User:MikeDunlavey/Difex Article

Differential execution refers to a method of executing a computer subroutine (See control flow) in such a way that differences from prior executions can be detected and acted upon. If the subroutine is one that walks through a data structure, differential execution can be used to detect changes in the data structure and produce notifications or take actions so as to communicate the changes. This has applications in algorithm animation or graphical user interfaces where a remote image must be kept in agreement with the data structure via a low-bandwidth connection. It has other uses as well.

Basic outline

The basic outline of the method is deceptively simple. It consists of repeatedly reading the data structure and comparing it against values saved in a sequential file. However, this basic method becomes more elaborate when conditional statements are added.

To illustrate the method, assume a normal programming language, in this case C. The statements of that language are shown in lower case, such as if and while. Further assume that there exists a pair of sequential files, one from which numbers are read (fOld), and one to which numbers are written (fNew), and a pair of boolean variables (initially false) read and write. Reading from fOld only occurs if read is true, and writing to fNew only occurs if write is true.

A particular subroutine (toplevel) can be written to scan the data structure of interest, and it is used in a series of repetitions thus:

   write = true;
   open fNew as a new file
   toplevel();
   copy fNew to fOld
   read = true;
   
   allow the data structure to change
   open fNew as a new file
   toplevel();
   copy fNew to fOld
   
   repeat the above 4 lines as many times as desired
   
   write = false;
   toplevel();
   read = false;

In other words, toplevel is invoked a number of times. write is true in all invocations but the last. read is true in all invocations but the first. Between any two invocations, fNew is copied to fOld, and fNew is opened as a new file.

Suppose there is a subroutine scannumber(int n) written as follows:

   void scannumber(int n){
       int oldn;
       if (read) read( &oldn, fOld);
       if (write) write( n, fNew);
       if (!read && write){
           n has come into existence. take appropriate action.
       }
       if (read && !write){
           n has gone out of existence. take appropriate action.
       }
       if (read && write && n != oldn){
           n has changed. take appropriate action.
       }
   }

The purpose of this routine is to be called from toplevel and to detect changes in numbers. Suppose the "data structure" simply consists of global integer variables a, b, c. Then toplevel could be written as:

   toplevel(){
      scannumber(a);
      scannumber(b);
      scannumber(c);
   }

Conditional statements

Suppose there is a new boolean global variable bValid indicating whether or not integer b exists. if bValid is false, the scan should skip b. bValid can also change between invocations of toplevel. This produces a complication that the file can have either 2 numbers in it, or 3, and how should this be handled?

The conditional is handled by also scanning the value of bValid, and using that to determine whether to read b, write b, or both. This requires a new boolean-valued function and some structured code. The boolean-valued function is called ifUtil. (Note that boolean values are treated as integers.)

   bool ifUtil(bool test){
       bool oldtest;
       if (read) fread( &oldtest, fOld);
       if (write) fwrite( test, fNew);
       if (read && write){
           if (oldtest==test) return test;
           if (!oldtest){read = false; return true;}
           if (!test){write = false; return true;}
       }
       if (read) return oldtest;
       if (write) return test;
       return false;
   }

Then the call to scannumber(b) is enclosed in the following structured conditional statement:

   {bool svread = read, svwrite = write; if ( ifUtil( bValid )){
   
       scannumber(b);
   
   }read = svread; write = svwrite;}

The effect of this is to read/write the value of bValid in the files, and use it to control the execution of scannumber(b). Since ifUtil can optionally turn off either the read or write flag, the flags must be restored when leaving the body of the if statement.

So what does this do? Consider the case where read and write are both true (i.e. all invocations but the first and last). There are four cases:

• bValid is unchanged and false. The call to scannumber(b) is skipped.
• bValid is unchanged and true. The call to scannumber(b) is performed.
• bValid is changed to false. write is turned off, and the call to scannumber(b) is performed, which notes that b has gone out of existence.
• bValid is changed to true. read is turned off, and the call to scannumber(b) is performed, which notes that b has come into existence.

Thus, if bValid was false last time, the b is not read. If bValue is false this time, then b is not written. In this way, the files grow or shrink as needed.

It should be obvious that the body of the conditional statement can contain any number of scannumber statements. It can also contain further conditional statements nested within it as the need arises. It should also be clear that further subroutines can be called from within toplevel, as long as they only call routines that are legal to call from toplevel. For example, to walk a tree structure, a recursive routine can be used.

It should be clear that this method can be generalized to walk any data structure. For example, if instead of bValid being a boolean, there could be a pointer p to a structure, and if that pointer is not null, then the structure can be walked.

Since the conditional as written above is tiresome to write, it can be turned into a pair of macros, as in:

   IF(bValid)
       statements
   END

Looping is accomplished with a slight variation on the macro above. Simply replace the if statement with while! This can also be turned into a macro, as in:

   WHILE( test )
       statements
   END

In order to use this to index over an array of n elements (where n can change), an additional concept is needed, the idea of protected computations.

• Protected computations If any significant computations are to be executed under toplevel but above the level of primitive such as scannumber, they must not be executed when write is false.

In this case it is necessary to initialize a counter variable i and increment it within the loop, but those should only be done it write is true. Fortunately in the C language this is easily done:

   (write ? (i = 0) : 0);
   WHILE( i < n)
       statements
       (write ? (i++) : 0);
   END

The reason for protecting computations is that, when write is false it is because data is going out of existence, so computations on nonexistent data can do unpredictable things. Division by zero, indexing out of arrays, following garbage pointers, and other more subtle errors are possible when computations are not protected.

Another rule is important:

• Data structure cannot change during the execution of toplevel.

The reason for this should be obvious. Any program that walks a data structure generally assumes that the data structure does not change beneath its feet.

When these rules are followed, differential execution is very robust.

Many variations on the theme of differential execution have been tried, such as additional modes for handling user input in graphical user interfaces.

Discussion

In the example above scannumber is an example of a routine that examines a number to see if it has changed. It is a primitive of differential execution, since it can only be called beneath toplevel and it must respect the convention of reading and writing. Another example could be in the computer graphics domain, where one could have a box(int x, int y, int w, int h) to draw a box on a remote graphic screen. By looking at the history of its arguments, it can decide if it is necessary to draw, erase, redraw the box, or leave it alone. If the files are stored in memory, the computational cost of reading and writing the values is insignificant compared to the cost of drawing the box.

This method works no matter how much the data structure changes between invocations of toplevel, meaning it is not necessary to check the data structure frequently for changes, but only when needed.

References

• Dunlavey, "Differential Evaluation: a Cache-based Technique for Incremental Update of Graphical Displays of Structures" Software Practice and Experience 23(8):871-893, August 1993.