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<div class="subTitle">org.apache.commons.math3.ode.events</div>
<h2 title="Class EventFilter" class="title">Class EventFilter</h2>
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<li>org.apache.commons.math3.ode.events.EventFilter</li>
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<dt>All Implemented Interfaces:</dt>
<dd><a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events">EventHandler</a></dd>
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<pre>public class <span class="strong">EventFilter</span>
extends <a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</a>
implements <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events">EventHandler</a></pre>
<div class="block">Wrapper used to detect only increasing or decreasing events.
<p>General <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events"><code>events</code></a> are defined implicitly
by a <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html#g(double,%20double[])"><code>g function</code></a> crossing
zero. This function needs to be continuous in the event neighborhood,
and its sign must remain consistent between events. This implies that
during an ODE integration, events triggered are alternately events
for which the function increases from negative to positive values,
and events for which the function decreases from positive to
negative values.
</p>
<p>Sometimes, users are only interested in one type of event (say
increasing events for example) and not in the other type. In these
cases, looking precisely for all events location and triggering
events that will later be ignored is a waste of computing time.</p>
<p>Users can wrap a regular <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events"><code>event handler</code></a> in
an instance of this class and provide this wrapping instance to
the <a href="../../../../../../org/apache/commons/math3/ode/FirstOrderIntegrator.html" title="interface in org.apache.commons.math3.ode"><code>ODE solver</code></a>
in order to avoid wasting time looking for uninteresting events.
The wrapper will intercept the calls to the <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html#g(double,%20double[])"><code>g function</code></a> and to the <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html#eventOccurred(double,%20double[],%20boolean)"><code>eventOccurred</code></a> method in order to ignore uninteresting events. The
wrapped regular <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events"><code>event handler</code></a> will the see only
the interesting events, i.e. either only <code>increasing</code> events or
<code>decreasing</code> events. the number of calls to the <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html#g(double,%20double[])"><code>g function</code></a> will also be reduced.</p></div>
<dl><dt><span class="strong">Since:</span></dt>
<dd>3.2</dd></dl>
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<h3>Nested classes/interfaces inherited from interface org.apache.commons.math3.ode.events.<a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events">EventHandler</a></h3>
<code><a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html" title="enum in org.apache.commons.math3.ode.events">EventHandler.Action</a></code></li>
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<th class="colOne" scope="col">Constructor and Description</th>
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<td class="colOne"><code><strong><a href="../../../../../../org/apache/commons/math3/ode/events/EventFilter.html#EventFilter(org.apache.commons.math3.ode.events.EventHandler,%20org.apache.commons.math3.ode.events.FilterType)">EventFilter</a></strong>(<a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events">EventHandler</a> rawHandler,
<a href="../../../../../../org/apache/commons/math3/ode/events/FilterType.html" title="enum in org.apache.commons.math3.ode.events">FilterType</a> filter)</code>
<div class="block">Wrap an <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events"><code>event handler</code></a>.</div>
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<th class="colFirst" scope="col">Modifier and Type</th>
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<td class="colFirst"><code><a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html" title="enum in org.apache.commons.math3.ode.events">EventHandler.Action</a></code></td>
<td class="colLast"><code><strong><a href="../../../../../../org/apache/commons/math3/ode/events/EventFilter.html#eventOccurred(double,%20double[],%20boolean)">eventOccurred</a></strong>(double t,
double[] y,
boolean increasing)</code>
<div class="block">Handle an event and choose what to do next.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><code>double</code></td>
<td class="colLast"><code><strong><a href="../../../../../../org/apache/commons/math3/ode/events/EventFilter.html#g(double,%20double[])">g</a></strong>(double t,
double[] y)</code>
<div class="block">Compute the value of the switching function.</div>
</td>
</tr>
<tr class="altColor">
<td class="colFirst"><code>void</code></td>
<td class="colLast"><code><strong><a href="../../../../../../org/apache/commons/math3/ode/events/EventFilter.html#init(double,%20double[],%20double)">init</a></strong>(double t0,
double[] y0,
double t)</code>
<div class="block">Initialize event handler at the start of an ODE integration.</div>
</td>
</tr>
<tr class="rowColor">
<td class="colFirst"><code>void</code></td>
<td class="colLast"><code><strong><a href="../../../../../../org/apache/commons/math3/ode/events/EventFilter.html#resetState(double,%20double[])">resetState</a></strong>(double t,
double[] y)</code>
<div class="block">Reset the state prior to continue the integration.</div>
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<a name="EventFilter(org.apache.commons.math3.ode.events.EventHandler, org.apache.commons.math3.ode.events.FilterType)">
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<h4>EventFilter</h4>
<pre>public EventFilter(<a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events">EventHandler</a> rawHandler,
<a href="../../../../../../org/apache/commons/math3/ode/events/FilterType.html" title="enum in org.apache.commons.math3.ode.events">FilterType</a> filter)</pre>
<div class="block">Wrap an <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events"><code>event handler</code></a>.</div>
<dl><dt><span class="strong">Parameters:</span></dt><dd><code>rawHandler</code> - event handler to wrap</dd><dd><code>filter</code> - filter to use</dd></dl>
</li>
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<h3>Method Detail</h3>
<a name="init(double, double[], double)">
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<h4>init</h4>
<pre>public void init(double t0,
double[] y0,
double t)</pre>
<div class="block">Initialize event handler at the start of an ODE integration.
