added nilakantha.js for the calculation of PI

nilakantha.js

@@ -0,0 +1,31 @@
+const add = require("./add.js");
+const long_addition = require("./long_addition.js");
+const divide = require("./divide.js");
+const multiply = require("./multiply.js");
+
+// calculate PI using Nilakantha Series
+function nilakantha(steps = 100, { divide_options } = {}) {
+  let sign = "+";
+  let pi = "3";
+  let a = "2";
+  let b = "3";
+  let c = "4";
+  for (let i = 1; i < steps; i++) {
+    const divisor = multiply(a, b, c);
+
+    const part = sign + divide("4", divisor, divide_options);
+
+    pi = add(pi, part);
+
+    // flip sign
+    sign = sign === "-" ? "+" : "-";
+
+    a = c;
+    b = long_addition(c, "1");
+    c = long_addition(b, "1");
+  }
+  return pi;
+}
+
+module.exports = nilakantha;
+module.exports.default = nilakantha;