prime-factors

rust/nth-prime/src/lib.rs

@@ -1,48 +1,91 @@
-const PRIME_LEN: usize = 10_010;
-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");
+    let mut num = 1;
+    for _ in 0..=n {
+        loop {
+            num += 1;
+            if is_prime(num as u64) {
+                break;
+            }
+        }
     }
-    CACHE.primes[nth]
+    num
+}
+
+pub fn is_prime(n: u64) -> bool {
+    if n < 2 {
+        return false;
+    }
+    if n == 2 || n == 3 {
+        return true;
+    }
+    if n % 2 == 0 {
+        return false;
+    }
+
+    let s = (n - 1).trailing_zeros();
+    let d = (n - 1) >> s;
+
+    let witnesses = get_witnesses(n);
+
+    for &a in witnesses {
+        if !miller_rabin_test(a, d, n, s) {
+            return false;
+        }
+    }
+    true
+}
+
+fn get_witnesses(n: u64) -> &'static [u64] {
+    const WITNESSES: &[(u64, &[u64])] = &[
+        (2_046, &[2]),
+        (1_373_652, &[2, 3]),
+        (9_080_190, &[31, 73]),
+        (25_326_000, &[2, 3, 5]),
+        (4_759_123_140, &[2, 7, 61]),
+        (1_112_004_669_632, &[2, 13, 23, 1662803]),
+        (2_152_302_898_746, &[2, 3, 5, 7, 11]),
+        (3_474_749_660_382, &[2, 3, 5, 7, 11, 13]),
+        (341_550_071_728_320, &[2, 3, 5, 7, 11, 13, 17]),
+        (3_825_123_056_546_413_050, &[2, 3, 5, 7, 11, 13, 17, 19, 23]),
+        (u64::MAX, &[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37]),
+    ];
+
+    WITNESSES
+        .iter()
+        .find(|(limit, _)| *limit >= n)
+        .map(|(_, w)| *w)
+        .unwrap()
+}
+
+fn miller_rabin_test(a: u64, d: u64, n: u64, s: u32) -> bool {
+    let mut x = mod_pow(a, d, n);
+    if x == 1 || x == n - 1 {
+        return true;
+    }
+
+    for _ in 1..s {
+        x = mod_mul(x, x, n);
+        if x == n - 1 {
+            return true;
+        }
+    }
+    false
+}
+
+#[inline]
+fn mod_mul(a: u64, b: u64, m: u64) -> u64 {
+    ((a as u128 * b as u128) % m as u128) as u64
+}
+
+fn mod_pow(mut base: u64, mut exp: u64, m: u64) -> u64 {
+    let mut res = 1;
+    base %= m;
+    while exp > 0 {
+        if exp % 2 == 1 {
+            res = mod_mul(res, base, m);
+        }
+        base = mod_mul(base, base, m);
+        exp /= 2;
+    }
+    res
 }

rust/nth-prime/src/main.rs

@@ -0,0 +1,82 @@
+fn main() {
+    let n: u64 = std::env::args().nth(1)
+        .and_then(|s| s.parse().ok())
+        .unwrap_or(35);
+
+    println!("{} -> {}", n, if is_prime(n) { "prime" } else { "composite" });
+}
+
+pub fn is_prime(n: u64) -> bool {
+    if n < 2 { return false; }
+    if n == 2 || n == 3 { return true; }
+    if n % 2 == 0 { return false; }
+
+    let s = (n - 1).trailing_zeros();
+    let d = (n - 1) >> s;
+
+    let witnesses = get_witnesses(n);
+
+    for &a in witnesses {
+        if !miller_rabin_test(a, d, n, s) {
+            return false;
+        }
+    }
+    true
+}
+
+// O(1) lookup based on input size
+fn get_witnesses(n: u64) -> &'static [u64] {
+    const WITNESSES: &[(u64, &[u64])] = &[
+        (2_046, &[2]),
+        (1_373_652, &[2, 3]),
+        (9_080_190, &[31, 73]),
+        (25_326_000, &[2, 3, 5]),
+        (4_759_123_140, &[2, 7, 61]),
+        (1_112_004_669_632, &[2, 13, 23, 1662803]),
+        (2_152_302_898_746, &[2, 3, 5, 7, 11]),
+        (3_474_749_660_382, &[2, 3, 5, 7, 11, 13]),
+        (341_550_071_728_320, &[2, 3, 5, 7, 11, 13, 17]),
+        (3_825_123_056_546_413_050, &[2, 3, 5, 7, 11, 13, 17, 19, 23]),
+        (u64::MAX, &[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37]),
+    ];
+
+    WITNESSES.iter()
+        .find(|(limit, _)| *limit >= n)
+        .map(|(_, w)| *w)
+        .unwrap()
+}
+
+fn miller_rabin_test(a: u64, d: u64, n: u64, s: u32) -> bool {
+    let mut x = mod_pow(a, d, n);
+    if x == 1 || x == n - 1 {
+        return true;
+    }
+
+    for _ in 1..s {
+        x = mod_mul(x, x, n);
+        if x == n - 1 {
+            return true;
+        }
+    }
+    false
+}
+
+// === Math Core ===
+
+#[inline]
+fn mod_mul(a: u64, b: u64, m: u64) -> u64 {
+    ((a as u128 * b as u128) % m as u128) as u64
+}
+
+fn mod_pow(mut base: u64, mut exp: u64, m: u64) -> u64 {
+    let mut res = 1;
+    base %= m;
+    while exp > 0 {
+        if exp % 2 == 1 {
+            res = mod_mul(res, base, m);
+        }
+        base = mod_mul(base, base, m);
+        exp /= 2;
+    }
+    res
+}