pow(power) and digit(n) functions on Int

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/extensions/Combinations.kt

@@ -1,6 +1,6 @@
 package de.ronny_h.aoc.extensions
 
-import java.lang.Math.pow
+import kotlin.math.pow
 
 
 /**
@@ -46,7 +46,7 @@ fun <E> permutationsOf(list: List<E>): Sequence<List<E>> = sequence {
 fun <E> allSublistsOf(list: List<E>): Sequence<List<E>> = sequence {
     // count a binary number with list.size digits from 0...0 to 1...1
     var number = 0
-    while (number < pow(2.0, list.size.toDouble())) {
+    while (number < 2.0.pow(list.size.toDouble())) {
         val sublist = ArrayList<E>(list.size)
         val binary = number.toString(2).reversed()
         // take the list elements at indices of the 1 bits

src/main/kotlin/de/ronny_h/aoc/extensions/numbers/Digits.kt

@@ -18,10 +18,18 @@ fun Long.digitCount(): Int {
 }
 
 /**
- * Returns the ones digit (the rightmost digit) of the given Long number.
+ * @return The ones digit (the rightmost digit) of the given Long number.
  */
 fun Long.onesDigit(): Int = abs(this % 10).toInt()
 
+/**
+ * @return The [n]th digit of the given Int number.
+ */
+fun Int.digit(n: Int): Int {
+    require(n > 0) { "n must be greater than zero, but was: $n" }
+    return abs(this / 10.pow(n - 1) % 10)
+}
+
 /**
  * Converts a digit character to a boolean value.
  * @return `true` if this is '1', `false` if this is '0'.

src/main/kotlin/de/ronny_h/aoc/extensions/numbers/IntegralNumbers.kt

@@ -25,3 +25,15 @@ fun Long.toIntChecked(): Int {
     }
     return this.toInt()
 }
+
+/**
+ * @return This Int to the power of [power].
+ */
+fun Int.pow(power: Int): Int {
+    if (power == 0) return 1
+    var result = this
+    repeat(power - 1) {
+        result *= this
+    }
+    return result
+}

src/test/kotlin/de/ronny_h/aoc/extensions/numbers/DigitsTest.kt

@@ -31,6 +31,21 @@ class DigitsTest : StringSpec({
         }
     }
 
+    "The nth digit is returned" {
+        forAll(
+            row(0, 1, 0),
+            row(0, 2, 0),
+            row(54321, 1, 1),
+            row(54321, 2, 2),
+            row(54321, 3, 3),
+            row(54321, 4, 4),
+            row(54321, 5, 5),
+            row(54321, 6, 0),
+        ) { number, n, digit ->
+            number.digit(n) shouldBe digit
+        }
+    }
+
     "toBoolean converts a char digit to Boolean" {
         '0'.toBoolean() shouldBe false
         '1'.toBoolean() shouldBe true

src/test/kotlin/de/ronny_h/aoc/extensions/numbers/IntegralNumbersTest.kt

@@ -76,4 +76,18 @@ class IntegralNumbersTest : StringSpec({
             }
         }
     }
+
+    "The power of Ints" {
+        forAll(
+            row(0, 0, 1),
+            row(1, 1, 1),
+            row(1, 2, 1),
+            row(10, 0, 1),
+            row(10, 1, 10),
+            row(10, 2, 100),
+            row(10, 5, 100000),
+        ) { number, power, result ->
+            number.pow(power) shouldBe result
+        }
+    }
 })