forked from kscience/kmath
feat(Core): add a permutation sorting prototype for buffers
This is a Buffer extension function to create a list of permuted indices that represent the sequence of naturally sorted buffer elements
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/*
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* Copyright 2018-2021 KMath contributors.
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* Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
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*/
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package space.kscience.kmath.misc
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import kotlin.comparisons.*
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import space.kscience.kmath.misc.UnstableKMathAPI
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import space.kscience.kmath.structures.Buffer
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import space.kscience.kmath.structures.indices
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/**
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* Return a new list filled with buffer indices. Indice order is defined by sorting associated buffer value.
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* This feature allows to sort buffer values without reordering its content.
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*
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* @param descending True to revert sort order from highest to lowest values. Default to ascending order.
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* @return List of buffer indices, sorted by associated value.
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*/
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@PerformancePitfall
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@UnstableKMathAPI
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public fun <V: Comparable<V>> Buffer<V>.permSort(descending : Boolean = false) : IntArray {
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if (size < 2) return IntArray(size)
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val comparator = if (descending) compareByDescending<Int> { get(it) } else compareBy<Int> { get(it) }
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/* TODO: optimisation : keep a constant big array of indices (Ex: from 0 to 4096), then create indice
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* arrays more efficiently by copying subpart of cached one. For bigger needs, we could copy entire
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* cached array, then fill remaining indices manually. Not done for now, because:
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* 1. doing it right would require some statistics about common used buffer sizes.
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* 2. Some benchmark would be needed to ensure it would really provide better performance
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*/
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val packedIndices = IntArray(size) { idx -> idx }
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/* TODO: find an efficient way to sort in-place instead, and return directly the IntArray.
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* Not done for now, because no standard utility is provided yet. An open issue exists for this.
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* See: https://youtrack.jetbrains.com/issue/KT-37860
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*/
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return packedIndices.sortedWith(comparator).toIntArray()
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}
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/*
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* Copyright 2018-2021 KMath contributors.
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* Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
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*/
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package space.kscience.kmath.misc
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import kotlin.collections.mutableListOf
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import kotlin.random.Random
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import kotlin.test.Test
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import kotlin.test.assertEquals
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import kotlin.test.assertTrue
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import kotlin.test.fail
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import space.kscience.kmath.structures.IntBuffer
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class PermSortTest {
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/**
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* Permutation on empty buffer should immediately return an empty array.
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*/
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@Test
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fun testOnEmptyBuffer() {
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val emptyBuffer = IntBuffer(0) {it}
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var permutations = emptyBuffer.permSort()
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assertTrue(permutations.isEmpty(), "permutation on an empty buffer should return an empty result")
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permutations = emptyBuffer.permSort(true)
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assertTrue(permutations.isEmpty(), "permutation on an empty buffer should return an empty result")
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}
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@Test
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fun testOnSingleValueBuffer() {
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testPermutation(1)
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}
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@Test
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public fun testOnSomeValues() {
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testPermutation(10)
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}
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private fun testPermutation(bufferSize: Int) {
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val seed = Random.nextLong()
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println("Test randomization seed: $seed")
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val buffer = Random(seed).buffer(bufferSize)
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val indices = buffer.permSort()
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assertEquals(bufferSize, indices.size)
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// Ensure no doublon is present in indices
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assertEquals(indices.toSet().size, indices.size)
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for (i in 0 until (bufferSize-1)) {
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val current = buffer[indices[i]]
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val next = buffer[indices[i+1]]
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assertTrue(current <= next, "Permutation indices not properly sorted")
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}
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val descIndices = buffer.permSort(true)
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assertEquals(bufferSize, descIndices.size)
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// Ensure no doublon is present in indices
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assertEquals(descIndices.toSet().size, descIndices.size)
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for (i in 0 until (bufferSize-1)) {
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val current = buffer[descIndices[i]]
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val next = buffer[descIndices[i+1]]
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assertTrue(current >= next, "Permutation indices not properly sorted in descending order")
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}
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}
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private fun Random.buffer(size : Int) = IntBuffer(size) { nextInt() }
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}
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