class RuboCop::Cop::Packaging::RequireHardcodingLib


require “baz/qux”
# good
require File.dirname(__FILE__) + “/../../lib/baz/qux”
# bad
require “foo/bar/baz/qux”
# good
require File.expand_path(“../../../lib/foo/bar/baz/qux”, __dir__)
# bad
require “foo”
# good
require File.expand_path(“../../lib/foo”, __FILE__)
# bad
require “foo/bar”
# good
require “../lib/foo/bar”
# bad
@example
the “lib” directory, except originating from lib/.
This cop flags the ‘require` calls, from anywhere mapping to
:nodoc:
:nodoc:
:nodoc:

def falls_in_lib?(str)

anywhere except the "lib" directory.
It flags an offense if the `require` call is made from
This method is called from inside `#def_node_matcher`. 
def falls_in_lib?(str)
  @str = str
  target_falls_in_lib?(str) && inspected_file_is_not_in_lib_or_gemspec?
end

def falls_in_lib_using_file?(str)

arguement) is made from anywhere except the "lib" directory.
It flags an offense if the `require` call (using the __FILE__
This method is called from inside `#def_node_matcher`. 
def falls_in_lib_using_file?(str)
  @str = str
  target_falls_in_lib_using_file?(str) && inspected_file_is_not_in_lib_or_gemspec?
end

def falls_in_lib_with_file_dirname_plus_str?(str)

And then determines if that call is made to "lib/".
This method preprends a "." to the string that starts with "/". 
def falls_in_lib_with_file_dirname_plus_str?(str)
  @str = str
  str.prepend(".")
  target_falls_in_lib?(str) && inspected_file_is_not_in_lib_or_gemspec?
end

def good_require_call

the "bad" require call with the "good" one.
Called from on_send, this method helps to replace 
def good_require_call
  good_call = @str.sub(%r{^.*/lib/}, "")
  %(require "#{good_call}")
end

def on_new_investigation

Processing of the AST happens here.

https://github.com/rubocop-hq/rubocop/blob/343f62e4555be0470326f47af219689e21c61a37/lib/rubocop/cop/base.rb
More about the `#on_new_investigation` method can be found here:
Extended from the Base class. 
def on_new_investigation
  @file_path = processed_source.file_path
  @file_directory = File.dirname(@file_path)
end

def on_send(node)

https://github.com/rubocop-hq/rubocop-ast/blob/08d0f49a47af1e9a30a6d8f67533ba793c843d67/lib/rubocop/ast/traversal.rb#L112
More about the `#on_send` method can be found here:
Extended from AST::Traversal. 
def on_send(node)
  return unless require?(node)
  add_offense(node) do |corrector|
    corrector.replace(node, good_require_call)
  end
end