add rate calculator

src/main/kotlin/dev/darktale/RateCalculator.kt

@@ -0,0 +1,38 @@
+package dev.darktale
+
+import dev.darktale.model.Rate
+import java.math.BigDecimal
+import java.time.LocalDate
+
+class AmountByRateCalculator {
+    private val leapYearDuration = BigDecimal.valueOf(366L)
+    private val nonLeapYearDuration = BigDecimal.valueOf(365L)
+    fun calculate(amount: BigDecimal, rate: Rate, dateFrom: LocalDate, dateTo: LocalDate): BigDecimal {
+        var result = amount
+        var curRate = rate.rateOnDate(dateFrom)
+        while (curRate != null && curRate.since < dateTo) {
+            val curDateFrom = max(curRate.since, dateFrom)
+            val curDateTo = min(dateTo, curRate.till)
+            result *= (BigDecimal.ONE + getRateForPeriod(curRate.rate!!, curDateFrom, curDateTo))
+            curRate = rate.rateOnDate(curDateTo.plusDays(1))?.let {
+                if (it === curRate) null else it
+            }
+        }
+        return result
+    }
+
+    private fun getRateForPeriod(rate: BigDecimal, dateFrom: LocalDate, dateTo: LocalDate): BigDecimal {
+        return if (dateFrom.year == dateTo.year) {
+            rate.setScale(20) *
+                    (dateTo.dayOfYear - dateFrom.dayOfYear + 1).toBigDecimal() /
+                    if (dateFrom.isLeapYear) leapYearDuration else nonLeapYearDuration
+        } else getRateForPeriod(rate, dateFrom, LocalDate.of(dateFrom.year, 12, 31)) +
+                getRateForPeriod(rate, LocalDate.of(dateTo.year, 1, 1), dateTo)
+    }
+}
+
+private fun max(localDate1: LocalDate, localDate2: LocalDate?) =
+    if (localDate2 == null || localDate1 >= localDate2) localDate1 else localDate2
+
+private fun min(localDate1: LocalDate, localDate2: LocalDate?) =
+    if (localDate2 == null || localDate1 <= localDate2) localDate1 else localDate2

src/main/kotlin/dev/darktale/model/Rates.kt

@@ -0,0 +1,44 @@
+package dev.darktale.model
+
+import java.math.BigDecimal
+import java.time.LocalDate
+import java.util.*
+
+sealed class Rate {
+    open val rate: BigDecimal? = null
+    abstract val since: LocalDate
+    open val till: LocalDate? = null
+    abstract fun rateOnDate(date: LocalDate): Rate?
+
+    override fun toString() = "Rate($rate since $since ${till?.let { "till $it" }}"
+}
+
+class SimpleRate(
+    override val rate: BigDecimal,
+    override val since: LocalDate,
+    override val till: LocalDate? = null,
+) : Rate() {
+
+    constructor(rate: Double, since: LocalDate, till: LocalDate?) :
+            this(BigDecimal.valueOf(rate), since, till)
+
+    override fun rateOnDate(date: LocalDate) =
+        if (date >= since && (till == null || date <= till)) this else null
+}
+
+class PeriodicRate(
+    private val rates: TreeMap<LocalDate, Rate>
+) : Rate() {
+
+    constructor(vararg rates: Rate) : this(TreeMap(rates.associateBy { it.since }))
+
+    override val since: LocalDate
+        get() = rates.firstKey()
+
+    override fun rateOnDate(date: LocalDate) =
+        rates.floorEntry(date)?.value?.let {
+            if (it.till == null || it.till!! >= date) it else null
+        }
+
+    override fun toString() = rates.values.toString()
+}

src/test/java/dev/darktale/AmountByRateCalculatorTest.kt

@@ -0,0 +1,98 @@
+package dev.darktale
+
+import dev.darktale.model.PeriodicRate
+import dev.darktale.model.Rate
+import dev.darktale.model.SimpleRate
+import org.junit.jupiter.api.extension.ExtensionContext
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.Arguments
+import org.junit.jupiter.params.provider.ArgumentsProvider
+import org.junit.jupiter.params.provider.ArgumentsSource
+import java.math.BigDecimal
+import java.time.LocalDate
+import java.util.stream.Stream
+import kotlin.math.abs
+import kotlin.math.pow
+import kotlin.test.DefaultAsserter.assertTrue
+
+class AmountByRateCalculatorTest {
+    private val amountByRateCalculator = AmountByRateCalculator()
+
+    @ParameterizedTest
+    @ArgumentsSource(RateProvider::class)
+    fun testRateCalculator(
+        rate: Rate,
+        sourceAmount: BigDecimal,
+        dateFrom: LocalDate,
+        dateTo: LocalDate,
+        expectedAmount: BigDecimal
+    ) {
+        val result = amountByRateCalculator.calculate(sourceAmount, rate, dateFrom, dateTo)
+        val maxDelta = 10.0.pow(-expectedAmount.scale().toDouble())
+        assertTrue(
+            "expected amount was $expectedAmount ± ${
+                BigDecimal.valueOf(maxDelta).toPlainString()
+            }, but actual is $result",
+            abs((expectedAmount - result).toDouble()) <= maxDelta,
+        )
+    }
+}
+
+class RateProvider : ArgumentsProvider {
+    override fun provideArguments(context: ExtensionContext): Stream<out Arguments> {
+        val baseAmount = BigDecimal.valueOf(1000L)
+        val startOfYear = LocalDate.of(2025, 1, 1)
+        val endOfMonth = LocalDate.of(2025, 1, 31)
+        val endOfMonth2 = LocalDate.of(2025, 2, 28)
+        val endOfYear = LocalDate.of(2025, 12, 31)
+        val middleOfYear = startOfYear.plusDays((endOfYear.dayOfYear / 2).toLong())
+        val simpleRate = SimpleRate(0.1, startOfYear, endOfYear)
+        val periodicRate = PeriodicRate(
+            SimpleRate(0.1, startOfYear, endOfMonth),
+            SimpleRate(0.1, endOfMonth.plusDays(1), endOfMonth2)
+        )
+        val decemberOfNonLeapYear = LocalDate.of(2023, 12, 1)
+        val januaryOfLeapYear = LocalDate.of(2024, 1, 31)
+        val nonLeapAndLeapYearRate = SimpleRate(
+            0.1,
+            decemberOfNonLeapYear,
+            januaryOfLeapYear
+        )
+
+        return Stream.of(
+            Arguments.of(
+                simpleRate, baseAmount, startOfYear, endOfYear, BigDecimal.valueOf(1100L)
+            ),
+            Arguments.of(
+                simpleRate,
+                baseAmount,
+                startOfYear,
+                middleOfYear,
+                BigDecimal.valueOf(1050.137)
+            ),
+            Arguments.of(
+                periodicRate,
+                baseAmount,
+                startOfYear,
+                endOfMonth2,
+                BigDecimal.valueOf(1016.2295)
+            ),
+            Arguments.of(
+                periodicRate,
+                baseAmount,
+                startOfYear,
+                middleOfYear,
+                BigDecimal.valueOf(1016.2295)
+            ),
+            Arguments.of(
+                nonLeapAndLeapYearRate,
+                baseAmount,
+                nonLeapAndLeapYearRate.since,
+                nonLeapAndLeapYearRate.till,
+                BigDecimal.valueOf(1016.963096)
+            )
+        )
+    }
+
+}
+