Optimize primality check a bit more

Add a benchmark to make sure an array inclusion check is beneficial.

package.json

@@ -45,8 +45,10 @@
     "posttest": "npm run lint",
     "test": "vitest",
     "doc": "typedoc src/index.ts . --name xen-dev-utils",
-    "prebenchmark": "tsc -p tsconfig-benchmark.json",
-    "benchmark": "node benchmarks/__benchmarks__/monzo.mark.js"
+    "premonzo-benchmark": "tsc -p tsconfig-benchmark.json",
+    "monzo-benchmark": "node benchmarks/__benchmarks__/monzo.mark.js",
+    "preprimes-benchmark": "tsc -p tsconfig-benchmark.json",
+    "primes-benchmark":  "node benchmarks/__benchmarks__/primes.mark.js"
   },
   "engines": {
     "node": ">=10.6.0"

src/__benchmarks__/primes.mark.ts

@@ -0,0 +1,41 @@
+// @ts-ignore
+import Benchmark = require('benchmark');
+import {PRIMES, isPrime} from '../primes';
+
+function withoutInclusionCheck(n: number) {
+  if (!Number.isInteger(n) || n < 2) {
+    return false;
+  }
+  if (n > 62837328) {
+    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;
+    }
+  }
+  return true;
+}
+
+function randSmallInt() {
+  return Math.floor(Math.random() * 10000);
+}
+
+const primalityCheckSuite = new Benchmark.Suite();
+primalityCheckSuite
+  .add('primality check (with array inclusion)', () => isPrime(randSmallInt()))
+  .add('primality check (without array inclusion)', () =>
+    withoutInclusionCheck(randSmallInt())
+  )
+  .on('cycle', (event: {target: any}) => {
+    console.log(String(event.target));
+  })
+  .on('complete', () => {
+    console.log(
+      'Fastest is ' + primalityCheckSuite.filter('fastest').map('name')
+    );
+  })
+  .run({async: true});

src/primes.ts

@@ -108,12 +108,12 @@ export const BIG_INT_PRIMES = PRIMES.map(BigInt);
  * @returns True if the number is prime, false otherwise.
  */
 export function isPrime(n: number) {
-  if (!Number.isInteger(n) || n < 2) {
-    return false;
-  }
   if (n < 7927) {
     return PRIMES.includes(n);
   }
+  if (!Number.isInteger(n)) {
+    return false;
+  }
   if (n > 62837328) {
     throw new Error('Prime check only implemented up to 62837328');
   }

tsconfig-benchmark.json

@@ -2,7 +2,11 @@
   "extends": "./node_modules/gts/tsconfig-google.json",
   "compilerOptions": {
     "rootDir": "./src/",
-    "outDir": "benchmarks"
+    "outDir": "benchmarks",
+    "lib": [
+      "es2020"
+    ],
+    "target": "es2020"
   },
   "include": [
     "src/__benchmarks__/*"