size if i.odd? }'
String#size called 50 times
Here's another more complex example:
COUNT_CALLS_TO='B#foo' ruby -r ./solution.rb -e 'module A; def foo; end; end; class B; include A; end; 10.times{B.new.foo}'
B#foo called 10 times