adventofcode/2019/04b.py

# Python 3.7

def main():
    rangeStart = 109165
    rangeEnd = 576723
    count = 0
    for pw in range(rangeStart, rangeEnd):
        count += validatePassword(pw)
    print(count)

def validatePassword(pw):
    pwStr = str(pw)
    # does it have a doubled digit
    #  but not a tripled etc. digit!
    # does it have a descending digit
    c, d, m = 0, 0, ""
    for i in range(1, len(pwStr)):
        if pwStr[i-1] == pwStr[i]:
            if c == 0 and m == "": # it has a doubled digit we haven't seen before
                c = 1
                m = pwStr[i]
            elif c == 1 and m == pwStr[i]: # it has a doubled digit we HAVE seen before
                c = 0                      # so it doesn't count :(
        else: # this digit isn't the same as the previous, so reset the memo
            m = ""
        if int(pwStr[i]) < int(pwStr[i-1]):
            d = 1
    if c == 0 or d == 1:
        return 0
    return 1

if __name__ == "__main__":
    main()