What is the most unusual-yet-effective technique you have discovered or used?

Question

Certainly there are valuable programming techniques yet to be discovered, or that have been discovered but are not well-known.

I'm interested in hearing about techniques that may be counterintuitive or seem strange, but have real benefit.

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Edit: It was suggested that I give an example.

Just to show what I mean, my personal favorite is something I discovered in '86, that I called differential execution. It's a control structure in the same way that recursion or backtrack is a control structure, and just as hard to understand for anyone who doesn't know it yet. But it has a benefit in building UIs with time-varying structure, in reducing source code by about an order of magnitude.

It is hard to understand, no question about it, but once it is understood, it pays off in productivity.

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Edit: Another favorite I've seen is a way to do propositional resolution theorem proving with bit patterns. It doesn't get much use, but when you need it it's pretty nifty. Each proposition corresponds to a bit-position in a word. Suppose you have two propositional clauses A and B stated in conjunctive normal form. Let word A0 be the set of propositions that are false in A, and A1 be the set that are true in A. Similarly for B0 and B1. Then you can resolve A and B into a new clause R by the bitwise operations:

// find the propositions in which A and B disagree
int temp = (A0 & B1) | (A1 & B0);
// if they disagree
if (temp != 0){
    // make a new clause by combining them
    // and removing the proposition where they disagree
    int R0 = (A0 | B0) & !temp;
    int R1 = (A1 | B1) & !temp;
    // do something with the result R
}

So, for example if A = (!X | Y), and B = X, then R = Y.

Answer 1

Duff's device is an optimized implementation of a serial copy that uses a technique widely applied in assembly language for loop unwinding.

Answer 2

One thing that surprised me the first time I saw it was an "automatic" variable in a C++ method that was never used. Like

  Foo &foo;

But when I took it out, things failed badly. It turns out the constructor for the variable grabbed a mutex lock, and the destructor released it, so as long as it was in scope, it had the lock.

I don't normally condone such "jedi mind tricks" in the code, but it was pretty cool.

Answer 3

ternary operators. They are second nature to some, a glaring run-time error to others. I feel the real power of them come from the fact you can assign variables to the evaluation of ther ternary operators.

var welcome_string = "good " + (Time.now > noon ? "evening!" : " morning!")

Pardon my pseudocode

Answer 4

String Mapping is an odd thing I've encountered for VB, where one overwrites the headers of an array in VB with the address of a string and accesses the string like an array. Sure, you could just copy a string to a byte array, but this is faster. And crashes VB if you don't untrash the headers before the array is free.

Answer 5

Monte carlo unit testing. If you have some really complex algorithm in your code that would be hard to test deterministically because of the huge number of code paths, also implement a naive algorithm that's hard to get wrong and is supposed to have the same observable behavior as your complex algorithm. Then, generate a bunch of random input, give it to both your complex and naive algorithm, and make sure the results match.

Answer 6

Try-Finally blocks:

I like these for functions that have to do a lot of setup and cleanup, and have multiple return paths. They let you consolidate your cleanup logic, while keeping your returns simple and direct.

private string Test(int arg) {
 try {
  //extensive setup

  if (arg > 5) return "too big";
  return "normal";
 } finally {
  // extensive clean up
 }
}

Answer 7

I know one. Deliberately slowing (throttling) down a system to make it perform better and scale higher.