FsIsLeapYear.fsx

(*
Rules - https://techcommunity.microsoft.com/t5/azure-developer-community-blog/it-s-2020-is-your-code-ready-for-leap-day/ba-p/1157279
… Is the year evenly divisible by 4? Then it is a leap year. (Examples: 2012, 2016, 2020, 2024)
… Unless it is also evenly divisible by 100. Those are not leap years. (Examples: 1800, 1900, 2100)
… Except that year that are evenly divisible by 400 are leap years. (Examples: 1600, 2000, 2400)
*)

let fmod (x:int) (y:int) : bool = x % y = 0
let isEvenlyDivisibleByFour (year:int) : bool = fmod year 4 
let isEvenlyDivisibleByOneHundred (year:int) : bool = fmod year 100
let isEvenlyDivisibleByFourHundred (year:int) : bool = fmod year 400

let isLeapYear year = 
    let divisibleByFour = isEvenlyDivisibleByFour year
    let divisibleByOneHundred = isEvenlyDivisibleByOneHundred year
    let divisibleByFourHundred = isEvenlyDivisibleByFourHundred year
    divisibleByFour && (not divisibleByOneHundred || divisibleByFourHundred)

isLeapYear 2016;;
isLeapYear 2020;;
isLeapYear 2021;; 
isLeapYear 2024;;
isLeapYear 1800;;
isLeapYear 1900;;
isLeapYear 2100;;
isLeapYear 1600;;
isLeapYear 2000;;
isLeapYear 2400;;