From e58d350613b7f010fb2472950c24f859410c94fd Mon Sep 17 00:00:00 2001
From: Hylke Faas <hylke.faas@atmosuav.com>
Date: Mon, 13 Mar 2023 10:47:40 +0100
Subject: [PATCH] Add method with option to not allow extrapolation.

---
 .../InterpolationTests/LinearSplineTest.cs     | 13 +++++++++++++
 src/Numerics/Interpolation/LinearSpline.cs     | 18 ++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/src/Numerics.Tests/InterpolationTests/LinearSplineTest.cs b/src/Numerics.Tests/InterpolationTests/LinearSplineTest.cs
index 79980ee31..ae0105678 100644
--- a/src/Numerics.Tests/InterpolationTests/LinearSplineTest.cs
+++ b/src/Numerics.Tests/InterpolationTests/LinearSplineTest.cs
@@ -29,6 +29,7 @@
 
 using MathNet.Numerics.Interpolation;
 using NUnit.Framework;
+using System;
 
 namespace MathNet.Numerics.Tests.InterpolationTests
 {
@@ -138,5 +139,17 @@ public void FewSamples()
             Assert.That(() => LinearSpline.Interpolate(new double[1], new double[1]), Throws.ArgumentException);
             Assert.That(LinearSpline.Interpolate(new[] { 1.0, 2.0 }, new[] { 2.0, 2.0 }).Interpolate(1.0), Is.EqualTo(2.0));
         }
+
+        [Test]
+        public void DoNotExtrapolate()
+        {
+            LinearSpline ip = LinearSpline.Interpolate(_t, _y);
+
+            Assert.Throws<ArgumentOutOfRangeException>(() => ip.Interpolate(-2.1, false));
+            Assert.Throws<ArgumentOutOfRangeException>(() => ip.Interpolate(10.0, false));
+
+            Assert.Throws<ArgumentOutOfRangeException>(() => LinearSpline.Interpolate(new[] { 1.0, 2.0 }, new[] { 2.0, 2.0 }).Interpolate(0.9, false));
+            Assert.Throws<ArgumentOutOfRangeException>(() => LinearSpline.Interpolate(new[] { 1.0, 2.0 }, new[] { 2.0, 2.0 }).Interpolate(3.0, false));
+        }
     }
 }
diff --git a/src/Numerics/Interpolation/LinearSpline.cs b/src/Numerics/Interpolation/LinearSpline.cs
index 0f192c3e3..8c127df60 100644
--- a/src/Numerics/Interpolation/LinearSpline.cs
+++ b/src/Numerics/Interpolation/LinearSpline.cs
@@ -130,6 +130,24 @@ public static LinearSpline Interpolate(IEnumerable<double> x, IEnumerable<double
         /// <returns>Interpolated value x(t).</returns>
         public double Interpolate(double t)
         {
+            return Interpolate(t, true);
+        }
+
+        /// <summary>
+        /// Interpolate at point t.
+        /// </summary>
+        /// <param name="t">Point t to interpolate at.</param>
+        /// <param name="allowExtrapolate">set to fals if extrapolate should not be allowed</param>
+        /// <returns>Interpolated value x(t).</returns>
+        /// <exception cref="ArgumentOutOfRangeException">Thrown when t is outside range and would result in Extrapolation!</exception>
+        public double Interpolate(double t, bool allowExtrapolate)
+        {
+            if (!allowExtrapolate &&
+                (t < _x[0] || t > _x[_x.Length - 1]))
+            {
+                throw new ArgumentOutOfRangeException("'t' is outside of the constructed array. This would result in Extrapolation!");
+            }
+
             int k = LeftSegmentIndex(t);
             return _c0[k] + (t - _x[k])*_c1[k];
         }