Optimize the primality check

xenharmonic-devs · Dec 19, 2023 · 68604ca · 68604ca
1 parent de21aa7
commit 68604ca
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diff --git a/src/__tests__/primes.spec.ts b/src/__tests__/primes.spec.ts
@@ -47,6 +47,12 @@ describe('Primeness detector', () => {
       expect(isPrime(n)).toBe(false);
     }
   });
+  it('works for 62837327', () => {
+    expect(isPrime(62837327)).toBe(false);
+  });
+  it('works for 62837303', () => {
+    expect(isPrime(62837303)).toBe(true);
+  });
 });
 
 describe('Lists of primes', () => {

diff --git a/src/primes.ts b/src/primes.ts
@@ -118,6 +118,9 @@ export function isPrime(n: number) {
     throw new Error('Prime check only implemented up to 62837328');
   }
   for (const prime of PRIMES) {
+    if (prime * prime > n) {
+      return true;
+    }
     if (n % prime === 0) {
       return false;
     }