Array::recurrences.coffee

# Returns a new Array containing only those elements that occur
# twice or more times in the original Array.
Array::recurrences = ->
  FirstOccurrences = [] # of the recurring elements
                        # in the original Array

  occurrences = {} # from an element to an Array of indexes
                   # where it occurs
  
  for element, index in @
    unless occurrences.hasOwnProperty element
      occurrences[element] = []

    occurrences[element].push index

    indexes = occurrences[element]
    if indexes.length is 2
      FirstOccurrences.push indexes[0]

  FirstOccurrences
    .sort()
    .map (index) =>
      @[index]

# It preserves the order, in which elements occur for the first
# time in the original Array.
console.log [1, 1, 2, 3, 4, 5, 5, 2, 3, 5, 6, 7].recurrences()
#=> [ 1, 2, 3, 5 ]
# and not [ 1, 5, 2, 3 ], for example