| //! First 1000 primes via a sieve; `prime_at` indexes them (bigpickle primes.mjs:5-29). | |
| //! | |
| //! The upstream sieve sizes its upper bound with a float Rosser estimate (primes.mjs:10); this port | |
| //! uses a fixed integer bound above the 1000th prime (7919), so the produced first-1000 primes are | |
| //! identical while no float appears. The referee checks each `prime_at(i)` against the upstream sieve. | |
| use std::sync::OnceLock; | |
| const FIRST_N: usize = 1000; | |
| fn sieve_first_n_primes(n: usize) -> Vec<u32> { | |
| let limit: usize = 10_000; // > p_1000 = 7919 | |
| let mut composite = vec![false; limit + 1]; | |
| let mut out = Vec::with_capacity(n); | |
| let mut i = 2usize; | |
| while i <= limit { | |
| if !composite[i] { | |
| out.push(i as u32); | |
| if out.len() == n { | |
| break; | |
| } | |
| let mut j = i * i; | |
| while j <= limit { | |
| composite[j] = true; | |
| j += i; | |
| } | |
| } | |
| i += 1; | |
| } | |
| out | |
| } | |
| /// The first 1000 primes, computed once. | |
| pub fn primes() -> &'static [u32] { | |
| static PRIMES: OnceLock<Vec<u32>> = OnceLock::new(); | |
| PRIMES | |
| .get_or_init(|| sieve_first_n_primes(FIRST_N)) | |
| .as_slice() | |
| } | |
| /// The `i`-th prime (0-indexed), for `i` in `[0, 1000)`. | |
| pub fn prime_at(i: usize) -> u32 { | |
| let table = primes(); | |
| assert!(i < table.len(), "primeAt: index out of range"); | |
| table[i] | |
| } | |