<p>
This method is called once at the start of the integration. It
may be used by the event handler to initialize some internal data
if needed.
</p></div>
<dl>
<dt><strong>Specified by:</strong></dt>
<dd><code><a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html#init(double,%20double[],%20double)">init</a></code> in interface <code><a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events">EventHandler</a></code></dd>
<dt><span class="strong">Parameters:</span></dt><dd><code>t0</code> - start value of the independent <i>time</i> variable</dd><dd><code>y0</code> - array containing the start value of the state vector</dd><dd><code>t</code> - target time for the integration</dd></dl>
</li>
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<a name="g(double, double[])">
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<h4>g</h4>
<pre>public double g(double t,
double[] y)</pre>
<div class="block">Compute the value of the switching function.
<p>The discrete events are generated when the sign of this
switching function changes. The integrator will take care to change
the stepsize in such a way these events occur exactly at step boundaries.
The switching function must be continuous in its roots neighborhood
(but not necessarily smooth), as the integrator will need to find its
roots to locate precisely the events.</p>
<p>Also note that the integrator expect that once an event has occurred,
the sign of the switching function at the start of the next step (i.e.
just after the event) is the opposite of the sign just before the event.
This consistency between the steps <string>must</strong> be preserved,
otherwise <a href="../../../../../../org/apache/commons/math3/exception/NoBracketingException.html" title="class in org.apache.commons.math3.exception"><code>exceptions</code></a> related to root not being bracketed will occur.</p>
<p>This need for consistency is sometimes tricky to achieve. A typical
example is using an event to model a ball bouncing on the floor. The first
idea to represent this would be to have <code>g(t) = h(t)</code> where h is the
height above the floor at time <code>t</code>. When <code>g(t)</code> reaches 0, the
ball is on the floor, so it should bounce and the typical way to do this is
to reverse its vertical velocity. However, this would mean that before the
event <code>g(t)</code> was decreasing from positive values to 0, and after the
event <code>g(t)</code> would be increasing from 0 to positive values again.
Consistency is broken here! The solution here is to have <code>g(t) = sign
* h(t)</code>, where sign is a variable with initial value set to <code>+1</code>. Each
time <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html#eventOccurred(double,%20double[],%20boolean)"><code>eventOccurred</code></a> is called,
<code>sign</code> is reset to <code>-sign</code>. This allows the <code>g(t)</code>
function to remain continuous (and even smooth) even across events, despite
<code>h(t)</code> is not. Basically, the event is used to <em>fold</em> <code>h(t)</code>
at bounce points, and <code>sign</code> is used to <em>unfold</em> it back, so the
solvers sees a <code>g(t)</code> function which behaves smoothly even across events.</p></div>
<dl>
<dt><strong>Specified by:</strong></dt>
<dd><code><a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html#g(double,%20double[])">g</a></code> in interface <code><a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events">EventHandler</a></code></dd>
<dt><span class="strong">Parameters:</span></dt><dd><code>t</code> - current value of the independent <i>time</i> variable</dd><dd><code>y</code> - array containing the current value of the state vector</dd>
<dt><span class="strong">Returns:</span></dt><dd>value of the g switching function</dd></dl>
</li>
</ul>
<a name="eventOccurred(double, double[], boolean)">
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<h4>eventOccurred</h4>
<pre>public <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html" title="enum in org.apache.commons.math3.ode.events">EventHandler.Action</a> eventOccurred(double t,
double[] y,
boolean increasing)</pre>
<div class="block">Handle an event and choose what to do next.
