[wpimath] Move SlewRateLimiter from wpilib to wpimath (#3399)

Timer was replaced with wpi::Now() to avoid a dependency on other wpilib
classes.

wpimath/src/main/native/include/frc/SlewRateLimiter.h

@@ -0,0 +1,73 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <algorithm>
+
+#include <wpi/timestamp.h>
+
+#include "units/time.h"
+
+namespace frc {
+/**
+ * A class that limits the rate of change of an input value.  Useful for
+ * implementing voltage, setpoint, and/or output ramps.  A slew-rate limit
+ * is most appropriate when the quantity being controlled is a velocity or
+ * a voltage; when controlling a position, consider using a TrapezoidProfile
+ * instead.
+ *
+ * @see TrapezoidProfile
+ */
+template <class Unit>
+class SlewRateLimiter {
+ public:
+  using Unit_t = units::unit_t<Unit>;
+  using Rate = units::compound_unit<Unit, units::inverse<units::seconds>>;
+  using Rate_t = units::unit_t<Rate>;
+
+  /**
+   * Creates a new SlewRateLimiter with the given rate limit and initial value.
+   *
+   * @param rateLimit The rate-of-change limit.
+   * @param initialValue The initial value of the input.
+   */
+  explicit SlewRateLimiter(Rate_t rateLimit, Unit_t initialValue = Unit_t{0})
+      : m_rateLimit{rateLimit},
+        m_prevVal{initialValue},
+        m_prevTime{units::microsecond_t(wpi::Now())} {}
+
+  /**
+   * Filters the input to limit its slew rate.
+   *
+   * @param input The input value whose slew rate is to be limited.
+   * @return The filtered value, which will not change faster than the slew
+   * rate.
+   */
+  Unit_t Calculate(Unit_t input) {
+    units::second_t currentTime = units::microsecond_t(wpi::Now());
+    units::second_t elapsedTime = currentTime - m_prevTime;
+    m_prevVal += std::clamp(input - m_prevVal, -m_rateLimit * elapsedTime,
+                            m_rateLimit * elapsedTime);
+    m_prevTime = currentTime;
+    return m_prevVal;
+  }
+
+  /**
+   * Resets the slew rate limiter to the specified value; ignores the rate limit
+   * when doing so.
+   *
+   * @param value The value to reset to.
+   */
+  void Reset(Unit_t value) {
+    m_prevVal = value;
+    m_prevTime = units::microsecond_t(wpi::Now());
+  }
+
+ private:
+  Rate_t m_rateLimit;
+  Unit_t m_prevVal;
+  units::second_t m_prevTime;
+};
+}  // namespace frc