Test with fewer witnesses for smaller numbers

Author: JSorngard

src/check.rs

@@ -44,14 +44,31 @@ pub const fn is_prime(n: u64) -> bool {
     // we can use the fact that there are known perfect bases
     // in order to make the test both fast and deterministic.
     // This list of witnesses was taken from
-    // <https://en.wikipedia.org/wiki/Miller%E2%80%93Rabin_primality_test#Testing_against_small_sets_of_bases>
-    // and is sufficient for all numbers smaller than 2^64.
-    const NUM_WITNESSES: usize = 12;
-    const WITNESSES: [u64; NUM_WITNESSES] = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37];
+    // <https://en.wikipedia.org/wiki/Miller%E2%80%93Rabin_primality_test#Testing_against_small_sets_of_bases>.
+    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]),
+    ];
 
     let mut i = 0;
-    while i < NUM_WITNESSES && WITNESSES[i] < n {
-        if !miller_test(d, n, WITNESSES[i]) {
+    while WITNESSES[i].0 < n {
+        i += 1;
+    }
+    let witnesses = WITNESSES[i].1;
+    let num_witnesses = witnesses.len();
+
+    let mut i = 0;
+    while i < num_witnesses && witnesses[i] < n {
+        if !miller_test(d, n, witnesses[i]) {
             return false;
         }
         i += 1;