comptime primes

rust/nth-prime/src/lib.rs

@@ -1,3 +1,48 @@
-pub fn nth(n: u32) -> u32 {
+const PRIME_LEN: usize = 10_010;
-    todo!("What is the 0-indexed {n}th prime number?")
+const LIMIT: usize = 106_000;
+
+struct PrimeCache {
+    primes: [u32; PRIME_LEN],
+}
+
+impl PrimeCache {
+    const fn new() -> Self {
+        let mut primes = [0; PRIME_LEN];
+        let mut sieve = [true; LIMIT];
+
+        sieve[0] = false;
+        sieve[1] = false;
+
+        let mut i = 2;
+        while i * i < LIMIT {
+            if sieve[i] {
+                let mut j = i * i;
+                while j < LIMIT {
+                    sieve[j] = false;
+                    j += i;
+                }
+            }
+            i += 1;
+        }
+        let mut num = 0;
+        let mut j = 0;
+        while num < LIMIT && j < PRIME_LEN {
+            if sieve[num] {
+                primes[j] = num as u32;
+                j += 1;
+            }
+            num += 1;
+        }
+        Self { primes }
+    }
+}
+
+static CACHE: PrimeCache = PrimeCache::new();
+
+pub fn nth(n: u32) -> u32 {
+    let nth = n as usize;
+    if nth >= PRIME_LEN {
+        panic!("N more than comptime was calculated");
+    }
+    CACHE.primes[nth]
 }