Add BigDecimal implementation

kmath-core/src/commonMain/kotlin/space/kscience/kmath/operations/BigDecimal.kt

@@ -0,0 +1,94 @@
+/*
+ * Copyright 2018-2023 KMath contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package space.kscience.kmath.operations
+
+import space.kscience.kmath.UnstableKMathAPI
+import kotlin.math.abs
+import kotlin.math.min
+
+/**
+ * Decimal value with unfixed length
+ *
+ * The value is computed as `intValue*10^scale`.
+ */
+@UnstableKMathAPI
+public data class BigDecimal(val intValue: BigInt, val scale: Int)
+
+/**
+ * Convert current [BigInt] to a [BigDecimal] with scale 0.
+ */
+@UnstableKMathAPI
+public fun BigInt.asDecimal(): BigDecimal = BigDecimal(this, 0)
+
+/**
+ * A field for [BigDecimal] on top of [BigIntField]
+ */
+@UnstableKMathAPI
+public object BigDecimalField : Field<BigDecimal> {
+
+    override val one: BigDecimal = BigIntField.one.asDecimal()
+    override val zero: BigDecimal = BigIntField.zero.asDecimal()
+
+    internal val ten: BigInt = BigIntField.number(10)
+
+    internal fun pow10(power: UInt): BigInt = ten.pow(power)
+
+    public fun Number.toDecimal(): BigDecimal = number(this)
+
+    /**
+     * Rescale a [BigDecimal] to have [targetScale]. If the target scale is larger than the current scale, loss of precision is possible.
+     */
+    public fun BigDecimal.rescale(targetScale: Int): BigDecimal = if (targetScale == scale) {
+        this
+    } else {
+        val scaleDif = scale - targetScale
+        if (scaleDif > 0) {
+            BigDecimal(BigIntField.multiply(intValue, pow10(scaleDif.toUInt())), targetScale)
+        } else {
+            BigDecimal(BigIntField.divide(intValue, pow10(abs(scaleDif).toUInt())), targetScale)
+        }
+    }
+
+    public fun BigDecimal.pow10(power: Int): BigDecimal = BigDecimal(intValue, scale + power)
+
+    override fun add(left: BigDecimal, right: BigDecimal): BigDecimal = if (left.scale == right.scale) {
+        BigDecimal(BigIntField.add(left.intValue, right.intValue), left.scale)
+    } else {
+        val minScale = min(left.scale, right.scale)
+        //rescale both to a minimal scale
+        add(left.rescale(minScale), right.rescale(minScale))
+    }
+
+    override fun BigDecimal.unaryMinus(): BigDecimal = BigDecimal(-intValue, scale)
+
+    override fun scale(a: BigDecimal, value: Double): BigDecimal = with(BigIntField) {
+        BigDecimal(a.intValue * value, a.scale)
+    }
+
+    override fun divide(left: BigDecimal, right: BigDecimal): BigDecimal =
+        BigDecimal(BigIntField.divide(left.intValue, right.intValue), left.scale - right.scale)
+
+    override fun multiply(left: BigDecimal, right: BigDecimal): BigDecimal =
+        BigDecimal(BigIntField.multiply(left.intValue, right.intValue), left.scale + right.scale)
+
+
+    public operator fun Double.times(other: BigDecimal): BigDecimal = toDecimal() * other
+
+    public operator fun Double.div(other: BigDecimal): BigDecimal = toDecimal() / other
+
+    public operator fun Double.plus(other: BigDecimal): BigDecimal = toDecimal() + other
+
+    public operator fun Double.minus(other: BigDecimal): BigDecimal = toDecimal() - other
+
+
+    public operator fun BigDecimal.times(other: Double): BigDecimal = this * other.toDecimal()
+
+    public operator fun BigDecimal.div(other: Double): BigDecimal = this / other.toDecimal()
+
+    public operator fun BigDecimal.plus(other: Double): BigDecimal = this + other.toDecimal()
+
+    public operator fun BigDecimal.minus(other: Double): BigDecimal = this - other.toDecimal()
+}

kmath-core/src/commonMain/kotlin/space/kscience/kmath/operations/BigInt.kt

@@ -16,7 +16,6 @@ import kotlin.math.min
 import kotlin.math.sign
 
 private typealias Magnitude = UIntArray
-private typealias TBase = ULong
 
 /**
  * Kotlin Multiplatform implementation of Big Integer numbers (KBigInteger).
@@ -215,6 +214,9 @@ public class BigInt internal constructor(
         }
     }
 
+    /**
+     * Convert this [BigInt] to a string using hexadecimal representation
+     */
     override fun toString(): String {
         if (this.sign == 0.toByte()) {
             return "0x0"

kmath-core/src/commonTest/kotlin/space/kscience/kmath/operations/BigDecimalTest.kt

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2018-2023 KMath contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package space.kscience.kmath.operations
+
+import space.kscience.kmath.UnstableKMathAPI
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+@OptIn(UnstableKMathAPI::class)
+class BigDecimalTest {
+
+    @Test
+    fun simpleExpression() = with(BigDecimalField) {
+        assertEquals( 1000.0.toDecimal(),22.2.toDecimal().pow10(2) / 1.11)
+    }
+}