QFunction.md

1. QFunction

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Definitive Page On QFunctions

A QFunction is a representation for a data function that tries to reuse as much of the QDataSet interface as possible. A QFunction takes N parameters as input and returns M parameters as output. The QFunction interface is simply a value() method that takes a rank 1 dataset as input and returns a rank 1 dataset as output. Also a method exampleInput() returns an example of an input used for the function. This allows for discovery of the function, identifying units, valid ranges, and cadences for inputs.

```

interface QFunction {
  QDataSet value( QDataSet in );   // rank 1 input of N parameters and rank 1 output of M values.
  QDataSet exampleInput();
}

```

```

class DemoFunction implements QFunction {
  public QDataSet value( QDataSet in ) {
      return Ops.sin( in );
  }
  public QDataSet exampleInput() {
      MutablePropertyDataSet bds= new DDataSet(2,1);
      bds.putProperty( 0, QDataSet.UNITS, Units.dimensionless );
      bds.putProperty( 0, QDataSet.TYPICAL_MIN, -Math.PI );
      bds.putProperty( 0, QDataSet.TYPICAL_MAX, Math.PI );
      DDataSet v= DDataSet.wrap( 0. );
      v.putProperty( QDataSet.BUNDLE_0, bds );        
      return v;
}

```

This is slightly extended to simply implementations and allow for more efficient use when multiple invocations will be made:

```

interface QFunction {
  QDataSet value( QDataSet in );   //
  QDataSet values( QDataSet in );  // rank 2 input and rank 2 output.
  QDataSet exampleInput();         // 
  QDataSet exampleOutput();        // call this when you just need the output.
}

```

Note that implementations of a QFunction may simply check the the BUNDLE\_0 pointer has not changed to know that none of the properties have changed.

Here is how QDataSet properties are interpreted in this case:

```

   * discover an example input.  Result is a rank 1 bundle QDataSet.
   *   QFunction ff= TestFunction();
   *   ff.exampleInput().length();  // how many parameters the function takes
   *   QDataSet bds= ff.exampleInput().property( QDataSet.BUNDLE_0 );  // get the bundle descriptor
   *   ; to discover properties of the first (0th) parameter:
   *   bds.slice(0).property( QDataSet.UNITS )       // function should handle convertible units (e.g. TimeAxes Ephemeris).
   *   bds.slice(0).property( QDataSet.VALID_MIN )   // absolute limits of domain of the function
   *   bds.slice(0).property( QDataSet.VALID_MAX )
   *   bds.slice(0).property( QDataSet.TYPICAL_MIN ) // domain of the function parameter
   *   bds.slice(0).property( QDataSet.TYPICAL_MAX )
   *   bds.slice(0).property( QDataSet.CADENCE ) // granularity of the function parameter
   *   bds.slice(0).property( QDataSet.LABEL )   // label for the parameter
   * slice(0) is the first argument, slice(1) would be the second, etc.
   * This would be a bundle.

```

1. Rank handling

QFunctions have two methods: value() and values(). `value` always takes a rank 1 bundle. Even if this is just one parameter, then this must be a bundle of the one parameter. `values` is always a rank 2. The class AbstractQFunction is intended to simply writing QFunctions by implementing a values method that calls the value method for each input of the rank 2 array. This allows the QFunctions to be implemented more efficiently when a lookup must be done before a result can be formed. For example, at the Plasma Wave group, we implement functions by posting the input to a server and then returning the result.

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