A moving average of the specified collection by Swift 3. Forked from https://gist.github.com/hirohitokato/5bfc5836480d68074135820d73c04794

Collection+movingAverage.swift

//
//  Collection+movingAverage.swift
//

// inspired by https://gist.github.com/hirohitokato/5bfc5836480d68074135820d73c04794

import Foundation

public protocol ArithmeticType: Comparable, IntegerLiteralConvertible {
    func +(lhs: Self, rhs: Self) -> Self
    func -(lhs: Self, rhs: Self) -> Self
    func *(lhs: Self, rhs: Self) -> Self
    func /(lhs: Self, rhs: Self) -> Self
}

extension Int: ArithmeticType {}
extension CGFloat: ArithmeticType {}
extension Double: ArithmeticType {}
extension Float: ArithmeticType {}

// Calculate moving average of the specified array.
// @see: https://ja.wikipedia.org/wiki/%E7%A7%BB%E5%8B%95%E5%B9%B3%E5%9D%87
#if swift(>=3)
    // tested on swift-DEVELOPMENT-SNAPSHOT-2016-05-09-a
    public extension Collection where Iterator.Element: ArithmeticType {
        @_specialize(Array<Int>)
        @_specialize(Array<CGFloat>)
        @_specialize(Array<Double>)
        @_specialize(Array<Float>)
        public func movingAverage() -> Iterator.Element {
            var i: Iterator.Element = 0
            let numer = reduce(i) {
                i = i + 1
                return $0 + $1 * i
            }
            let denom = i * (i + 1) / 2
            return numer / denom
        }
    }
#else
    public extension CollectionType where Generator.Element: ArithmeticType {
        public func movingAverage() -> Generator.Element {
            var i: Generator.Element = 0
            let numer = reduce(i) {
                i = i + 1
                return $0 + $1 * i
            }
            let denom = i * (i + 1) / 2
            return numer / denom
        }
    }
#endif

Could you give some instruction on this