feat(Core): add new flavors of permSort: allow user to specify a comparator (sort with) or a custom field to use in buffer values (sort by).

kmath-core/src/commonMain/kotlin/space/kscience/kmath/misc/sorting.kt

@@ -6,23 +6,38 @@
 package space.kscience.kmath.misc
 
 import kotlin.comparisons.*
-import space.kscience.kmath.misc.UnstableKMathAPI
 import space.kscience.kmath.structures.Buffer
-import space.kscience.kmath.structures.indices
 
 /**
  * Return a new list filled with buffer indices. Indice order is defined by sorting associated buffer value.
  * This feature allows to sort buffer values without reordering its content.
- * 
+ *
- * @param descending True to revert sort order from highest to lowest values. Default to ascending order.
  * @return List of buffer indices, sorted by associated value.
  */
 @PerformancePitfall
 @UnstableKMathAPI
-public fun <V: Comparable<V>> Buffer<V>.permSort(descending : Boolean = false) : IntArray {
+public fun <V: Comparable<V>> Buffer<V>.permSort() : IntArray = _permSortWith(compareBy<Int> { get(it) })
-    if (size < 2) return IntArray(size)
 
-    val comparator = if (descending) compareByDescending<Int> { get(it) } else compareBy<Int> { get(it) }
+@PerformancePitfall
+@UnstableKMathAPI
+public fun <V: Comparable<V>> Buffer<V>.permSortDescending() : IntArray = _permSortWith(compareByDescending<Int> { get(it) })
+
+@PerformancePitfall
+@UnstableKMathAPI
+public fun <V, C: Comparable<C>> Buffer<V>.permSortBy(selector: (V) -> C) : IntArray = _permSortWith(compareBy<Int> { selector(get(it)) })
+
+@PerformancePitfall
+@UnstableKMathAPI
+public fun <V, C: Comparable<C>> Buffer<V>.permSortByDescending(selector: (V) -> C) : IntArray = _permSortWith(compareByDescending<Int> { selector(get(it)) })
+
+@PerformancePitfall
+@UnstableKMathAPI
+public fun <V> Buffer<V>.permSortWith(comparator : Comparator<V>) : IntArray = _permSortWith { i1, i2 -> comparator.compare(get(i1), get(i2)) }
+
+@PerformancePitfall
+@UnstableKMathAPI
+private fun <V> Buffer<V>._permSortWith(comparator : Comparator<Int>) : IntArray {
+    if (size < 2) return IntArray(size)
 
     /* TODO: optimisation : keep a constant big array of indices (Ex: from 0 to 4096), then create indice
      * arrays more efficiently by copying subpart of cached one. For bigger needs, we could copy entire
@@ -31,10 +46,10 @@ public fun <V: Comparable<V>> Buffer<V>.permSort(descending : Boolean = false) :
      *  2. Some benchmark would be needed to ensure it would really provide better performance
      */
     val packedIndices = IntArray(size) { idx -> idx }
-    
+
-    /* TODO: find an efficient way to sort in-place instead, and return directly the IntArray. 
+    /* TODO: find an efficient way to sort in-place instead, and return directly the IntArray.
      * Not done for now, because no standard utility is provided yet. An open issue exists for this.
-     * See: https://youtrack.jetbrains.com/issue/KT-37860 
+     * See: https://youtrack.jetbrains.com/issue/KT-37860
      */
     return packedIndices.sortedWith(comparator).toIntArray()
 }

kmath-core/src/commonTest/kotlin/space/kscience/kmath/misc/PermSortTest.kt

@@ -5,17 +5,24 @@
 
 package space.kscience.kmath.misc
 
-import kotlin.collections.mutableListOf
+import space.kscience.kmath.misc.PermSortTest.Platform.*
 import kotlin.random.Random
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertTrue
-import kotlin.test.fail
 
 import space.kscience.kmath.structures.IntBuffer
+import space.kscience.kmath.structures.asBuffer
+import kotlin.test.assertContentEquals
 
 class PermSortTest {
 
+    private enum class Platform {
+        ANDROID, JVM, JS, NATIVE, WASM
+    }
+
+    private val platforms = Platform.values().asBuffer()
+
     /**
      * Permutation on empty buffer should immediately return an empty array.
      */
@@ -24,7 +31,7 @@ class PermSortTest {
         val emptyBuffer = IntBuffer(0) {it}
         var permutations = emptyBuffer.permSort()
         assertTrue(permutations.isEmpty(), "permutation on an empty buffer should return an empty result")
-        permutations = emptyBuffer.permSort(true)
+        permutations = emptyBuffer.permSortDescending()
         assertTrue(permutations.isEmpty(), "permutation on an empty buffer should return an empty result")
     }
 
@@ -34,10 +41,34 @@ class PermSortTest {
     }
 
     @Test
-    public fun testOnSomeValues() {
+    fun testOnSomeValues() {
         testPermutation(10)
     }
 
+    @Test
+    fun testPermSortBy() {
+        val permutations = platforms.permSortBy { it.name }
+        val expected = listOf(ANDROID, JS, JVM, NATIVE, WASM)
+        assertContentEquals(expected, permutations.map { platforms[it] }, "Ascending PermSort by name")
+    }
+
+    @Test
+    fun testPermSortByDescending() {
+        val permutations = platforms.permSortByDescending { it.name }
+        val expected = listOf(WASM, NATIVE, JVM, JS, ANDROID)
+        assertContentEquals(expected, permutations.map { platforms[it] }, "Descending PermSort by name")
+    }
+
+    @Test
+    fun testPermSortWith() {
+        var permutations = platforms.permSortWith { p1, p2 -> p1.name.length.compareTo(p2.name.length) }
+        val expected = listOf(JS, JVM, WASM, NATIVE, ANDROID)
+        assertContentEquals(expected, permutations.map { platforms[it] }, "PermSort using custom ascending comparator")
+
+        permutations = platforms.permSortWith(compareByDescending { it.name.length })
+        assertContentEquals(expected.reversed(), permutations.map { platforms[it] }, "PermSort using custom descending comparator")
+    }
+
     private fun testPermutation(bufferSize: Int) {   
 
         val seed = Random.nextLong()
@@ -56,7 +87,7 @@ class PermSortTest {
             assertTrue(current <= next, "Permutation indices not properly sorted")
         }
 
-        val descIndices = buffer.permSort(true)
+        val descIndices = buffer.permSortDescending()
         assertEquals(bufferSize, descIndices.size) 
         // Ensure no doublon is present in indices
         assertEquals(descIndices.toSet().size, descIndices.size)