use binarySearch.frink

// This class generates random elements from a discrete set with a
// known distribution, for example, the frequency of letters in the English
// language.
class randomDistribution
{
   // An array of elements
   var elements

   // The sum of probabilities
   var sum
   
   new[] :=
   {
      elements = new array
      sum = 0
   }

   // Adds an item with the specified probability.
   add[item, prob] :=
   {
      sum = sum + prob
      elements.push[ [item, prob, sum] ]
   }

   // Selects a random item based on the specified probabilities.
   random[] :=
   {
      z = randomFloat[0, sum]
      index = binarySearch[elements, z, {|a,b| a <=> b@2}]
      return elements@index@0
   }
}