SophieDeBenedetto

defp get_two([first | [_second | rest]]) do
  get_two([first | rest])
end

SophieDeBenedetto

defp get_two([first | [second | _rest]]) when first + second == @sum do
  first * second
end

SophieDeBenedetto

defmodule Accountant do
  @sum 2020
  defguard equals_twenty_twenty(first, second) when first + second == @sum

  def product_of_equals_twenty_twenty(list, product \\ nil)
  def product_of_equals_twenty_twenty(_list, product) when product != nil, do: product

  def product_of_equals_twenty_twenty([_head | tail] = list, _product) do
    product_of_equals_twenty_twenty(tail, get_two(list))
  end

SophieDeBenedetto

defmodule Accountant do
  @sum 2020

  def product_of_equals_twenty_twenty(list, product \\ nil)
  def product_of_equals_twenty_twenty(_list, product) when product != nil, do: product

  def product_of_equals_twenty_twenty([_head | tail] = list, _product) do
    product_of_equals_twenty_twenty(tail, get_two(list))
  end

SophieDeBenedetto

defmodule Accountant do
  @sum 2020

  def product_of_equals_twenty_twenty(list, product \\ nil)
  def product_of_equals_twenty_twenty(_list, product) when product != nil, do: product

  def product_of_equals_twenty_twenty([_head | tail] = list, _product) do
    product_of_equals_twenty_twenty(tail, get_two(list))
  end
  # ...

SophieDeBenedetto

defmodule Accountant do
  @sum 2020
  def product_of_equals_twenty_twenty([]), do: :noop
  def product_of_equals_twenty_twenty([_head | tail] = list) do
    product = get_two(list)
    if product do
      product
    else
      product_of_equals_twenty_twenty(tail)
    end

SophieDeBenedetto

defmodule Accountant do
  @sum 2020
  def product_of_equals_twenty_twenty([]), do: :noop
  def product_of_equals_twenty_twenty([_head | tail] = list) do
    product = get_two(list)
    if product do
      product
    else
      product_of_equals_twenty_twenty(tail)
    end

SophieDeBenedetto

defmodule Accountant do
  @sum 2020

  def product_of_equals_twenty_twenty([_head | tail] = list) do
    product = get_two(list)
    if product do
      product
    else
      product_of_equals_twenty_twenty(tail)
    end

SophieDeBenedetto

defmodule Accountant do
  @sum 2020

  def product_of_equals_twenty_twenty([_head | tail] = list) do
    product = get_two(list)
    if product do
      product
    else
      # keep going
    end

SophieDeBenedetto

defmodule Accountant do
  @sum 2020
  # ...
  defp get_two(list) when length(list) == 1, do: nil

  defp get_two([first | [second | _rest]]) when first + second == @sum do
    first * second
  end

  defp get_two([first | [_second | rest]]) do