## org.apache.commons.math.optimization.fitting
Class HarmonicCoefficientsGuesser

    java.lang.Object
      org.apache.commons.math.optimization.fitting.HarmonicCoefficientsGuesser

    public class HarmonicCoefficientsGuesser

extends java.lang.Object

This class guesses harmonic coefficients from a sample.

The algorithm used to guess the coefficients is as follows:

We know f (t) at some sampling points ti and want to find a, ω and φ such that f (t) = a cos (ω t + φ).

From the analytical expression, we can compute two primitives :

         If2  (t) = ∫ f2  = a2 × [t + S (t)] / 2
         If'2 (t) = ∫ f'2 = a2 ω2 × [t - S (t)] / 2
         where S (t) = sin (2 (ω t + φ)) / (2 ω)

We can remove S between these expressions :

         If'2 (t) = a2 ω2 t - ω2 If2 (t)

The preceding expression shows that If'2 (t) is a linear combination of both t and If2 (t): If'2 (t) = A × t + B × If2 (t)

From the primitive, we can deduce the same form for definite integrals between t1 and ti for each ti :

       If2 (ti) - If2 (t1) = A × (ti - t1) + B × (If2 (ti) - If2 (t1))

We can find the coefficients A and B that best fit the sample to this linear expression by computing the definite integrals for each sample points.

For a bilinear expression z (xi, yi) = A × xi + B × yi, the coefficients A and B that minimize a least square criterion ∑ (zi - z (xi, yi))2 are given by these expressions:

             ∑yiyi ∑xizi - ∑xiyi ∑yizi
         A = ------------------------
             ∑xixi ∑yiyi - ∑xiyi ∑xiyi

             ∑xixi ∑yizi - ∑xiyi ∑xizi
         B = ------------------------
             ∑xixi ∑yiyi - ∑xiyi ∑xiyi

In fact, we can assume both a and ω are positive and compute them directly, knowing that A = a2 ω2 and that B = - ω2. The complete algorithm is therefore:

     for each ti from t1 to tn-1, compute:
       f  (ti)
       f' (ti) = (f (ti+1) - f(ti-1)) / (ti+1 - ti-1)
       xi = ti - t1
       yi = ∫ f2 from t1 to ti
       zi = ∫ f'2 from t1 to ti
       update the sums ∑xixi, ∑yiyi, ∑xiyi, ∑xizi and ∑yizi
     end for

                |--------------------------
             \  | ∑yiyi ∑xizi - ∑xiyi ∑yizi
     a     =  \ | ------------------------
               \| ∑xiyi ∑xizi - ∑xixi ∑yizi

                |--------------------------
             \  | ∑xiyi ∑xizi - ∑xixi ∑yizi
     ω     =  \ | ------------------------
               \| ∑xixi ∑yiyi - ∑xiyi ∑xiyi

Once we know ω, we can compute:

        fc = ω f (t) cos (ω t) - f' (t) sin (ω t)
        fs = ω f (t) sin (ω t) + f' (t) cos (ω t)

It appears that `fc = a ω cos (φ)` and `fs = -a ω sin (φ)`, so we can use these expressions to compute φ. The best estimate over the sample is given by averaging these expressions.

Since integrals and means are involved in the preceding estimations, these operations run in O(n) time, where n is the number of measurements.

Since:
  2.0
Version:
  $Revision: 786479 $ $Date: 2009-06-19 08:36:16 -0400 (Fri, 19 Jun 2009) $

Field Summary
`private double` `a` Guessed amplitude.
`private WeightedObservedPoint[]` `observations` Sampled observations.
`private double` `omega` Guessed pulsation ω.
`private double` `phi` Guessed phase φ.

Constructor Summary
`HarmonicCoefficientsGuesser(WeightedObservedPoint[] observations)` Simple constructor.

Method Summary
`double` `getGuessedAmplitude()` Get the guessed amplitude a.
`double` `getGuessedPhase()` Get the guessed phase φ.
`double` `getGuessedPulsation()` Get the guessed pulsation ω.
`void` `guess()` Estimate a first guess of the coefficients.
`private void` `guessAOmega()` Estimate a first guess of the a and ω coefficients.
`private void` `guessPhi()` Estimate a first guess of the φ coefficient.
`private void` `sortObservations()` Sort the observations with respect to the abscissa.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Field Detail

### observations

    private final WeightedObservedPoint[] observations

  Sampled observations.

### a

    private double a

  Guessed amplitude.

### omega

    private double omega

  Guessed pulsation ω.

### phi

    private double phi

  Guessed phase φ.

Constructor Detail

### HarmonicCoefficientsGuesser

    public HarmonicCoefficientsGuesser(WeightedObservedPoint[] observations)

  Simple constructor.

  Parameters:
    `observations` - sampled observations

Method Detail

### guess

    public void guess()
               throws OptimizationException

  Estimate a first guess of the coefficients.

  Throws:
    `OptimizationException` - if the sample is too short or if the first guess cannot be computed (when the elements under the square roots are negative).

### sortObservations

    private void sortObservations()

  Sort the observations with respect to the abscissa.

### guessAOmega

    private void guessAOmega()
                      throws OptimizationException

  Estimate a first guess of the a and ω coefficients.

  Throws:
    `OptimizationException` - if the sample is too short or if the first guess cannot be computed (when the elements under the square roots are negative).

### guessPhi

    private void guessPhi()

  Estimate a first guess of the φ coefficient.

### getGuessedAmplitude

    public double getGuessedAmplitude()

  Get the guessed amplitude a.

  Returns:
    guessed amplitude a;

### getGuessedPulsation

    public double getGuessedPulsation()

  Get the guessed pulsation ω.

  Returns:
    guessed pulsation ω

### getGuessedPhase

    public double getGuessedPhase()

  Get the guessed phase φ.

  Returns:
    guessed phase φ

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