Add test for ParabolicEquation (need to add analytic solution)

Remove Main class

src/main/java/by/andd3dfx/math/pde/Equation.java

@@ -169,7 +169,7 @@ protected void prepare(double h, double tau) {
      * @param t        time
      */
     public void sUt(String fileName, double t) {
-        sUt(fileName, new double[]{0, t});
+        sUt(fileName, new double[]{t});
     }
 
     /**
@@ -202,7 +202,7 @@ public void sUt(String fileName, double t[]) {
      * @param x        space coordinate
      */
     public void sUx(String fileName, double x) {
-        sUx(fileName, new double[]{0, x});
+        sUx(fileName, new double[]{x});
     }
 
     /**

src/main/java/by/andd3dfx/math/pde/ParabolicEquation.java

@@ -2,7 +2,7 @@
 
 /**
  * Parabolic equation:
- * M(x,t,U)*dU_dt = K(x,t,U)*d2U_dt2 + V(x,t,U)*dU_dt + F(x,t,U) where U = U(x,t)
+ * M(x,t,U)*dU_dt = dU(K(x,t,U)*dU_dt)_dt + V(x,t,U)*dU_dt + F(x,t,U) where U = U(x,t)
  */
 public class ParabolicEquation extends Equation {
 

src/test/java/by/andd3dfx/math/pde/ParabolicEquationTest.java

@@ -0,0 +1,45 @@
+package by.andd3dfx.math.pde;
+
+import by.andd3dfx.math.Interval;
+import by.andd3dfx.util.FileUtil;
+import org.junit.jupiter.api.Test;
+
+class ParabolicEquationTest {
+
+    private final double C_LEFT = 1.0;
+    private final double THICKNESS = 1e-3;  // 1mm
+    private final double TIME = 1;          // 1sec
+    private final double DIFFUSION_COEFFICIENT = 1e-4;
+
+    @Test
+    void solve() {
+        var h = THICKNESS / 1000.0;
+        var tau = TIME / 1000.0;
+
+        var diffusionEquation = new ParabolicEquation(0, THICKNESS, TIME) {
+            @Override
+            protected double gK(double x, double t, double U) {
+                return DIFFUSION_COEFFICIENT;
+            }
+
+            @Override
+            protected double gLU(double t) {
+                return C_LEFT;
+            }
+        };
+
+        diffusionEquation.solve(h, tau);
+
+        diffusionEquation.sUt("./build/res-numeric.txt", TIME);
+        FileUtil.saveFunc(
+                new Interval(0, THICKNESS, h),
+                (x) -> analyticSolution(x, TIME),
+                "./build/res-analytic.txt"
+        );
+    }
+
+    private double analyticSolution(double x, double t) {
+        // TODO add analytic solution
+        return 0;
+    }
+}