merge: Algorithm to calculate the Arithmetic Geometric Mean (#897)

* Create ArithmeticGeometricMean.js

* Finally added the test script for AGM

* Better doc, and corrected some formatting

* Fixed syntax typos

* Added more tests and made FP comparison more "loose"

* Patched bugs

* Fixed-0 bug

* Again, tried to fix minus zero

* Finally fixed all bugs (probably)

* Fixed style (probably)

* Fixed style

* Fixed all style

Maths/ArithmeticGeometricMean.js

@@ -0,0 +1,28 @@
+/**
+ * @function agm
+ * @description This finds the Arithmetic-Geometric Mean between any 2 numbers.
+ * @param {Number} a - 1st number, also used to store Arithmetic Mean.
+ * @param {Number} g - 2nd number, also used to store Geometric Mean.
+ * @return {Number} - AGM of both numbers.
+ * @see [AGM](https://en.wikipedia.org/wiki/Arithmetic%E2%80%93geometric_mean)
+ */
+
+export const agm = (a, g) => {
+  if (a === Infinity && g === 0) return NaN
+  if (Object.is(a, -0) && !Object.is(g, -0)) return 0
+  if (a === g) return a // avoid rounding errors, and increase efficiency
+  let x // temp var
+  do {
+    [a, g, x] = [(a + g) / 2, Math.sqrt(a * g), a]
+  } while (a !== x && !isNaN(a))
+  /*
+  `x !== a` ensures the return value has full precision,
+  and prevents infinite loops caused by rounding differences between `div` and `sqrt` (no need for "epsilon").
+  If we were to compare `a` with `g`, some input combinations (not all) can cause an infinite loop,
+  because the rounding mode never changes at runtime.
+  Precision is not the same as accuracy, but they're related.
+  This function isn't always 100% accurate (round-errors), but at least is more than 95% accurate.
+  `!isNaN(x)` prevents infinite loops caused by invalid inputs like: negatives, NaNs and Infinities.
+  */
+  return a
+}