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defmodule Fib do
@moduledoc """
Lazy Fibonacci Sequence
"""
defmodule FibVal do
defstruct val: 0, next: 1
end
@doc """
Return a lazy sequence of FibVals.
To get the values, use map &(&1.val)
iex> Fib.fib |> Stream.map(&(&1.val)) |> Enum.take(10)
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
"""
def fib do
Stream.iterate %FibVal{}, fn %FibVal{val: val, next: next} ->
%FibVal{val: next, next: val + next}
end
end
@doc """
Return a lazy sequence of Fibonacci numbers
This one is better as it returns the actual integer value
and doesn't use FibVal, thanks to Stream.unfold/2
iex> Fib.fib2 |> Enum.take(10)
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
"""
def fib2 do
Stream.unfold({0, 1}, fn {a, b} -> {a, {b, a + b}} end)
end
end
ExUnit.start
defmodule FibTest do
use ExUnit.Case
test "fib" do
fib = Fib.fib |> Stream.map(&(&1.val)) |> Enum.take(10)
assert fib == [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
end
test "fib2" do
fib = Fib.fib2 |> Enum.take(10)
assert fib == [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
end
end