Sparse: fix bug in submatrix-clear

src/Numerics/LinearAlgebra/Storage/SparseCompressedRowMatrixStorage.cs

@@ -152,17 +152,17 @@ namespace MathNet.Numerics.LinearAlgebra.Storage
                 return;
             }
 
-            for (int i = rowIndex, row = 0; i < rowIndex + rowCount; i++, row++)
+            for (int i = rowIndex + rowCount - 1, row = rowCount - 1; i >= rowIndex; i--, row--)
             {
                 var startIndex = RowPointers[i];
                 var endIndex = i < RowPointers.Length - 1 ? RowPointers[i + 1] : ValueCount;
 
-                for (int j = startIndex; j < endIndex; j++)
+                for (int j = endIndex - 1; j >= startIndex; j--)
                 {
                     // check if the column index is in the range
                     if ((ColumnIndices[j] >= columnIndex) && (ColumnIndices[j] < columnIndex + columnCount))
                     {
-                        var column = ColumnIndices[j] - columnIndex;
+                        var column = ColumnIndices[j];
 
                         // NOTE: potential for more efficient implementation
                         At(row, column, _zero);

src/UnitTests/LinearAlgebraTests/MatrixStructureTheory.cs

@@ -120,6 +120,20 @@ namespace MathNet.Numerics.UnitTests.LinearAlgebraTests
             Assert.That(cleared, Is.EqualTo(matrix.CreateMatrix(matrix.RowCount, matrix.ColumnCount)));
         }
 
+        [Theory, Timeout(200)]
+        public void CanClearSubMatrix(Matrix<T> matrix)
+        {
+            var cleared = matrix.Clone();
+            Assume.That(cleared.RowCount, Is.GreaterThanOrEqualTo(2));
+            Assume.That(cleared.ColumnCount, Is.GreaterThanOrEqualTo(2));
+
+            cleared.Storage.Clear(0,2,1,1);
+            Assert.That(cleared.At(0, 0), Is.EqualTo(matrix.At(0, 0)));
+            Assert.That(cleared.At(1, 0), Is.EqualTo(matrix.At(1, 0)));
+            Assert.That(cleared.At(0, 1), Is.EqualTo(Zero));
+            Assert.That(cleared.At(1, 1), Is.EqualTo(Zero));
+        }
+
         [Theory, Timeout(200)]
         public void CanToArray(Matrix<T> matrix)
         {