euler/python/e045.py

"""After 40755, what is the next triangle number that is also pentagonal and hexagonal?

Triangle, pentagonal, and hexagonal numbers are generated by the following formulae:

Triangle    Tn=n(n+1)/2    1, 3, 6, 10, 15, ...
Pentagonal  Pn=n(3n-1)/2    1, 5, 12, 22, 35, ...
Hexagonal   Hn=n(2n-1)      1, 6, 15, 28, 45, ...

It can be verified that T285 = P165 = H143 = 40755.

Find the next triangle number that is also pentagonal and hexagonal.
"""

def triangle_generator(start = 0):
    n = start
    while True:
        n = n + 1
        yield n * (n + 1) / 2
def pentagonal_generator(start = 0):
    n = start
    while True:
        n = n + 1
        yield n * (3 * n - 1) / 2
def hexagonal_generator(start = 0):
    n = start
    while True:
        n = n + 1
        yield n * (2 * n - 1)

MIN = 40755

def main():
    pent_count = 0
    tri_count = 0
    for hex in hexagonal_generator():
        if hex <= MIN:
            continue
        # Catch up the other generators
        for pent in pentagonal_generator(pent_count):
            pent_count = pent_count + 1
            if pent >= hex:
                break
        if pent > hex:
            continue
        for tri in triangle_generator(tri_count):
            tri_count = tri_count + 1
            if tri >= hex:
                break
        if tri > hex:
            continue
        print 'Next number is:', hex
        break
if __name__ == '__main__':
    main()