refactor common logic

modules/math.js

@@ -53,14 +53,19 @@ function modpow(base, exponent, modulus) {
 	return result;
 }
 
-function witness(a, n) {
-	let d = n - 1n;
+function factorTwos(n) {
 	let s = 0;
-	while (d % 2n === 0n) {
-		d /= 2n;
+	while (n % 2n === 0n) {
+		n /= 2n;
 		s++;
 	}
 
+	return [n, s];
+}
+
+function witness(a, n) {
+	const [d, s] = factorTwos(n - 1n);
+
 	let x = modpow(a, d, n);
 	let y = null;
 	for (let i = 0; i < s; i++) {
@@ -106,13 +111,7 @@ function quadraticNonResidue(p) {
 }
 
 function tonelliShanks(n, p) {
-	let q = p - 1n;
-	let s = 0;
-	while (q % 2n === 0n) {
-		q /= 2n;
-		s++;
-	}
-
+	const [q, s] = factorTwos(p - 1n);
 	const z = quadraticNonResidue(p);
 
 	let m = s;