+class Array
+ # Chooses a random array element from the receiver based on the weights
+ # provided. If _weights_ is nil, then each element is weighed equally.
+ #
+ # [1,2,3].random #=> 2
+ # [1,2,3].random #=> 1
+ # [1,2,3].random #=> 3
+ #
+ # If _weights_ is an array, then each element of the receiver gets its
+ # weight from the corresponding element of _weights_. Notice that it
+ # favors the element with the highest weight.
+ #
+ # [1,2,3].random([1,4,1]) #=> 2
+ # [1,2,3].random([1,4,1]) #=> 1
+ # [1,2,3].random([1,4,1]) #=> 2
+ # [1,2,3].random([1,4,1]) #=> 2
+ # [1,2,3].random([1,4,1]) #=> 3
+ #
+ # If _weights_ is a symbol, the weight array is constructed by calling
+ # the appropriate method on each array element in turn. Notice that
+ # it favors the longer word when using :length.
+ #
+ # ['dog', 'cat', 'hippopotamus'].random(:length) #=> "hippopotamus"
+ # ['dog', 'cat', 'hippopotamus'].random(:length) #=> "dog"
+ # ['dog', 'cat', 'hippopotamus'].random(:length) #=> "hippopotamus"
+ # ['dog', 'cat', 'hippopotamus'].random(:length) #=> "hippopotamus"
+ # ['dog', 'cat', 'hippopotamus'].random(:length) #=> "cat"
+ def random(weights=nil)
+ return random(map {|n| n.send(weights)}) if weights.is_a? Symbol
+
+ weights ||= Array.new(length, 1.0)
+ total = weights.inject(0.0) {|t,w| t+w}
+ point = rand * total
+
+ zip(weights).each do |n,w|
+ return n if w >= point
+ point -= w
+ end
+ end
+
+ # Generates a permutation of the receiver based on _weights_ as in
+ # Array#random. Notice that it favors the element with the highest
+ # weight.
+ #
+ # [1,2,3].randomize #=> [2,1,3]
+ # [1,2,3].randomize #=> [1,3,2]
+ # [1,2,3].randomize([1,4,1]) #=> [2,1,3]
+ # [1,2,3].randomize([1,4,1]) #=> [2,3,1]
+ # [1,2,3].randomize([1,4,1]) #=> [1,2,3]
+ # [1,2,3].randomize([1,4,1]) #=> [2,3,1]
+ # [1,2,3].randomize([1,4,1]) #=> [3,2,1]
+ # [1,2,3].randomize([1,4,1]) #=> [2,1,3]
+ def randomize(weights=nil)
+ return randomize(map {|n| n.send(weights)}) if weights.is_a? Symbol
+
+ weights = weights.nil? ? Array.new(length, 1.0) : weights.dup
+
+ # pick out elements until there are none left
+ list, result = self.dup, []
+ until list.empty?
+ # pick an element
+ result << list.random(weights)
+ # remove the element from the temporary list and its weight
+ weights.delete_at(list.index(result.last))
+ list.delete result.last
+ end
+
+ result
+ end
+end
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