CodeSOD: Is It the Same?

A common source of bad code is when you have a developer who understands one thing very well, but is forced- either through organizational changes or the tides of history- to adapt to a new tool which they don't understand. But a possibly more severe problem is modern developers not fully understanding why certain choices may have been made. Today's code isn't a WTF, it's actually very smart.

Eric P was digging through some antique Fortran code, just exploring some retrocomputing history, and found a block which needed to check if two values were the same.

The normal way to do that in Fortran would be to use the .EQ. operator, e.g.:

LSAME = ( (LOUTP(IOUTP)).EQ.(LPHAS1(IOUTP)) )

Now, in this specific case, I happen to know that LOUTP(IOUTP) and LPHAS1(IOUTP) happen to be boolean expressions. I know this, in part, because of how the original developer actually wrote an equality comparison:

 LSAME = (( LOUTP(IOUTP)).AND.( LPHAS1(IOUTP)).OR. (.NOT.LOUTP(IOUTP)).AND.(.NOT.LPHAS1(IOUTP)) )

Now, Eric sent us two messages. In their first message:

This type of comparison appears in at least 5 different places and the result is then used in other unnecessarily complicated comparisons and assignments.

But that doesn't tell the whole story. We need to understand the actual underlying purpose of this code. And the purpose of this block of code is to translate symbolic formula expressions to execute on Programmable Array Logic (PAL) devices.

PAL's were an early form of programmable ROM, and to describe the logic you wanted them to perform, you had to give them instructions essentially in terms of gates. Essentially, you 'd throw a binary representation of the gate arrangements at the chip, and it would now perform computations for you.

So Eric, upon further review, followed up with a fresh message:

The program it is from was co-written by the manager of the project to create the PAL (Programmable Array Logic) device. So, of course, this is exactly, down to the hardware logic gate, how you would implement an equality comparison in a hardware PAL!
It's all NOTs, ANDs, and ORs!

Programming is about building a model. Most of the time, we want our model to be clear to humans, and we focus on finding ways to describe that model in clear, unsurprising ways. But what's clear" and unsurprising" can vary depending on what specifically we're trying to model. Here, we're modeling low-level hardware, really low-level, and what looks weird at first is actually pretty darn smart.

Eric also included a link to the code he was reading through, for the PAL24 Assembler.

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