A-M baby

Task/Factors-of-a-Mersenne-number/JavaScript/factors-of-a-mersenne-number.js

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+function mersenne_factor(p){
+  var limit, k, q
+  limit = Math.sqrt(Math.pow(2,p) - 1)
+  k = 1
+  while ((2*k*p - 1) < limit){
+    q = 2*k*p + 1
+    if (isPrime(q) && (q % 8 == 1 || q % 8 == 7) && trial_factor(2,p,q)){
+      return q // q is a factor of 2**p-1
+    }
+    k++
+  }
+  return null
+}
+
+function isPrime(value){
+  for (var i=2; i < value; i++){
+    if (value % i == 0){
+      return false
+    }
+    if (value % i != 0){
+      return true;
+	 }
+  }
+}
+
+function trial_factor(base, exp, mod){
+  var square, bits
+  square = 1
+  bits = exp.toString(2).split('')
+  for (var i=0,ln=bits.length; i<ln; i++){
+    square = Math.pow(square, 2) * (bits[i] == 1 ? base : 1) % mod
+  }
+  return (square == 1)
+}
+
+function check_mersenne(p){
+  var f, result
+  console.log("M"+p+" = 2^"+p+"-1 is ")
+  f = mersenne_factor(p)
+  console.log(f == null ? "prime" : "composite with factor "+f)
+}