nth fib with Matrix exponentiation in Ruby

matrix_fib.rb

require 'matrix'

def fib n
  matrix = Matrix[[0, 1], [1, 1]] ** n.pred
  matrix[1, 1].to_i
end

require 'minitest/autorun'
require 'minitest/pride'

describe 'fib' do
  it 'does nth positive fib' do
    assert_equal fib(0), 0
    assert_equal fib(1), 1
    assert_equal fib(2), 1
    assert_equal fib(3), 2
    assert_equal fib(4), 3
    assert_equal fib(5), 5
    assert_equal fib(6), 8
    assert_equal fib(7), 13
    assert_equal fib(8), 21
  end

  it 'does nth negative fib' do
    assert_equal fib(-1), 1
    assert_equal fib(-2), -1
    assert_equal fib(-3), 2
    assert_equal fib(-4), -3
    assert_equal fib(-5), 5
    assert_equal fib(-6), -8
    assert_equal fib(-7), 13
    assert_equal fib(-8), -21
  end
end