001 - Duff’s device
Duff’s device is a famous snippet of C code that manually performs loop unrolling.
Manually unrolling loops is nowadays considered a bad practice, as most compilers perform this automatically behind the scenes.
However, the historical significance of Duff’s device remains: it reminds us that creativity can help us overcome limits introduced by abstraction created in a higher level language (in this case C as opposed to assembly).
At least two good explanations are available that break this code down in detail: