Calculates regular saver effective interest rate, assuming you pay in 250 each first day of month over 1 year period.

Regular saver interest calculator

class Rounding {
    public BigDecimal round(int n) {
        return setScale(n, BigDecimal.ROUND_HALF_UP);
    }
}
class Taxable {
    def defaultTaxRate = 0.2
    public afterTax(taxRate=defaultTaxRate) {
        return this * (1-taxRate);
    }
}
BigDecimal.mixin Rounding
BigDecimal.mixin Taxable
def months = ["January" : 31,
            "February" : 28,
            "March" : 31 ,
            "April" : 30 ,
            "May" : 31 ,
            "June" : 30 ,
            "July" : 31 ,
            "August" : 31 ,
            "September" : 30 ,
            "October" : 31 ,
            "November" : 30 ,
            "December" : 31 ]

def grossRate = 0.06
def dailyRate = grossRate/365
def payInAmount = 250
def payInCount = 0
def interestsTotal =0 
BigDecimal accountValue = 0
println 'DATA IN ------------------'
println 'Calculating interest rate...'
println 'Interest GROSS AER rate = ' << grossRate
println 'Daily rate = ' << dailyRate  
println '--------------------------'
def dailyInterest=0
months.each{String month, int days ->
     //pay in first day of month
     ++payInCount
     accountValue = accountValue + payInAmount
     for (dayOfMonth in (1..days)) {
         dailyInterest = accountValue*dailyRate
         accountValue = dailyInterest + accountValue
         interestsTotal += dailyInterest
     }
     //println 'account value in the end of ' << month << ' is ' << accountValue.round(2)
}
println 'RESULT --------------------'
println 'Number of pay ins ' << payInCount
println 'Total interests ' << interestsTotal.round(2)
println 'Total interests after tax ' << (interestsTotal.afterTax()).round(2)
println 'Your account value after 12 months and after tax is ' << (payInCount*payInAmount+interestsTotal.afterTax()).round(2)
def grossInterest = (interestsTotal/accountValue*100)
println 'Effective GROSS interest rate is '  << grossInterest.round(2)
println 'Effective NET interest rate is '  << grossInterest.afterTax().round(2)