MyList.exs

defmodule MyList do
  def len([]), do: 0
  def len([_head | tail]), do: 1 + len(tail)

  def square([]), do: []
  def square([head | tail]), do: [head*head | square(tail)]

  def map([], _func), do: []
  def map([head | tail], func), do: [func.(head) | map(tail, func)]

  def sum(list), do: _sum(list, 0)

  # private function
  defp _sum([], total), do: total
  defp _sum([head | tail], total), do: _sum([tail], head + total)

  def reduce([], value, _) do
    value
  end
  def reduce([head | tail], value, func) do
    reduce(tail, func.(head, value), func)
  end

  def mapsum(list, func) do
    reduce(map(list, func), 0, (&(&1 + &2)))
  end

  def max([]), do: 0
  def max([head | tail]), do: reduce([head | tail], head, &(compare(&1, &2)))

  def compare(p1, p2) when p1 > p2, do: p1
  def compare(p1, p2) when p1 <= p2, do: p2

  def caesar(list, n) do
    map(list, fn x -> _decode_char(x, n) end)
  end

  defp _decode_char(char, num) when char + num > 122 do
    char + num - 26
  end
  defp _decode_char(char, num) when char + num < 122 do
      char + num
  end

  def span(from, to) when from < to do
    [from | span(from + 1, to)]
  end
  def span(from, to) when from == to, do: [to]

end