[jira] Commented: (MATH-325) Improvement of Romberg extrapolation

["Luc Maisonobe (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/MATH-325?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12794719#action_12794719 ] 

Luc Maisonobe commented on MATH-325:
------------------------------------

I think this issue has already been solved since 2009-10-13. Could you check the current implementation from subversion repository ?
The fix was introduced in r824822 with the following comment:
{quote}
improved array structure in Romberg integrator
since the integrator uses only two rows at a time, we allocate only two arrays and reuse them
{quote}


> Improvement of Romberg extrapolation
> ------------------------------------
>
>                 Key: MATH-325
>                 URL: https://issues.apache.org/jira/browse/MATH-325
>             Project: Commons Math
>          Issue Type: Improvement
>    Affects Versions: 2.0
>            Reporter: Andreas mueller
>             Fix For: 2.1
>
>   Original Estimate: 2h
>  Remaining Estimate: 2h
>
> One can use a one-dimensional array (instead of Romberg's tableau) for extrapolating subsequent values.
> Please have a look at following code fragments (which I've taken form the class RombergExtrapolator of
> my MathLibrary). Feel free to use this code.
> 	/**
> 	 * Default number of maximal extrapolation steps.
> 	 */
> 	public static int DEF_MAXIMAL_EXTRAPOLATION_COUNT = 8;
> 	/**
> 	 * The approximation order. <br>
> 	 * Assume that f(h) is approximated by a function a(h), so that f(h) = a(h) +
> 	 * O(h<sup>p</sup>). We say that p is the approximation order.
> 	 */
> 	private int approximationOrder;
> 	private int extrapolationCount = 0;
> 	private double prevResult;
> 	
> 	/**
> 	 * The estimate and tolerance may be used to deside wether to finalize the
> 	 * iteration process (|estimate| < tolerance).
> 	 */
> 	 
> 	/** Holds the current estimated error. */
> 	private double estimate;
> 	/** Holds the current reached tolerance. */
> 	private double tolerance;
> 	
> 	private double result[] = new double[DEF_MAXIMAL_EXTRAPOLATION_COUNT + 1];;
> 	/**
> 	 * Set the maximal number of subsequent extrapolation steps.
> 	 * 
> 	 * @param maximalExtrapolationCount
> 	 *            maximal extrapolation steps
> 	 */
> 	public void setMaximalExtrapolationCount(int maximalExtrapolationCount)
> 	{
> 		result = new double[maximalExtrapolationCount + 1];
> 	}
> 	/**
> 	 * Extrapolate a sequence of values by means of Romberg's algorithm.
> 	 * Therefore a polynomial of degree maximalExtraploationCount
> 	 * is used. Calculates the current estimate and tolerance using the
> 	 * approximation order.
> 	 * 
> 	 * @param value
> 	 *            value to extrapolate
> 	 * @return extrapolated value
> 	 */
> 	public double extrapolate(double value)
> 	{
> 		if (extrapolationCount == 0) {
> 			// first estimate
> 			estimate = value;
> 			tolerance = -1.0;
> 			prevResult = 0;
> 		}
>  
> 		int i, m, m1 = idx(extrapolationCount);
> 		long k = (1 << approximationOrder);
> 		int imin = Math.max(0, extrapolationCount - (result.length - 1));
> 		result[m1] = value;
> 		for (i = extrapolationCount - 1; i >= imin; i--) {
> 			m = idx(i);
> 			m1 = idx(i + 1);
> 			result[m] = (k * result[m1] - result[m]) / (k - 1);
> 			k <<= approximationOrder;
> 		}
> 		m1 = idx(i + 1);
> 		estimate = result[m1] - prevResult;
> 		tolerance = Math.abs(result[m1]) * relativeAccuracy + absoluteAccuracy;
> 		prevResult = result[m1];
> 		extrapolationCount++;
> 		return result[m1];
> 	}
> 	/**
> 	 * Ring buffer index
> 	 */
> 	private int idx(int i)
> 	{
> 		return (i % result.length);
> 	}

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