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## org.apache.commons.math.transform
Class FastHadamardTransformer

    java.lang.Object
      org.apache.commons.math.transform.FastHadamardTransformer

All Implemented Interfaces:
  RealTransformer

    public class FastHadamardTransformer

extends Object
implements RealTransformer

Implements the Fast Hadamard Transform (FHT). Transformation of an input vector x to the output vector y.

In addition to transformation of real vectors, the Hadamard transform can transform integer vectors into integer vectors. However, this integer transform cannot be inverted directly. Due to a scaling factor it may lead to rational results. As an example, the inverse transform of integer vector (0, 1, 0, 1) is rational vector (1/2, -1/2, 0, 0).

Since:
  2.0
Version:
  $Revision: 1070725 $ $Date: 2011-02-15 02:31:12 +0100 (mar. 15 févr. 2011) $

Constructor Summary
`FastHadamardTransformer()`

Method Summary
`protected double[]` | `fht(double[] x)` The FHT (Fast Hadamard Transformation) which uses only subtraction and addition.
`protected int[]` | `fht(int[] x)` The FHT (Fast Hadamard Transformation) which uses only subtraction and addition.
`double[]` | `inversetransform(double[] f)` Inversely transform the given real data set.
`double[]` | `inversetransform(UnivariateRealFunction f, double min, double max, int n)` Inversely transform the given real function, sampled on the given interval.
`double[]` | `transform(double[] f)` Transform the given real data set.
`int[]` | `transform(int[] f)` Transform the given real data set.
`double[]` | `transform(UnivariateRealFunction f, double min, double max, int n)` Transform the given real function, sampled on the given interval.

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

Constructor Detail

### FastHadamardTransformer

    public FastHadamardTransformer()

Method Detail

### transform

    public double[] transform(double[] f)
                       throws IllegalArgumentException

  Transform the given real data set.

  Specified by:
    `transform` in interface `RealTransformer`
  Parameters:
    `f` - the real data array to be transformed (signal)
  Returns:
    the real transformed array (spectrum)
  Throws:
    `IllegalArgumentException` - if any parameters are invalid

### transform

    public double[] transform(UnivariateRealFunction f,
                              double min,
                              double max,
                              int n)
                       throws FunctionEvaluationException,
                              IllegalArgumentException

  Transform the given real function, sampled on the given interval.

  Specified by:
    `transform` in interface `RealTransformer`
  Parameters:
    `f` - the function to be sampled and transformed
    `min` - the lower bound for the interval
    `max` - the upper bound for the interval
    `n` - the number of sample points
  Returns:
    the real transformed array
  Throws:
    `FunctionEvaluationException` - if function cannot be evaluated at some point
    `IllegalArgumentException` - if any parameters are invalid

### inversetransform

    public double[] inversetransform(double[] f)
                              throws IllegalArgumentException

  Inversely transform the given real data set.

  Specified by:
    `inversetransform` in interface `RealTransformer`
  Parameters:
    `f` - the real data array to be inversely transformed (spectrum)
  Returns:
    the real inversely transformed array (signal)
  Throws:
    `IllegalArgumentException` - if any parameters are invalid

### inversetransform

    public double[] inversetransform(UnivariateRealFunction f,
                                     double min,
                                     double max,
                                     int n)
                              throws FunctionEvaluationException,
                                     IllegalArgumentException

  Inversely transform the given real function, sampled on the given interval.

  Specified by:
    `inversetransform` in interface `RealTransformer`
  Parameters:
    `f` - the function to be sampled and inversely transformed
    `min` - the lower bound for the interval
    `max` - the upper bound for the interval
    `n` - the number of sample points
  Returns:
    the real inversely transformed array
  Throws:
    `FunctionEvaluationException` - if function cannot be evaluated at some point
    `IllegalArgumentException` - if any parameters are invalid

### transform

    public int[] transform(int[] f)
                    throws IllegalArgumentException

  Transform the given real data set.

