This week, I finally continue the series of posts on the new finite-difference framework. It seems we’re on the home stretch.
After you read it, or before, or even in the meantime, you might download QuantLib 1.9.2. It’s exactly the same as QuantLib 1.9.1, except that the tests aren’t broken. For some reason, that seemed to confuse people.
In the old framework, as you might remember, base engines declared a
virtual method that derived classes had to implement and which applied
the initial condition of the problem. In the new framework, in
keeping with its plug-and-play nature, calculating the initial
condition is left instead to separate objects that can be reused in
different engines. The base class for such objects is
FdmInnerValueCalculator, shown in the listing below.
Its interface contains the
innerValue method, which returns the
initial value at a given point in the mesh (denoted by an iterator)
avgInnerValue method, which returns an average of the
initial value around a given point and thus might provide a way to
smooth discontinuities in the payoff. They’re both declared as pure
virtual; this makes complete sense for
innerValue, and is not wrong
avgInnerValue either. In hindsight, though, a few derived
classes give up the calculation of the average as too difficult and
return the value at the central point, i.e., the return value of
innerValue, as an approximation; this might have been a default
implementation. (But you could also argue that the approximation
should be a deliberate choice of whoever writes the derived class, and
you wouldn’t be wrong either.)
The next listing contains an examples of concrete class implementing
the interface. The
FdmLogInnerValue class is used to calculate a
payoff, depending on a single underlying variable, on a logarithmic
It takes the payoff itself, the mesh, and the index of the axis along which the underlying varies; the latter is needed because the mesh might be multi-dimensional. For instance, the underlying might be a stock value and the problem might model the stock and its volatility, or the stock and the interest rate, or all three. There’s no guarantee that the stock varies along the first axis.
As you probably expect, the
innerValue method extracts from the mesh
the value of the underlying variable corresponding to the passed
iterator, and then passes it to the payoff. The
method does something similar, but with a combination of numerical
boost::bind that I won’t bother to show here (and that
I hope to eradicate when we can use lambdas instead).
A second example,
FdmLogBasketInnerValue, is used for a payoff
depending on the values of multiple underlying variables (and not on
the total basket value, which probably makes the class name a
innerValue method collects the underlying values from the
several axis of the mesh—which, in this case, is assumed not to
model any other variable—and passes them to the payoff. The
avgInnerValue method bails out (poor thing) and calls
Aside: dispelling magic.
If you look back at the creation of the inner-value calculator in the engine code shown in an earlier post, you’ll see that we pass the index of the dimension as the magic number 0. This, of course, is not ideal. The problem is that the info is hard-coded in the definition of the mesher, but it’s not accessible by code, and it’s not easy to think of a meaningful common interface that meshers might provide to retrieve it.
I don’t know if there’s a solution, other than the time-honored
mitigation method of giving magic numbers a name; for instance, the
FdmBlackScholesMesher class could provide a specific static variable
x_dimension. However, good names for these variables might
be hard to define; and the idea runs counter to a possible
simplification I mentioned in yet another post, in which meshers
could be non-polymorphic and derived classes would be replaced by
The new framework doesn’t declare any new interface for boundary
FdmBoundaryConditionSet seen in the engine is
OperatorTraits comes from the old framework and thus makes use
BoundaryCondition class template I described
back in the day; the only thing new is
that we’re instantiating it with the
Of course, specific boundary conditions are implemented in this
framework so that they work with the new classes. For instance, the
next listing shows the interface of the
Its constructor takes the mesher that defines the domain of the
problem, the value of the instrument on the boundary, the index of the
axis whose boundary we want to control and an enumeration to specify
the lower or upper side of the mesh. The implementation, not shown,
also uses the mesher to identify the array elements to condition.
Finally, step conditions also inherit from a class template defined in
the old framework (
StepCondition, described here); and of course,
they also use the classes defined in the new framework. One such
condition is implemented in the
shown in the listing below, which determines if an American option
should be exercised.
The implementation is how you would expect: the
iterates over the mesh, and at each point it compares the intrinsic
value given by the option payoff with the current value of the option
and it updates the corresponding array element. The constructor takes
and stores the information that
The framework also defines the
FdmStepConditionComposite class. It
is a convenience class, which uses the Composite pattern to group
multiple step conditions and pass them around as a single entity. Its
interface is shown in the listing that follows; it defines the
applyTo method, which applies all conditions in sequence,
as well as a number of utility methods. Most of them are generic
enough; there are inspectors for the stored information, as well as a
method to join multiple conditions (or groups thereof).
vanillaComposite static method is a bit different. It
is a factory method that takes a bunch of arguments and makes a
ready-to-go composite condition that can be used in different
problems; which is good, but it’s more specific than the rest of the
class. Not that specific, mind you, since a quick search shows that
it’s used in engines for a few instruments as different as barrier
options and swaptions; but still, if we were to write more of them, I
wouldn’t like them to proliferate inside this class. It might be
cleaner to define them as free functions.
And we’re done for this week. In next post in the series, we’ll look at finite-difference schemes.
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