package edu.northwestern.at.utils.math.rootfinders;
/* Please see the license information at the end of this file. */
import edu.northwestern.at.utils.math.*;
/** Find interval bracketing a root.
*/
public class BracketRoot
{
/** Find interval bracketing a root.
*
* @param bracket Array containing initial estimate for
* of bracketing interval.
*
* bracket[0] is left hand estimate.
* bracket[1] is right hand estimate.
*
* @param function Function whose root is to be bracketed.
*
* @param maxIter Maximum number of iterations to try
* expanding bracket.
*
* @param expansionFactor Factor by which to expand bracket interval
* on each iteration.
*
* @return True if bracket successfully found
* The new bracket will be placed in
* "bracket".
*
* False if bracket could not be found within
* maxIter attempts.
*/
public static boolean bracketRoot
(
double bracket[],
MonadicFunction function,
int maxIter,
double expansionFactor
)
{
// Check for valid initial bracket.
if ( ( bracket == null ) || ( bracket.length < 2 ) ||
( bracket[ 0 ] == bracket[ 1 ] ) )
{
throw new IllegalArgumentException( "initial bracket bad" );
}
// Compute function at initial
// left and right bracket points.
double fLeft = function.f( bracket[ 0 ] );
double fRight = function.f( bracket[ 1 ] );
for ( int iter = 1; iter <= maxIter; iter++ )
{
// If the left and right function
// values are of different sign,
// a zero is bracketed between them.
if ( ( fLeft * fRight ) < 0.0 ) return true;
// Determine which end of the bracket
// to expand.
if ( Math.abs( fLeft ) < Math.abs( fRight ) )
{
bracket[ 0 ] =
expansionFactor * ( bracket[ 0 ] - bracket[ 1 ] );
fLeft = function.f( bracket[ 0 ] );
}
else
{
bracket[ 1 ] =
expansionFactor * ( bracket[ 1 ] - bracket[ 0 ] );
fRight = function.f( bracket[ 1 ] );
}
}
return false;
}
}
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