Add sqrt function.

lib/bn.js

@@ -2016,6 +2016,22 @@
     return this.clone().imuln(num);
   };
 
+  // https://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Babylonian_method
+  BN.prototype.sqrt = function sqrt () {
+    var z = new BN(0);
+    if (this.gt(new BN(3))) {
+      z = this;
+      var x = this.div(new BN(2)).add(new BN(1));
+      while (x.lt(z)) {
+        z = x;
+        x = this.div(x).add(x).div(new BN(2));
+      }
+    } else if (!this.eq(new BN(0))) {
+      z = new BN(1);
+    }
+    return z;
+  };
+
   // `this` * `this`
   BN.prototype.sqr = function sqr () {
     return this.mul(this);

test/arithmetic-test.js

@@ -317,6 +317,15 @@ describe('BN.js/Arithmetic', function () {
     });
   });
 
+  describe('.sqrt()', function () {
+    it('should raise number to the sqrt', function () {
+      var a = new BN('123456', 10);
+      var b = a.mul(a).sqrt();
+
+      assert.equal(b.toString(10), a.toString(10));
+    });
+  });
+
   describe('.div()', function () {
     it('should divide small numbers (<=26 bits)', function () {
       assert.equal(new BN('256').div(new BN(10)).toString(10),