ParkingMeterKata.fs

module ParkingMeterKata
open System

// coins accepted by the system
type Coin =
    | TenCents = 10
    | TwentyCents = 20
    | FiftyCents = 50
    | OneEuro = 100
    | TwoEuro = 200

// convert a coint into it's amount in Euro's
let amountOfCoin (coin: Coin) =
    Convert.ToDecimal (coin |> int) / 100m

// calculate the total amount of a list of coins
let amountOfCoins (coins: seq<Coin>) =
    coins |> Seq.map amountOfCoin |> Seq.sum

// defines how many minutes (second column) can be bought for an amount in euros (first column
// it is important that it's in decending order
let priceTable =
    [ 8.M, 600M
      5.M, 300M
      3.M, 120M
      2.M, 60M
      1.M, 20M ]

// calculate how many minutes an amount in euros will buy you
let getMinutesBought (amount: Decimal) =
    // find which price range this amount falls into
    let priceRange = priceTable |> Seq.tryFind (fun (price, time) -> amount >= price)
    match priceRange with
    | Some (price, time) ->
        // amount of time we can buy is the ratio of the amount we've given and the cost of range
        // times the number of minutes in this price range
        (amount /  price) * time 
    | None -> failwith "Not enough"

// calculates wether this is a day where we need to pay
let isExcludeDay (date: DateTime) =
    date.DayOfWeek = DayOfWeek.Sunday
    || date.Month = 8
    // TODO public holidates

// number of minutes in a day, a "day" is 10 hours long as we pay between 9h and 19h
let minutesInADay = 600M

// calculate what day we've paid up to if it's n days later
let getNthPayingDay n (startDate: DateTime) =
    // creates an infinite sequence of the paying days starting at given date
    let payingDays (startDate: DateTime) = 
        // create an infinite sequence of all dates from given date
        let allDates = Seq.initInfinite (fun x -> startDate.AddDays( x |> float)) 
        // filter out any dates that aren't paying dates
        allDates |> Seq.filter (fun x -> not (isExcludeDay x))
    // find the nth element in this list
    Seq.nth n (payingDays startDate)

// test to see if given date is outside the paying range
let afterPayingTime (date: DateTime) =
    date.Hour > 19

// test to see if we're before the time where we need to pay
let beforePayingTime (date: DateTime) =
    date.Hour < 9

// Add the minutes we bought to current time, taking into account opening 
// hours and excluded days
let addTime (startDate: DateTime) (minutes: Decimal) =
    // first caculate how many days we need to add
    let days = 
        (Math.Floor (minutes / minutesInADay)|> int) +
        (if afterPayingTime startDate then 1 else 0)
    // then calculate how many minutes during this day we need to add
    let remainingMinutes = minutes % minutesInADay |> float
    // add on the number of days
    let date = if days > 0 then getNthPayingDay days startDate else startDate
    // if we're before the time were we need to pay bring the date up to the paying time
    let date = if beforePayingTime date then date.Date.AddHours(9.) else date
    // add all the minutes we've bought for today (this can't be > minutesInADay)
    let dateWithAllMinsAdded = date.AddMinutes(remainingMinutes)
    // see if we've passed the hours we're we need to pay
    if afterPayingTime dateWithAllMinsAdded then
        // find the next paying day and add remaing minutes
        let nextDay = getNthPayingDay 1 date
        let nextDayAt9 = nextDay.Date.AddHours(9.)
        let remainingMins = 
            dateWithAllMinsAdded - date.Date.AddHours(19.)
        nextDayAt9.Add( remainingMins)
    else
        // otherwise just return this date
        dateWithAllMinsAdded

// compose getMinutesBought and addTime to find the end time
let calcuateEndDateFromAmount (time: DateTime) (amount: Decimal) =
    getMinutesBought amount
    |> addTime time 

// compose amountOfCoins and calcuateEndDateFromAmount to find what the end date
// for the coins we've used to pay with
let calcumateEndDateFromCoins (time: DateTime) (coins: seq<Coin>) =
     coins |> amountOfCoins |> calcuateEndDateFromAmount time