Mortgage Calculator — Kotlin source

// mortgage-calculator — amortized monthly payment.
//
// Language: Kotlin (1.9, standard library only)
// Source:   CosmoDev polyglot showcase port of the Mortgage Calculator
//           tool, ported from src/lib/mortgage.ts (the canonical
//           TypeScript implementation).
// Tool:     https://dev.cosmolabs.org/tools/mortgage-calculator
// License:  display source — part of CosmoDev's polyglot tool pages.
//
// Display snippet: ports the core monthlyPayment() formula from the
// TypeScript lib. The full amortization schedule (integer-cent tracking,
// extra payments, payoff dates) lives in the TypeScript/Go sources.
//
// Semantics: standard amortization formula P*r*(1+r)^n / ((1+r)^n - 1)
// with r = ratePct/100/12 and n = years*12. A zero rate divides the
// principal evenly across n months. Returns the payment rounded to
// cents, or null on invalid input (non-positive principal, non-positive
// years, non-finite values).

import kotlin.math.floor
import kotlin.math.pow

/** Round to 2 decimals, half-up (mirrors the TS Math.round). */
fun round2(n: Double): Double = floor(n * 100.0 + 0.5) / 100.0

fun monthlyPayment(principal: Double, ratePct: Double, years: Int): Double? {
    if (!principal.isFinite() || !ratePct.isFinite()) return null
    if (principal <= 0.0 || years <= 0) return null

    val r = ratePct / 100.0 / 12.0
    val n = years * 12.0

    val payment = if (r == 0.0) {
        principal / n
    } else {
        val growth = (1.0 + r).pow(n)
        principal * r * growth / (growth - 1.0)
    }
    return round2(payment)
}

fun main() {
    // Example:
    //   monthlyPayment(300000.0, 6.0, 30) -> 1798.65
    val result = monthlyPayment(300000.0, 6.0, 30)
    check(result == 1798.65) { "expected 1798.65, got $result" }

    // A zero rate divides the principal evenly.
    check(monthlyPayment(300000.0, 0.0, 30) == 833.33) { "expected 833.33 at a zero rate" }

    // Invalid inputs are rejected, never clamped.
    check(monthlyPayment(-1.0, 6.0, 30) == null) { "non-positive principal" }
    check(monthlyPayment(300000.0, 6.0, 0) == null) { "non-positive years" }
    check(monthlyPayment(Double.POSITIVE_INFINITY, 6.0, 30) == null) { "non-finite value" }

    println("ok")
}