  The integer transform cannot be inverted directly, due to a scaling factor it may lead to double results.

  Parameters:
    `f` - the integer data array to be transformed (signal)
  Returns:
    the integer transformed array (spectrum)
  Throws:
    `IllegalArgumentException` - if any parameters are invalid

### fht

    protected double[] fht(double[] x)
                    throws IllegalArgumentException

  The FHT (Fast Hadamard Transformation) which uses only subtraction and addition.
  Requires Nlog2N = n2n additions.
  Short Table of manual calculation for N=8:
  1. x is the input vector we want to transform
  2. y is the output vector which is our desired result
  3. a and b are just helper rows

       +----+----------+---------+----------+
       | x  |    a     |    b    |    y     |
       +----+----------+---------+----------+
       | x0 | a0=x0+x1 | b0=a0+a1 | y0=b0+b1 |
       +----+----------+---------+----------+
       | x1 | a1=x2+x3 | b0=a2+a3 | y0=b2+b3 |
       +----+----------+---------+----------+
       | x2 | a2=x4+x5 | b0=a4+a5 | y0=b4+b5 |
       +----+----------+---------+----------+
       | x3 | a3=x6+x7 | b0=a6+a7 | y0=b6+b7 |
       +----+----------+---------+----------+
       | x4 | a0=x0-x1 | b0=a0-a1 | y0=b0-b1 |
       +----+----------+---------+----------+
       | x5 | a1=x2-x3 | b0=a2-a3 | y0=b2-b3 |
       +----+----------+---------+----------+
       | x6 | a2=x4-x5 | b0=a4-a5 | y0=b4-b5 |
       +----+----------+---------+----------+
       | x7 | a3=x6-x7 | b0=a6-a7 | y0=b6-b7 |
       +----+----------+---------+----------+

  How it works
  1. Construct a matrix with N rows and n+1 columns
     hadm[n+1][N]
     (If I use [x][y] it always means [row-offset][column-offset] of a Matrix with n rows and m columns. Its entries go from M[0][0] to M[n][m])
  2. Place the input vector x[N] in the first column of the matrix hadm
  3. The entries of the submatrix Dtop are calculated as follows.
     Dtop goes from entry [0][1] to [N/2-1][n+1].
     The columns of Dtop are the pairwise mutually exclusive sums of the previous column
  4. The entries of the submatrix Dbottom are calculated as follows.
     Dbottom goes from entry [N/2][1] to [N][n+1].
     The columns of Dbottom are the pairwise differences of the previous column
  5. How Dtop and Dbottom you can understand best with the example for N=8 above.
  6. The output vector y is now in the last column of hadm
  7. Algorithm from: http://www.archive.chipcenter.com/dsp/DSP000517F1.html
  Visually

              +--------+---+---+---+-----+---+
              |   0    | 1 | 2 | 3 | ... |n+1|
       +------+--------+---+---+---+-----+---+
       |0     | x0     |       /\            |
       |1     | x1     |       ||            |
       |2     | x2     |   <= Dtop  =>       |
       |...   | ...    |       ||            |
       |N/2-1 | xN/2-1  |       \/            |
       +------+--------+---+---+---+-----+---+
       |N/2   | xN/2   |       /\            |
       |N/2+1 | xN/2+1  |       ||            |
       |N/2+2 | xN/2+2  |  <= Dbottom  =>      | which is in the last column of the matrix
       |...   | ...    |       ||            |
       |N     | xN/2   |        \/           |
       +------+--------+---+---+---+-----+---+

  Parameters:
    `x` - input vector
  Returns:
    y output vector
  Throws:
    `IllegalArgumentException` - if input array is not a power of 2

### fht

    protected int[] fht(int[] x)
                 throws IllegalArgumentException

  The FHT (Fast Hadamard Transformation) which uses only subtraction and addition.

  Parameters:
    `x` - input vector
  Returns:
    y output vector
  Throws:
    `IllegalArgumentException` - if input array is not a power of 2

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