September 2017 Update

Task/Arithmetic-geometric-mean/Swift/arithmetic-geometric-mean.swift

@@ -0,0 +1,31 @@
+import Darwin
+
+enum AGRError : Error {
+	case undefined
+}
+
+func agm(_ a: Double, _ g: Double, _ iota: Double = 1e-8) throws -> Double {
+	var a = a
+	var g = g
+	var a1: Double = 0
+	var g1: Double = 0
+	
+	guard a * g >= 0 else {
+		throw AGRError.undefined
+	}
+	
+	while abs(a - g) > iota {
+		a1 = (a + g) / 2
+		g1 = sqrt(a * g)
+		a = a1
+		g = g1
+	}
+	
+	return a
+}
+
+do {
+	try print(agm(1, 1 / sqrt(2)))
+} catch {
+	print("agr is undefined when a * g < 0")
+}