#!/usr/bin/env perl
use feature qw{say state} ;
use bigint;
my $max = 4_000_000 ;

my $sum = 0 ;
while (1){
    state $i = 0;
    $i++;
    my $f = fibonacci($i);
    my $e = $f %2==0?1:0;
    last if $f > $max ;
    $sum += $f if $e;
    }
say $sum;

sub fibonacci {
    my $i = shift ;
    state $cache ;
    $cache->[0]=1;
    $cache->[1]=1;
    if ( $cache->[$i] ) { return $cache->[$i] }
    $cache->[$i] = $cache->[$i-1] + $cache->[$i-2] ;
    return $cache->[$i] ;
    }

"""
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))

"""Find the sum of even Fibonacci numbers less than 4,000,000"""

def fibonacci(cap):
    """Generates fibonacci numbers less than cap"""
    a, b, c = 0, 1, 1
    while c < cap:
        yield c
        a, b = b, c
        c = a + b

def even_fib_sum(cap):
    """Returns the sum of even fibonacci numbers less than cap"""
    return sum(fib for fib in fibonacci(cap) if fib % 2 == 0)

if __name__ == "__main__":
    print(even_fib_sum(4000000))
    print(list(fibonacci(4000000))

# => 4613732
# => [1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, 75025, 121393, 196418, 317811, 514229, 832040, 1346269, 2178309, 3524578] 

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

sumEven a sum =
  if a % 2 == 0 then
    a + sum
  else
    sum

sumfibs max a b sum =
  if a > max then
    sum
  else
    sumfibs max b (a + b) (sumEven a sum)

sumfibs 4000000 1 2 0

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

def solve(n):
    even_terms = []
    i = 2
    current = 0
    while current <= n:
        current = fib(i)
        even_terms.append(current)
        i += 3
    return sum(even_terms)


def fib(n, cache={1: 1, 2: 2}):
    if n not in cache:
        cache[n] = fib(n-1, cache) + fib(n-2, cache)
    return cache[n]

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

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

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

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))

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

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
}