Calculate interest

InterestTests.swift

//
//  InterestTests.swift
//  iOSHookTests
//
//  Created by Yanni Wang on 25/4/20.
//  Copyright © 2020 Yanni. All rights reserved.
//

import XCTest

class InterestTests: XCTestCase {
    
    func testGetInterestPeriodRate() {
        for _ in 0 ... 100 {
            let principalAmount: Double = Double.random(in: 1 ... 1000 * 10000)
            let interestAmount: Double = Double.random(in: 0 ... principalAmount)
            let period: Int = Int.random(in: 1 ... 200)
            let interestPeriodRate = getInterestPeriodRate(principalAmount: principalAmount, interestAmount: interestAmount, period: period)
            let periodPaidByFormula = getPeriodPaidByFormula(principalAmount: principalAmount, interestPeriodRate: interestPeriodRate, period: period)
            let interestAmountByFormula = periodPaidByFormula * Double(period) - principalAmount
            XCTAssertTrue(fabs(interestAmountByFormula - interestAmount) < 0.01, "diff is \(fabs(interestAmountByFormula - interestAmount))")
        }
    }
    
    func testGetPeriodPaid() {
        for _ in 0 ... 100 {
            let interestPeriodRate: Double = Double.random(in: 0 ... 1 / 12)
            let principalAmount: Double = Double.random(in: 1 ... 1000 * 10000)
            let period: Int = Int.random(in: 1 ... 200)
            let periodPaid = getPeriodPaid(principalAmount: principalAmount, interestPeriodRate: interestPeriodRate, period: period)
            let periodPaidByFormula = getPeriodPaidByFormula(principalAmount: principalAmount, interestPeriodRate: interestPeriodRate, period: period)
            XCTAssertTrue(fabs(periodPaid - periodPaidByFormula) < 0.01, "diff is \(fabs(periodPaid - periodPaidByFormula))")
        }
    }
    
    func testBestPeriod() {
        let principalAmount: Double = 500000
        let interestPeriodRate: Double = 6.3/100/12
        let periodMax: Int = 95
        var programs = [(period: Int, interestYearRate: Double)]()
        for period in 1 ... periodMax {
            let interestAmount = principalAmount * interestPeriodRate * Double(period)
            let interestYearRate = getInterestPeriodRate(principalAmount: principalAmount, interestAmount: interestAmount, period: period) * 12
            programs.append((period, interestYearRate))
        }

        for (period, interestYearRate) in programs {
            print("期数: \(period), 年利率: \(String(format: "%.2f", interestYearRate * 100))%")
        }
    }
    
    // MARK: utilities
    
    func getPeriodPaid(principalAmount: Double, interestPeriodRate: Double, period: Int) -> Double {
        let tolerance: Double = 0.01
        
        var periodPaidMin = principalAmount / Double(period)
        var periodPaidMax = (principalAmount + principalAmount * interestPeriodRate * Double(period)) / Double(period)
        var periodPaid = periodPaidMin
        
        while true {
            var paidPrincipal: Double = 0
            var paidInterest: Double = 0
            for _ in 1 ... period {
                let currentInterest = (principalAmount - paidPrincipal) * interestPeriodRate
                paidInterest += currentInterest
                paidPrincipal += periodPaid - currentInterest
            }
            if fabs(paidPrincipal - principalAmount) <= tolerance {
                break
            } else if (paidPrincipal > principalAmount){
                periodPaidMax = periodPaid
                periodPaid -= (periodPaid - periodPaidMin) / 2
            } else {
                periodPaidMin = periodPaid
                periodPaid += (periodPaidMax - periodPaid) / 2
            }
        }
        return periodPaid
    }
    
    func getInterestPeriodRate(principalAmount: Double, interestAmount: Double, period: Int) -> Double {
        let tolerance: Double = 0.00000000001
        let periodPaid = (principalAmount + interestAmount) / Double(period)
        
        var interestPeriodRateMin = 0 as Double
        var interestPeriodRateMax = periodPaid / principalAmount
        var interestPeriodRate = interestPeriodRateMin
        while true {
            var paidPrincipal: Double = 0
            var paidInterest: Double = 0
            for _ in 1 ... period {
                let currentInterest = (principalAmount - paidPrincipal) * interestPeriodRate
                paidInterest += currentInterest
                paidPrincipal += periodPaid - currentInterest
            }
            if fabs(interestPeriodRateMax - interestPeriodRateMin) <= tolerance {
                break
            } else if (paidPrincipal < principalAmount){
                interestPeriodRateMax = interestPeriodRate
                interestPeriodRate -= (interestPeriodRate - interestPeriodRateMin) / 2
            } else {
                interestPeriodRateMin = interestPeriodRate
                interestPeriodRate += (interestPeriodRateMax - interestPeriodRate) / 2
            }
        }
        return interestPeriodRate
    }
    
    func getPeriodPaidByFormula(principalAmount: Double, interestPeriodRate: Double, period: Int) -> Double {
        return principalAmount * interestPeriodRate * pow((1 + interestPeriodRate), Double(period)) / (pow((1 + interestPeriodRate), Double(period)) - 1)
    }
    
}