Euler #315

def primes(n):
    sieve = [True] * n
    for i in range(3, int(n**0.5)+1, 2):
        if sieve[i]:
            sieve[i*i::2*i] = [False]*((n-i*i-1)//(2*i)+1)
    return [2] + [i for i in range(3, n, 2) if sieve[i]]


def digsum(n):
    total = 0
    for digit in str(n):
        total += int(digit)
    return total


def energysave(before, after):
    before, safe = str(before)[-len(str(after)):], 0
    for digit in range(len(before)):
        num1, num2 = lights[int(before[digit])], lights[(int(str(after)[digit]))]
        for digit2 in range(7):
            if num1[digit2] == num2[digit2] == "1":
                safe += 1
    return safe*2


totalsafe = 0
lights = ["1110111", "0000011", "0111110", "0011111", "1001011", "1011101", "1111101", "1010011", "1111111", "1011111"]
for x in primes(2*10**7+1)[len(primes(10**7+1)):]:
    while True:
        totalsafe += energysave(x, digsum(x))
        if len(str(digsum(x))) == 1:
            break
        x = digsum(x)
print(totalsafe)