Add MedianFilter class for moving-window median (#2136)

This kind of filter is extremely useful for signals that are susceptible to sudden
outliers - ultrasonics, 1-D LIDAR, and results from vision processing are all
good use-cases.

This also modifies the existing ultrasonic examples accordingly.

wpilibc/src/main/native/include/frc/MedianFilter.h

@@ -0,0 +1,80 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 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.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#pragma once
+
+#include <algorithm>
+#include <vector>
+
+#include <wpi/Algorithm.h>
+#include <wpi/circular_buffer.h>
+
+namespace frc {
+/**
+ * A class that implements a moving-window median filter.  Useful for reducing
+ * measurement noise, especially with processes that generate occasional,
+ * extreme outliers (such as values from vision processing, LIDAR, or ultrasonic
+ * sensors).
+ */
+template <class T>
+class MedianFilter {
+ public:
+  /**
+   * Creates a new MedianFilter.
+   *
+   * @param size The number of samples in the moving window.
+   */
+  explicit MedianFilter(size_t size) : m_valueBuffer(size), m_size{size} {}
+
+  /**
+   * Calculates the moving-window median for the next value of the input stream.
+   *
+   * @param next The next input value.
+   * @return The median of the moving window, updated to include the next value.
+   */
+  T Calculate(T next) {
+    // Insert next value at proper point in sorted array
+    wpi::insert_sorted(m_orderedValues, next);
+
+    size_t curSize = m_orderedValues.size();
+
+    // If buffer is at max size, pop element off of end of circular buffer
+    // and remove from ordered list
+    if (curSize > m_size) {
+      m_orderedValues.erase(std::find(m_orderedValues.begin(),
+                                      m_orderedValues.end(),
+                                      m_valueBuffer.pop_back()));
+      curSize = curSize - 1;
+    }
+
+    // Add next value to circular buffer
+    m_valueBuffer.push_front(next);
+
+    if (curSize % 2 == 1) {
+      // If size is odd, return middle element of sorted list
+      return m_orderedValues[curSize / 2];
+    } else {
+      // If size is even, return average of middle elements
+      return (m_orderedValues[curSize / 2 - 1] + m_orderedValues[curSize / 2]) /
+             2.0;
+    }
+  }
+
+  /**
+   * Resets the filter, clearing the window of all elements.
+   */
+  void Reset() {
+    m_orderedValues.clear();
+    m_valueBuffer.reset();
+  }
+
+ private:
+  wpi::circular_buffer<T> m_valueBuffer;
+  std::vector<T> m_orderedValues;
+  size_t m_size;
+};
+}  // namespace frc