diff --git a/project_euler/problem_074/__init__.py b/project_euler/problem_074/__init__.py new file mode 100644 index 000000000..e69de29bb diff --git a/project_euler/problem_074/sol1.py b/project_euler/problem_074/sol1.py new file mode 100644 index 000000000..5e6aff6f5 --- /dev/null +++ b/project_euler/problem_074/sol1.py @@ -0,0 +1,111 @@ +""" +Project Euler Problem 74: https://projecteuler.net/problem=74 + +The number 145 is well known for the property that the sum of the factorial of its +digits is equal to 145: + +1! + 4! + 5! = 1 + 24 + 120 = 145 + +Perhaps less well known is 169, in that it produces the longest chain of numbers that +link back to 169; it turns out that there are only three such loops that exist: + +169 → 363601 → 1454 → 169 +871 → 45361 → 871 +872 → 45362 → 872 + +It is not difficult to prove that EVERY starting number will eventually get stuck in +a loop. For example, + +69 → 363600 → 1454 → 169 → 363601 (→ 1454) +78 → 45360 → 871 → 45361 (→ 871) +540 → 145 (→ 145) + +Starting with 69 produces a chain of five non-repeating terms, but the longest +non-repeating chain with a starting number below one million is sixty terms. + +How many chains, with a starting number below one million, contain exactly sixty +non-repeating terms? +""" + + +DIGIT_FACTORIALS = { + "0": 1, + "1": 1, + "2": 2, + "3": 6, + "4": 24, + "5": 120, + "6": 720, + "7": 5040, + "8": 40320, + "9": 362880, +} + +CACHE_SUM_DIGIT_FACTORIALS = {145: 145} + +CHAIN_LENGTH_CACHE = { + 145: 0, + 169: 3, + 36301: 3, + 1454: 3, + 871: 2, + 45361: 2, + 872: 2, + 45361: 2, +} + + +def sum_digit_factorials(n: int) -> int: + """ + Return the sum of the factorial of the digits of n. + >>> sum_digit_factorials(145) + 145 + >>> sum_digit_factorials(45361) + 871 + >>> sum_digit_factorials(540) + 145 + """ + if n in CACHE_SUM_DIGIT_FACTORIALS: + return CACHE_SUM_DIGIT_FACTORIALS[n] + ret = sum([DIGIT_FACTORIALS[let] for let in str(n)]) + CACHE_SUM_DIGIT_FACTORIALS[n] = ret + return ret + + +def chain_length(n: int, previous: set = None) -> int: + """ + Calculate the length of the chain of non-repeating terms starting with n. + Previous is a set containing the previous member of the chain. + >>> chain_length(10101) + 11 + >>> chain_length(555) + 20 + >>> chain_length(178924) + 39 + """ + previous = previous or set() + if n in CHAIN_LENGTH_CACHE: + return CHAIN_LENGTH_CACHE[n] + next_number = sum_digit_factorials(n) + if next_number in previous: + CHAIN_LENGTH_CACHE[n] = 0 + return 0 + else: + previous.add(n) + ret = 1 + chain_length(next_number, previous) + CHAIN_LENGTH_CACHE[n] = ret + return ret + + +def solution(num_terms: int = 60, max_start: int = 1000000) -> int: + """ + Return the number of chains with a starting number below one million which + contain exactly n non-repeating terms. + >>> solution(10,1000) + 28 + """ + return sum(1 for i in range(1, max_start) if chain_length(i) == num_terms) + + +if __name__ == "__main__": + print(f"{solution() = }")