Clean up LinearDigitalFilter class (#782)

* Renamed LinearDigitalFilter to LinearFilter
* Filter base class removed since it wasn't useful
* C++: std::shared_ptr<> replaced with double parameter

wpilibj/src/test/java/edu/wpi/first/wpilibj/LinearFilterNoiseTest.java

@@ -0,0 +1,65 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2015-2019 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+package edu.wpi.first.wpilibj;
+
+import java.util.Random;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.EnumSource;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+
+public class LinearFilterNoiseTest {
+  public enum TestType {
+    kSinglePoleIIR, kMovAvg
+  }
+
+  // Filter constants
+  public static final double kFilterStep = 0.005;
+  public static final double kFilterTime = 2.0;
+  public static final double kSinglePoleIIRTimeConstant = 0.015915;
+  public static final int kMovAvgTaps = 6;
+
+  @SuppressWarnings("ParameterName")
+  public static double getData(double t) {
+    return 100.0 * Math.sin(2.0 * Math.PI * t);
+  }
+
+  /**
+   * Test if the filter reduces the noise produced by a signal generator.
+   */
+  @ParameterizedTest
+  @EnumSource(TestType.class)
+  public void testNoiseReduce(TestType type) {
+    final LinearFilter filter;
+
+    if (type == TestType.kSinglePoleIIR) {
+      filter = LinearFilter.singlePoleIIR(kSinglePoleIIRTimeConstant, kFilterStep);
+    } else {
+      filter = LinearFilter.movingAverage(kMovAvgTaps);
+    }
+
+    double noiseGenError = 0.0;
+    double filterError = 0.0;
+
+    final Random gen = new Random();
+    final double kStdDev = 10.0;
+
+    for (double t = 0; t < kFilterTime; t += kFilterStep) {
+      final double theory = getData(t);
+      final double noise = gen.nextGaussian() * kStdDev;
+      filterError += Math.abs(filter.calculate(theory + noise) - theory);
+      noiseGenError += Math.abs(noise - theory);
+    }
+
+    assertTrue(noiseGenError > filterError,
+        "Filter should have reduced noise accumulation from " + noiseGenError
+        + " but failed. The filter error was " + filterError);
+  }
+}

wpilibj/src/test/java/edu/wpi/first/wpilibj/LinearFilterOutputTest.java

@@ -0,0 +1,85 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2015-2019 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+package edu.wpi.first.wpilibj;
+
+import java.util.function.DoubleFunction;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.EnumSource;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+
+public class LinearFilterOutputTest {
+  public enum TestType {
+    kSinglePoleIIR,
+    kHighPass,
+    kMovAvg,
+    kPulse
+  }
+
+  // Filter constants
+  public static final double kFilterStep = 0.005;
+  public static final double kFilterTime = 2.0;
+  public static final double kSinglePoleIIRTimeConstant = 0.015915;
+  public static final double kSinglePoleIIRExpectedOutput = -3.2172003;
+  public static final double kHighPassTimeConstant = 0.006631;
+  public static final double kHighPassExpectedOutput = 10.074717;
+  public static final int kMovAvgTaps = 6;
+  public static final double kMovAvgExpectedOutput = -10.191644;
+
+  @SuppressWarnings("ParameterName")
+  public static double getData(double t) {
+    return 100.0 * Math.sin(2.0 * Math.PI * t) + 20.0 * Math.cos(50.0 * Math.PI * t);
+  }
+
+  @SuppressWarnings("ParameterName")
+  public static double getPulseData(double t) {
+    if (Math.abs(t - 1.0) < 0.001) {
+      return 1.0;
+    } else {
+      return 0.0;
+    }
+  }
+
+  /**
+   * Test if the linear filters produce consistent output for a given data set.
+   */
+  @ParameterizedTest
+  @EnumSource(TestType.class)
+  public void testOutput(TestType type) {
+    final LinearFilter filter;
+    final DoubleFunction<Double> data;
+    final double expectedOutput;
+
+    if (type == TestType.kSinglePoleIIR) {
+      filter = LinearFilter.singlePoleIIR(kSinglePoleIIRTimeConstant, kFilterStep);
+      data = (double t) -> getData(t);
+      expectedOutput = kSinglePoleIIRExpectedOutput;
+    } else if (type == TestType.kHighPass) {
+      filter = LinearFilter.highPass(kHighPassTimeConstant, kFilterStep);
+      data = (double t) -> getData(t);
+      expectedOutput = kHighPassExpectedOutput;
+    } else if (type == TestType.kMovAvg) {
+      filter = LinearFilter.movingAverage(kMovAvgTaps);
+      data = (double t) -> getData(t);
+      expectedOutput = kMovAvgExpectedOutput;
+    } else {
+      filter = LinearFilter.movingAverage(kMovAvgTaps);
+      data = (double t) -> getPulseData(t);
+      expectedOutput = 0.0;
+    }
+
+    double filterOutput = 0.0;
+    for (double t = 0.0; t < kFilterTime; t += kFilterStep) {
+      filterOutput = filter.calculate(data.apply(t));
+    }
+
+    assertEquals(expectedOutput, filterOutput, 0.00005, "Filter output was incorrect.");
+  }
+}