Question

Recently I refactored the code of a 3rd party hash function from C++ to C. The process was relatively painless, with only a few changes of note. Now I want to write the same function in Java and I came upon a slight issue.

In the C/C++ code there is a C preprocessor macro that takes a few integer variables names as arguments and performs a bunch of bitwise operations with their contents and a few constants. That macro is used in several different places, therefore its presence avoids a fair bit of code duplication.

In Java, however, there is no equivalent for the C preprocessor. There is also no way to affect any basic type passed as an argument to a method – even autoboxing produces immutable objects. Coupled with the fact that Java methods return a single value, I can’t seem to find a simple way to rewrite the macro.

Avenues that I considered:

• Expand the macro by hand everywhere: It would work, but the code duplication could make things interesting in the long run.

• Write a method that returns an array: This would also work, but it would repeatedly result into code like this:

  long tmp[] = bitops(k, l, m, x, y, z);
  
  k = tmp[0];
  l = tmp[1];
  m = tmp[2];
  x = tmp[3];
  y = tmp[4];
  z = tmp[5];
  

• Write a method that takes an array as an argument: This would mean that all variable names would be reduced to array element references – it would be rather hard to keep track of which index corresponds to which variable.

• Create a separate class e.g. State with public fields of the appropriate type and use that as an argument to a method: This is my current solution. It allows the method to alter the variables, while still keeping their names. It has the disadvantage, however, that the State class will get more and more complex, as more macros and variables are added, in order to avoid copying values back and forth among different State objects.

How would you rewrite such a C macro in Java? Is there a more appropriate way to deal with this, using the facilities provided by the standard Java 6 Development Kit (i.e. without 3rd party libraries or a separate preprocessor)?

Answer 1

Option 3, create you own MutableInteger wrapper class.

struct MutableInteger{
  public MutableInteger(int v) { this.value = value;}
  public int value;
}

public void swap3( MutableInteger k, MutableInteger l, MutableInteger m) {
  int t = m.value;
  m.value = l.value
  l.value=k.value;
  k.value=t;
}