Added linear digital filters

Linear digital filter class based on code from FRC team 341

Change-Id: I4c5198e36a089e08a6d054bf1bf80392def27e23

wpilibj/src/shared/java/edu/wpi/first/wpilibj/CircularBuffer.java

@@ -0,0 +1,125 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) FIRST 2015. 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;
+
+/**
+ * This is a simple circular buffer so we don't need to "bucket brigade" copy
+ * old values.
+ */
+public class CircularBuffer {
+  private double[] m_data;
+  private int m_front = 0;
+  private int m_length = 0;
+
+  public CircularBuffer(int size) {
+    m_data = new double[size];
+    for (double i : m_data) {
+      i = 0.0;
+    }
+  }
+
+  /**
+   * Push new value onto front of the buffer. The value at the back is
+   * overwritten if the buffer is full.
+   */
+  public void pushFront(double value) {
+    if (m_data.length == 0) {
+      return;
+    }
+
+    m_front = moduloDec(m_front);
+
+    m_data[m_front] = value;
+
+    if (m_length < m_data.length) {
+      m_length++;
+    }
+  }
+
+  /**
+   * Push new value onto back of the buffer. The value at the front is
+   * overwritten if the buffer is full.
+   */
+  public void pushBack(double value) {
+    if (m_data.length == 0) {
+      return;
+    }
+
+    m_data[(m_front + m_length) % m_data.length] = value;
+
+    if (m_length < m_data.length) {
+      m_length++;
+    } else {
+      // Increment front if buffer is full to maintain size
+      m_front = moduloInc(m_front);
+    }
+  }
+
+  /**
+   * Pop value at front of buffer.
+   *
+   * @return value at front of buffer
+   */
+  public double popFront() {
+    // If there are no elements in the buffer, do nothing
+    if (m_length == 0) {
+      return 0.0;
+    }
+
+    double temp = m_data[m_front];
+    m_front = moduloInc(m_front);
+    m_length--;
+    return temp;
+  }
+
+
+  /**
+   * Pop value at back of buffer.
+   */
+  public double popBack() {
+    // If there are no elements in the buffer, do nothing
+    if (m_length == 0) {
+      return 0.0;
+    }
+
+    m_length--;
+    return m_data[(m_front + m_length) % m_data.length];
+  }
+
+  public void reset() {
+    for (double i : m_data) {
+      i = 0.0;
+    }
+    m_front = 0;
+    m_length = 0;
+  }
+
+  /**
+   * @return element at index starting from front of buffer.
+   */
+  public double get(int index) {
+    return m_data[(m_front + index) % m_data.length];
+  }
+
+  /**
+   * Increment an index modulo the length of the m_data buffer
+   */
+  private int moduloInc(int index) {
+    return (index + 1) % m_data.length;
+  }
+
+  /**
+   * Decrement an index modulo the length of the m_data buffer
+   */
+  private int moduloDec(int index) {
+    if (index == 0) {
+      return m_data.length - 1;
+    } else {
+      return index - 1;
+    }
+  }
+}