javascript:isPrime

isprime

// Copyright (c) 2011 Alexei Kourbatov, www.JavaScripter.net 

// function leastFactor(n) returns:
// * the smallest prime that divides n
// * NaN if n is NaN or Infinity
// *  0  if n is 0
// *  1  if n=1, n=-1, or n is not an integer

leastFactor = function(n) {
 if (isNaN(n) || !isFinite(n)) return NaN;  
 if (n==0) return 0;  
 if (n%1 || n*n<2) return 1;
 if (n%2==0) return 2;  
 if (n%3==0) return 3;  
 if (n%5==0) return 5;  
 var m = Math.sqrt(n);
 for (var i=7;i<=m;i+=30) {
  if (n%i==0)      return i;
  if (n%(i+4)==0)  return i+4;
  if (n%(i+6)==0)  return i+6;
  if (n%(i+10)==0) return i+10;
  if (n%(i+12)==0) return i+12;
  if (n%(i+16)==0) return i+16;
  if (n%(i+22)==0) return i+22;
  if (n%(i+24)==0) return i+24;
 }
 return n;
}

// Optimized version of leastFactor for Opera, Chrome, Firefox.
// In these browsers, "i divides n" is much faster as
// (q=n/i)==Math.floor(q)  than  n%i==0
if (
    navigator.userAgent.indexOf('Opera')  !=-1
 || navigator.userAgent.indexOf('Chrome') !=-1
 || navigator.userAgent.indexOf('Firefox')!=-1 )
{
 leastFactor = function(n) {
  if (isNaN(n) || !isFinite(n)) return NaN;   
  if (n==0) return 0;  
  if (n%1 || n*n<2) return 1;
  if (n%2==0) return 2;  
  if (n%3==0) return 3;  
  if (n%5==0) return 5;  
  var q, m = Math.sqrt(n);
  for (var i=7;i<=m;i+=30) {
   if ((q=n/i)==Math.floor(q))      return i;
   if ((q=n/(i+4))==Math.floor(q))  return i+4;
   if ((q=n/(i+6))==Math.floor(q))  return i+6;
   if ((q=n/(i+10))==Math.floor(q)) return i+10;
   if ((q=n/(i+12))==Math.floor(q)) return i+12;
   if ((q=n/(i+16))==Math.floor(q)) return i+16;
   if ((q=n/(i+22))==Math.floor(q)) return i+22;
   if ((q=n/(i+24))==Math.floor(q)) return i+24;
  }
  return n;
 }
}

// Optimized version for Internet Explorer avoids IE's 
// "slow script" warning at 5000000 script statements
// by grouping 48 divisibility checks into a single statement

if (navigator.userAgent.indexOf('MSIE')!=-1)
{
 leastFactor = function(n){
  if (isNaN(n)) return NaN;  
  if (n==0) return 0;  
  if (!isFinite(n) || n%1 || n*n<2) return 1;
  if (n%2==0) return 2;  
  if (n%3==0) return 3;  
  if (n%5==0) return 5;  
  if (n%7==0) return 7;  
  var m = Math.sqrt(n);
  for (var i=11;i<=m;i+=210) {
   if (n%i && n%(i+2) && n%(i+6) && n%(i+8)&& n%(i+12)&& n%(i+18)&& n%(i+20)&& n%(i+26)
   && n%(i+30) && n%(i+32) && n%(i+36) && n%(i+42) && n%(i+48) && n%(i+50) && n%(i+56)
   && n%(i+60) && n%(i+62) && n%(i+68) && n%(i+72) && n%(i+78) && n%(i+86)
   && n%(i+90) && n%(i+92) && n%(i+96) && n%(i+98) && n%(i+102)&& n%(i+110)&& n%(i+116)
   && n%(i+120)&& n%(i+126)&& n%(i+128)&& n%(i+132)&& n%(i+138)&& n%(i+140)&& n%(i+146)
   && n%(i+152)&& n%(i+156)&& n%(i+158)&& n%(i+162)&& n%(i+168)&& n%(i+170)&& n%(i+176)
   && n%(i+180)&& n%(i+182)&& n%(i+186)&& n%(i+188)&& n%(i+198)&& n%(i+200)
   ) continue;
   for (var j=0;j<210;j+=2) {if (n%(i+j)==0) return i+j; }
  }
  return n;
 }
}

// function isPrime(n) returns:
// - false if n is NaN or not a finite integer
// - true  if n is prime
// - false otherwise

isPrime = function(n) {
 if (isNaN(n) || !isFinite(n) || n%1 || n<2) return false; 
 if (n==leastFactor(n)) return true;
 return false;
}

// function factor(n) returns:
// * a string containing the prime factorization of n
// * n.toString() if the factrization cannot be found

function factor(n){
 if (isNaN(n) || !isFinite(n) || n%1 || n==0) return n.toString();
 if (n<0) return '-'+factor(-n);
 var minFactor = leastFactor(n);
 if (n==minFactor) return n.toString();
 return minFactor+'*'+factor(n/minFactor);
}

// function nextPrime(n) returns:
// * the smallest prime greater than n
// * NaN if this prime is not a representable integer

function nextPrime(n){
 if (isNaN(n) || !isFinite(n)) return NaN; 
 if (n<2) return 2;
 n = Math.floor(n);
 for (var i=n+n%2+1; i<9007199254740992; i+=2) {
  if (isPrime(i)) return i;
 }
 return NaN;
}

// function nextPrimeTwin(n) returns:
// * 2 if n<2 or
// * 3 if n<3 or
// * 5 if n<5 or
// * the smallest twin prime 6i-1 greater than n, for an integer i
// * NaN if such a prime is not a representable integer

function nextPrimeTwin(n) {
 if (isNaN(n) || !isFinite(n)) return NaN; 
 if (n<2) return 2;
 if (n<3) return 3;
 if (n<5) return 5;
 for (var i=6*Math.ceil(Math.floor(n+2)/6); i<9007199254740880; i+=6) {
  if (pscreen(i-1) && pscreen(i+1) && isPrime(i-1) && isPrime(i+1))
    return i-1;
 }
 return NaN;
}

// function nextPrimeQuad(n) returns:
// * the smallest prime in the next prime quadruplet greater than n
// * NaN if such a prime is not a representable integer

function nextPrimeQuad(n) {
 if (isNaN(n) || !isFinite(n)) return NaN; 
 if (n<11) return 11;
 for (var i=30*Math.ceil(Math.floor(n-10)/30); i<9007199254740880; i+=30) {
  if (pscreen(i+11) && pscreen(i+13) && pscreen(i+17) && pscreen(i+19)
   && isPrime(i+11) && isPrime(i+13) && isPrime(i+17) && isPrime(i+19))
    return i+11;
 }
 return NaN;
}

function pscreen(n) {
 if (n<=19 || n%3 && n%5 && n%7 && n%11 && n%13 && n%17 && n%19) return true;
 return false;
}