Project Euler #2 - Even Fibonacci numbers

const SQ5 = 5 ** .5;
const PHI = (1 + SQ5) / 2;
function allEvenFibonacciUpTo(limit) {
  const highestIndexBelowLimit = Math.floor(Math.log(limit * SQ5) / Math.log(PHI));
  // I love expressive variable names, but the formula could get too long!
  const n = Math.floor(highestIndexBelowLimit / 3);
  return ((PHI ** (3 * n + 3) - 1) / (PHI ** 3 - 1)
    - ((1 - PHI) ** (3 * n + 3) - 1) / ((1 - PHI) ** 3 - 1)) / SQ5;
}

console.log(allEvenFibonacciUpTo(4e6)); // 4613731.999999999

"""
Even Fibonacci numbers: Project Euler Problem 2
Solution in Python

https://projecteuler.net/problem=2

Takes 0.4s on my computer
"""
def fibonacci_sequence(n):
    numbers = [0, 1]

    while numbers[-1] < n:
        numbers.append(numbers[-1] + numbers[-2])

    return numbers


def sum_even_fibonacci_numbers(n):
    # O(N) Complexity
    sequence = fibonacci_sequence(n)
    sequence = [n for n in sequence if n % 2 == 0]
    return sum(sequence)

print(sum_even_fibonacci_numbers(4000000))

def sum_even_fibonacci_numbers_up_to(max)
  range = 1..Float::INFINITY
  sum = 0
  last_two_fibs = [0,1]

  range.lazy.take_while do
    next_fib = last_two_fibs[0] + last_two_fibs[1] 
    sum += next_fib if next_fib % 2 == 0
    last_two_fibs = [last_two_fibs[1], next_fib]
    next_fib < max
  end.force

  sum
end

sum_even_fibonacci_numbers_up_to(4_000_000)
# => 4613732

with FIBONACCI (i, FIBO, PREV) as 
(
   select  
      1 i, 0 FIBO, cast(null as number) PREV 
   from 
      DUAL
   union all
   select 
      f1.i + 1  i, f1.FIBO + nvl(f1.PREV,1) FIBO, f1.FIBO PREV
   from 
      FIBONACCI f1
   where 
      f1.i < 35
)

select 
   sum(FIBO) as "Answer"
from 
   FIBONACCI
where 
   mod(FIBO, 2) = 0 
order by i;

-----------
Answer: 4613732

a, b, sum = 1, -1, 0
while a <= 4_000_000
  a, b = a+b, a 
  sum += a if a.even?
end
sum # => 4613732

a, b = 1, -1
loop.lazy
    .map { a, b = a+b, a; a }
    .take_while { |n| n <= 4_000_000 }
    .select(&:even?)
    .sum # => 4613732

var memo = [0, 1];
function fib (n) {
    if(memo.length-1 < n) {
        memo[n] = fib(n-1)+fib(n-2);
    }
    return memo[n];
}

// creates array with elements from function that takes the index as argument while a given condition holds
function takeWhile (fromFunc, cond, arr=[]) {
    var n = arr.length;
    var val = fromFunc(n);
    if(cond(val)) {
        arr.push(val);
        return takeWhile(fromFunc, cond, arr);
    }
    return arr;
}

var sum = 
    takeWhile(fib, n => n < 4000000)
    .filter(n => n%2==0)
    .reduce((acc, c) => acc+c, 0);

console.log(sum);

fibo a b = a:fibo b (a+b)
isEven = (== 0) . flip mod 2

fibSumEvenUnder n = sum $ filter isEven $ takeWhile (<n) $ fibo 1 2
ans = fibSumEvenUnder 4000000

fn fibo(mut a: i64, mut b: i64, max: i64) -> i64 {
    let mut sum = 0;
    while a < max {
        if a % 2 == 0 {
            sum += a;
        }
        let c = a+b;
        a = b;
        b = c;
    }
    sum
}

fn main() {
    println!("{}", fibo(1,2,4000000))  // 4613732
}

def fibonacci(num, prev_num, sum=0):
    print(num, prev_num, sum)
    if num % 2 == 0:
        sum += num
    if num < 4000000:
        return fibonacci(num + prev_num, num, sum)
    return sum


print(fibonacci(1, 1))

puts Enumerator.new { |y|
  f1, f2 = 0, 1
  f1, f2 = f1 + f2, f1 while y << f1
}.take_while {|x| x <= 4_000_000 }.select(&:even?).sum