gistfile1.ex

#
# Given a list, return a new list with all occurrences of consecutive 
# duplicated elements replaced by `{element, count}`
#
#    compress [ 1,2,2,3,4,4,4,5,6,6]
#        → [1, {2, 2}, 3, {4, 3}, 5, {6, 2}]
#

# This version uses the head of the result to give the effect of lookahead

defmodule Demo do
  def compress(list), do: _compress(list, [])

  def _compress([], result), do: Enum.reverse(result)

  def _compress([a | list], [a | rest]) do
    _compress(list, [{a, 2} | rest])
  end

  def _compress([a | list], [{a, n} | rest]) do
    _compress(list, [ {a, n+1} | rest ])
  end

  def _compress([a | list], result) do
    _compress(list,  [ a | result ])
  end
end

# this version instead peeks at two elements from the source

defmodule Demo1 do
  def compress(list), do: _compress(list, [])

  def _compress([], result), do: Enum.reverse(result)

  def _compress([a, a | list], rest) do
    _compress([ {a,2} | list ], rest)
  end

  def _compress([{a,n}, a | list], [rest]) do
    _compress([ {a,n+1} | list], rest)
  end

  def _compress([a | list], result) do
    _compress(list,  [ a | result ])
  end
end