Updated standard source header. Created a time function mechanism that can be

used to easily plug in different methods of smoothly varying a value over time.


git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@3834 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/java/com/threerings/jme/util/TimeFunction.java

@@ -0,0 +1,93 @@
+//
+// $Id: TimeFunction.java 3122 2004-09-18 22:57:08Z mdb $
+//
+// Narya library - tools for developing networked games
+// Copyright (C) 2002-2006 Three Rings Design, Inc., All Rights Reserved
+// http://www.threerings.net/code/narya/
+//
+// This library is free software; you can redistribute it and/or modify it
+// under the terms of the GNU Lesser General Public License as published
+// by the Free Software Foundation; either version 2.1 of the License, or
+// (at your option) any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+// Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public
+// License along with this library; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+
+package com.threerings.jme.util;
+
+/**
+ * Used to vary a value over time where time is provided at discrete increments
+ * (on the frame tick) and the value is computed appropriately. This can be
+ * used for fades and other effects where different functions (linear, ease-in
+ * ease-out, etc.) should be easy to plug in.
+ */
+public abstract class TimeFunction
+{
+    /**
+     * Every time function varies a value from some starting value to some
+     * ending value over some duration. The way in which it varies
+     * (linearly, for example) is up to the derived class.
+     *
+     * @param start the starting value.
+     * @param end the ending value.
+     * @param duration the duration in seconds.
+     */
+    public TimeFunction (float start, float end, float duration)
+    {
+        _start = start;
+        _end = end;
+        _duration = duration;
+    }
+
+    /**
+     * Returns the current value given the supplied elapsed time. The value
+     * will be bounded to the originally supplied starting and ending values at
+     * times 0 (and below) and {@link #_duration} (and above) respectively.
+     *
+     * @param deltaTime the amount of time that has elapsed since the last call
+     * to this method.
+     */
+    public float getValue (float deltaTime)
+    {
+        _elapsed += deltaTime;
+
+        if (_elapsed <= 0) {
+            return _start;
+        } else if (_elapsed >= _duration) {
+            return _end;
+        } else {
+            return computeValue();
+        }
+    }
+
+    /**
+     * Returns true if this function has proceeded the full length of its
+     * duration. This should be called after a call to {@link #getValue} has
+     * been made to update our internal elapsed time.
+     */
+    public boolean isComplete ()
+    {
+        return _elapsed >= _duration;
+    }
+
+    /**
+     * This must be implemented by our derived class to compute our value given
+     * the currently stored {@link #_elapsed} time.
+     */
+    protected abstract float computeValue ();
+
+    /** Our starting and ending values. */
+    protected float _start, _end;
+
+    /** The number of milliseconds over which we vary our value. */
+    protected float _duration;
+
+    /** The number of seconds that have elapsed since we started. */
+    protected float _elapsed;
+}