Coders at Work: Reflections on the Craft of Programming

by Peter Seibel

But now I use change files all the time because I'm writing programs for myself that I'm using writing my book—I've got lots of problems that I want to solve and I want to experiment with different versions. Like yesterday I wanted to find out how big a Boolean circuit is for multiplication of n-bit numbers. I have a program that takes any Boolean function and finds out how big its BDD is. So I've got a program that takes any Boolean function and computes its BDD. In my original program you input the truth table of the function online—it says, “Give me a truth table,” and I type in a hexadecimal number, because I had a lot of small functions that I'm using as examples. But it only works for small functions, the ones that I want to type in the truth table. Then I've got a big function like, “Multiply all pairs of 8-bit numbers.” This is a function of 16 variables—there are 8 bits in x and 8 bits in y. So I write a little change file that takes out this interactive dialog and replaces it with a program that makes a truth table for multiplication. Then I changed that by saying, “Let's read the bits from right to left instead of from left to right, which gives you a different BDD.” Or, “Let me try all Boolean functions of six variables and I'll run through them all and find out which one has the largest BDD.” But all of these are customizations of my original thing. I'll have maybe 15 variants of that program that are easily understood. This was an unexpected spin-off from lit...