project-euler/094_almost_equilateral_triangles.py

#!/usr/bin/env python3

def pell3():
    x, y = 2, 1
    while True:
        yield x, y
        x, y = 2*x + 3*y, 2*y + x

max_perimiter = 10**9
s = 0

for x, y in pell3():
    p = 12*(2*y**2 + x*y) + 2
    if p <= max_perimiter:
        s += p
    p = 6*(x**2 - y**2) - 2
    if p > max_perimiter:
        break
    s += p

print(s)