From f7b06e2955492e590ff47decace92fed93b2a527 Mon Sep 17 00:00:00 2001
From: Daniel Sword <dsword@gmail.com>
Date: Mon, 19 Nov 2018 19:59:37 -0800
Subject: [PATCH] Combinatorics: add a method to invert a permutation.

---
 src/Numerics/Combinatorics.cs | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/src/Numerics/Combinatorics.cs b/src/Numerics/Combinatorics.cs
index 63e66c9d..fe0503d8 100644
--- a/src/Numerics/Combinatorics.cs
+++ b/src/Numerics/Combinatorics.cs
@@ -146,6 +146,20 @@ namespace MathNet.Numerics
             return indices;
         }
 
+        /// <summary>
+        /// Inverts a permutation, returning a new permutation.
+        /// No checks are performed to validate the input (apart from null checks).
+        /// </summary>
+        /// <param name="permutation">Permutation to invert.</param>
+        /// <returns>The inverted permutation.</returns>
+        public static int[] InvertPermutation(int[] permutation)
+        {
+            if (permutation == null) throw new ArgumentNullException(nameof(permutation));
+            var result = Enumerable.Range(0, permutation.Length).ToArray();
+            Sorting.Sort(permutation, result);
+            return result;
+        }
+
         /// <summary>
         /// Select a random permutation, without repetition, from a data array by reordering the provided array in-place.
         /// Implemented using Fisher-Yates Shuffling. The provided data array will be modified.