From 04d0a76d84e5e6476dc5766e5c961eb215495db4 Mon Sep 17 00:00:00 2001
From: Christoph Ruegg <git@cdrnet.ch>
Date: Fri, 1 Jan 2021 12:25:19 +0100
Subject: [PATCH] Linear Algebra: Matrix Resize

---
 .../MatrixStructureTheory.Reform.cs           | 36 +++++++++++++++++++
 src/Numerics/LinearAlgebra/Matrix.cs          | 14 ++++++++
 .../LinearAlgebra/Storage/MatrixStorage.cs    | 11 ++++--
 3 files changed, 59 insertions(+), 2 deletions(-)

diff --git a/src/Numerics.Tests/LinearAlgebraTests/MatrixStructureTheory.Reform.cs b/src/Numerics.Tests/LinearAlgebraTests/MatrixStructureTheory.Reform.cs
index 76275a15..7dd6ed66 100644
--- a/src/Numerics.Tests/LinearAlgebraTests/MatrixStructureTheory.Reform.cs
+++ b/src/Numerics.Tests/LinearAlgebraTests/MatrixStructureTheory.Reform.cs
@@ -377,5 +377,41 @@ namespace MathNet.Numerics.UnitTests.LinearAlgebraTests
             Assert.That(() => left.DiagonalStack(right, Matrix<T>.Build.Dense(left.RowCount + right.RowCount, left.ColumnCount + right.ColumnCount + 1)), Throws.ArgumentException);
             Assert.That(() => left.DiagonalStack(right, Matrix<T>.Build.Dense(left.RowCount + right.RowCount, left.ColumnCount + right.ColumnCount - 1)), Throws.ArgumentException);
         }
+
+        [Theory]
+        public void CanResize(TestMatrix testMatrix)
+        {
+            Matrix<T> matrix = Get(testMatrix);
+
+            var moreRows = matrix.Resize(matrix.RowCount + 2, matrix.ColumnCount);
+            Assert.That(moreRows.RowCount, Is.EqualTo(matrix.RowCount + 2));
+            Assert.That(moreRows.ColumnCount, Is.EqualTo(matrix.ColumnCount));
+            Assert.That(moreRows.SubMatrix(0, matrix.RowCount, 0, matrix.ColumnCount), Is.EqualTo(matrix));
+
+            var moreCols = matrix.Resize(matrix.RowCount, matrix.ColumnCount + 2);
+            Assert.That(moreCols.RowCount, Is.EqualTo(matrix.RowCount));
+            Assert.That(moreCols.ColumnCount, Is.EqualTo(matrix.ColumnCount + 2));
+            Assert.That(moreCols.SubMatrix(0, matrix.RowCount, 0, matrix.ColumnCount), Is.EqualTo(matrix));
+
+            if (matrix.RowCount >= 1)
+            {
+                var lessRows = matrix.Resize(matrix.RowCount - 1, matrix.ColumnCount);
+                Assert.That(lessRows.RowCount, Is.EqualTo(matrix.RowCount - 1));
+                Assert.That(lessRows.ColumnCount, Is.EqualTo(matrix.ColumnCount));
+                Assert.That(lessRows, Is.EqualTo(matrix.SubMatrix(0, matrix.RowCount - 1, 0, matrix.ColumnCount)));
+            }
+
+            if (matrix.ColumnCount >= 1)
+            {
+                var lessCols = matrix.Resize(matrix.RowCount, matrix.ColumnCount - 1);
+                Assert.That(lessCols.RowCount, Is.EqualTo(matrix.RowCount));
+                Assert.That(lessCols.ColumnCount, Is.EqualTo(matrix.ColumnCount - 1));
+                Assert.That(lessCols, Is.EqualTo(matrix.SubMatrix(0, matrix.RowCount, 0, matrix.ColumnCount - 1)));
+            }
+
+            var empty = matrix.Resize(0, 0);
+            Assert.That(empty.RowCount, Is.EqualTo(0));
+            Assert.That(empty.ColumnCount, Is.EqualTo(0));
+        }
     }
 }
diff --git a/src/Numerics/LinearAlgebra/Matrix.cs b/src/Numerics/LinearAlgebra/Matrix.cs
index 2cb7091b..880fb040 100644
--- a/src/Numerics/LinearAlgebra/Matrix.cs
+++ b/src/Numerics/LinearAlgebra/Matrix.cs
@@ -999,6 +999,20 @@ namespace MathNet.Numerics.LinearAlgebra
             }
         }
 
+        /// <summary>
+        /// Creates a new matrix with the desired size and copies this matrix to it.
+        /// Values which no longer exist in the new matrix are ignored, new values are set to zero.
+        /// </summary>
+        /// <param name="rowCount">The number of rows of the new matrix.</param>
+        /// <param name="columnCount">The number of columns of the new matrix.</param>
+        /// <returns>A new matrix with the desired rows and columns.</returns>
+        public Matrix<T> Resize(int rowCount, int columnCount)
+        {
+            var result = Build.SameAs(this, rowCount, columnCount, fullyMutable: true);
+            Storage.CopySubMatrixTo(result.Storage, 0, 0, Math.Min(RowCount, rowCount), 0, 0, Math.Min(ColumnCount, columnCount), ExistingData.AssumeZeros);
+            return result;
+        }
+
         /// <summary>
         /// Returns the transpose of this matrix.
         /// </summary>
diff --git a/src/Numerics/LinearAlgebra/Storage/MatrixStorage.cs b/src/Numerics/LinearAlgebra/Storage/MatrixStorage.cs
index 7d07c53e..63663650 100644
--- a/src/Numerics/LinearAlgebra/Storage/MatrixStorage.cs
+++ b/src/Numerics/LinearAlgebra/Storage/MatrixStorage.cs
@@ -52,12 +52,12 @@ namespace MathNet.Numerics.LinearAlgebra.Storage
         {
             if (rowCount < 0)
             {
-                throw new ArgumentOutOfRangeException(nameof(rowCount), "The number of rows of a matrix must be positive.");
+                throw new ArgumentOutOfRangeException(nameof(rowCount), "The number of rows of a matrix must be non-negative.");
             }
 
             if (columnCount < 0)
             {
-                throw new ArgumentOutOfRangeException(nameof(columnCount), "The number of columns of a matrix must be positive.");
+                throw new ArgumentOutOfRangeException(nameof(columnCount), "The number of columns of a matrix must be non-negative.");
             }
 
             RowCount = rowCount;
@@ -355,6 +355,13 @@ namespace MathNet.Numerics.LinearAlgebra.Storage
                 return;
             }
 
+            if (sourceRowIndex == 0 && targetRowIndex == 0 && rowCount == RowCount && rowCount == target.RowCount
+                && sourceColumnIndex == 0 && targetColumnIndex == 0 && columnCount == ColumnCount && columnCount == target.ColumnCount)
+            {
+                CopyTo(target);
+                return;
+            }
+
             if (ReferenceEquals(this, target))
             {
                 throw new NotSupportedException();