<p>This method is called when the integrator has accepted a step
ending exactly on a sign change of the function, just <em>before</em>
the step handler itself is called (see below for scheduling). It
allows the user to update his internal data to acknowledge the fact
the event has been handled (for example setting a flag in the <a href="../../../../../../org/apache/commons/math3/ode/FirstOrderDifferentialEquations.html" title="interface in org.apache.commons.math3.ode"><code>differential equations</code></a> to switch the derivatives computation in
case of discontinuity), or to direct the integrator to either stop
or continue integration, possibly with a reset state or derivatives.</p>
<ul>
<li>if <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html#STOP"><code>EventHandler.Action.STOP</code></a> is returned, the step handler will be called
with the <code>isLast</code> flag of the <a href="../../../../../../org/apache/commons/math3/ode/sampling/StepHandler.html#handleStep(org.apache.commons.math3.ode.sampling.StepInterpolator,%20boolean)"><code>handleStep</code></a>
method set to true and the integration will be stopped,</li>
<li>if <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html#RESET_STATE"><code>EventHandler.Action.RESET_STATE</code></a> is returned, the <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html#resetState(double,%20double[])"><code>resetState</code></a> method will be called once the step handler has
finished its task, and the integrator will also recompute the
derivatives,</li>
<li>if <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html#RESET_DERIVATIVES"><code>EventHandler.Action.RESET_DERIVATIVES</code></a> is returned, the integrator
will recompute the derivatives,
<li>if <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html#CONTINUE"><code>EventHandler.Action.CONTINUE</code></a> is returned, no specific action will
be taken (apart from having called this method) and integration
will continue.</li>
</ul>
<p>The scheduling between this method and the <a href="../../../../../../org/apache/commons/math3/ode/sampling/StepHandler.html" title="interface in org.apache.commons.math3.ode.sampling"><code>StepHandler</code></a> method <a href="../../../../../../org/apache/commons/math3/ode/sampling/StepHandler.html#handleStep(org.apache.commons.math3.ode.sampling.StepInterpolator,%20boolean)"><code>handleStep(interpolator, isLast)</code></a> is to call this method first and
<code>handleStep</code> afterwards. This scheduling allows the integrator to
pass <code>true</code> as the <code>isLast</code> parameter to the step
handler to make it aware the step will be the last one if this method
returns <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html#STOP"><code>EventHandler.Action.STOP</code></a>. As the interpolator may be used to navigate back
throughout the last step (as <a href="../../../../../../org/apache/commons/math3/ode/sampling/StepNormalizer.html" title="class in org.apache.commons.math3.ode.sampling"><code>StepNormalizer</code></a>
does for example), user code called by this method and user
code called by step handlers may experience apparently out of order values
of the independent time variable. As an example, if the same user object
implements both this <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events"><code>EventHandler</code></a> interface and the
<a href="../../../../../../org/apache/commons/math3/ode/sampling/FixedStepHandler.html" title="interface in org.apache.commons.math3.ode.sampling"><code>FixedStepHandler</code></a>
interface, a <em>forward</em> integration may call its
<code>eventOccurred</code> method with t = 10 first and call its
<code>handleStep</code> method with t = 9 afterwards. Such out of order
calls are limited to the size of the integration step for <a href="../../../../../../org/apache/commons/math3/ode/sampling/StepHandler.html" title="interface in org.apache.commons.math3.ode.sampling"><code>variable step handlers</code></a> and
to the size of the fixed step for <a href="../../../../../../org/apache/commons/math3/ode/sampling/FixedStepHandler.html" title="interface in org.apache.commons.math3.ode.sampling"><code>fixed step handlers</code></a>.</p></div>
<dl>
<dt><strong>Specified by:</strong></dt>
<dd><code><a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html#eventOccurred(double,%20double[],%20boolean)">eventOccurred</a></code> in interface <code><a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events">EventHandler</a></code></dd>
<dt><span class="strong">Parameters:</span></dt><dd><code>t</code> - current value of the independent <i>time</i> variable</dd><dd><code>y</code> - array containing the current value of the state vector</dd><dd><code>increasing</code> - if true, the value of the switching function increases
when times increases around event (note that increase is measured with respect
to physical time, not with respect to integration which may go backward in time)</dd>
<dt><span class="strong">Returns:</span></dt><dd>indication of what the integrator should do next, this
value must be one of <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html#STOP"><code>EventHandler.Action.STOP</code></a>, <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html#RESET_STATE"><code>EventHandler.Action.RESET_STATE</code></a>,
<a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html#RESET_DERIVATIVES"><code>EventHandler.Action.RESET_DERIVATIVES</code></a> or <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html#CONTINUE"><code>EventHandler.Action.CONTINUE</code></a></dd></dl>
</li>
</ul>
<a name="resetState(double, double[])">
<!-- -->
</a>
<ul class="blockListLast">
<li class="blockList">
<h4>resetState</h4>
<pre>public void resetState(double t,
double[] y)</pre>
<div class="block">Reset the state prior to continue the integration.
<p>This method is called after the step handler has returned and
before the next step is started, but only when <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html#eventOccurred(double,%20double[],%20boolean)"><code>EventHandler.eventOccurred(double, double[], boolean)</code></a> has itself returned the <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html#RESET_STATE"><code>EventHandler.Action.RESET_STATE</code></a>
indicator. It allows the user to reset the state vector for the
next step, without perturbing the step handler of the finishing
step. If the <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html#eventOccurred(double,%20double[],%20boolean)"><code>EventHandler.eventOccurred(double, double[], boolean)</code></a> never returns the <a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.Action.html#RESET_STATE"><code>EventHandler.Action.RESET_STATE</code></a> indicator, this function will never be called, and it is
safe to leave its body empty.</p></div>
<dl>
<dt><strong>Specified by:</strong></dt>
<dd><code><a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html#resetState(double,%20double[])">resetState</a></code> in interface <code><a href="../../../../../../org/apache/commons/math3/ode/events/EventHandler.html" title="interface in org.apache.commons.math3.ode.events">EventHandler</a></code></dd>
<dt><span class="strong">Parameters:</span></dt><dd><code>t</code> - current value of the independent <i>time</i> variable</dd><dd><code>y</code> - array containing the current value of the state vector
the new state should be put in the same array</dd></dl>
</li>
</ul>
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