Bug fix: no crash for zero vector in ResidualStopCriterion

Author: jkalias

src/Numerics.Tests/LinearAlgebraTests/Double/DenseMatrixTests.cs

@@ -31,6 +31,8 @@
 using System.Collections.Generic;
 using MathNet.Numerics.LinearAlgebra;
 using MathNet.Numerics.LinearAlgebra.Double;
+using MathNet.Numerics.LinearAlgebra.Double.Solvers;
+using MathNet.Numerics.LinearAlgebra.Solvers;
 using NUnit.Framework;
 
 namespace MathNet.Numerics.Tests.LinearAlgebraTests.Double
@@ -209,5 +211,19 @@ public void MatrixToMatrixString()
                 }
             }
         }
+
+        [Test]
+        public void ResidualStopCriterionDoesNotCrashForZeroVector()
+        {
+            var A = new DenseMatrix(2, 2, new double[] { 1, 2, 3, 4 });
+            var b = new DenseVector(2);
+            var x = new DenseVector(2);
+
+            var iterator = new Iterator<double>(new ResidualStopCriterion<double>(1e-5));
+            var solver = new BiCgStab();
+
+            solver.Solve(A, b, x, iterator, null);
+            Assert.AreEqual(new DenseVector(2), x);
+        }
     }
 }

src/Numerics/LinearAlgebra/Solvers/ResidualStopCriterion.cs

@@ -195,7 +195,7 @@ public IterationStatus DetermineStatus(int iterationNumber, Vector<T> solutionVe
 
             // ||r_i|| <= stop_tol * ||b||
             // Stop the calculation if it's clearly smaller than the tolerance
-            if (residualNorm < stopCriterion)
+            if (residualNorm <= stopCriterion)
             {
                 if (_lastIteration <= iterationNumber)
                 {