|
| 1 | +""" |
| 2 | +Project Euler Problem 50: https://projecteuler.net/problem=50 |
| 3 | +
|
| 4 | +Consecutive prime sum |
| 5 | +
|
| 6 | +The prime 41, can be written as the sum of six consecutive primes: |
| 7 | +41 = 2 + 3 + 5 + 7 + 11 + 13 |
| 8 | +
|
| 9 | +This is the longest sum of consecutive primes that adds to a prime below |
| 10 | +one-hundred. |
| 11 | +
|
| 12 | +The longest sum of consecutive primes below one-thousand that adds to a prime, |
| 13 | +contains 21 terms, and is equal to 953. |
| 14 | +
|
| 15 | +Which prime, below one-million, can be written as the sum of the most |
| 16 | +consecutive primes? |
| 17 | +""" |
| 18 | +from typing import List |
| 19 | + |
| 20 | + |
| 21 | +def prime_sieve(limit: int) -> List[int]: |
| 22 | + """ |
| 23 | + Sieve of Erotosthenes |
| 24 | + Function to return all the prime numbers up to a number 'limit' |
| 25 | + https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes |
| 26 | +
|
| 27 | + >>> prime_sieve(3) |
| 28 | + [2] |
| 29 | +
|
| 30 | + >>> prime_sieve(50) |
| 31 | + [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47] |
| 32 | + """ |
| 33 | + is_prime = [True] * limit |
| 34 | + is_prime[0] = False |
| 35 | + is_prime[1] = False |
| 36 | + is_prime[2] = True |
| 37 | + |
| 38 | + for i in range(3, int(limit ** 0.5 + 1), 2): |
| 39 | + index = i * 2 |
| 40 | + while index < limit: |
| 41 | + is_prime[index] = False |
| 42 | + index = index + i |
| 43 | + |
| 44 | + primes = [2] |
| 45 | + |
| 46 | + for i in range(3, limit, 2): |
| 47 | + if is_prime[i]: |
| 48 | + primes.append(i) |
| 49 | + |
| 50 | + return primes |
| 51 | + |
| 52 | + |
| 53 | +def solution(ceiling: int = 1_000_000) -> int: |
| 54 | + """ |
| 55 | + Returns the biggest prime, below the celing, that can be written as the sum |
| 56 | + of consecutive the most consecutive primes. |
| 57 | +
|
| 58 | + >>> solution(500) |
| 59 | + 499 |
| 60 | +
|
| 61 | + >>> solution(1_000) |
| 62 | + 953 |
| 63 | +
|
| 64 | + >>> solution(10_000) |
| 65 | + 9521 |
| 66 | + """ |
| 67 | + primes = prime_sieve(ceiling) |
| 68 | + length = 0 |
| 69 | + largest = 0 |
| 70 | + |
| 71 | + for i in range(len(primes)): |
| 72 | + for j in range(i + length, len(primes)): |
| 73 | + sol = sum(primes[i:j]) |
| 74 | +
7251
if sol >= ceiling: |
| 75 | + break |
| 76 | + |
| 77 | + if sol in primes: |
| 78 | + length = j - i |
| 79 | + largest = sol |
| 80 | + |
| 81 | + return largest |
| 82 | + |
| 83 | + |
| 84 | +if __name__ == "__main__": |
| 85 | + print(f"{solution() = }") |
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