[adrianratnapala]

Actually the first example looks perfectly reasonable to me. I would have used changes of "if () return" instead of a huge OR, but the result would have been the same.

The second example I can't judge. Probalby the variable name is bad, but not extraordinarily so. It's a lookup table, I can't tell if it is a good or bad idea without know what is being looked up.

[Houshalter]

Would could possibly be the advantage of hard coding divisibility checks against every prime number? There's so many problems with this. The simplest thing would be to just make an array of primes and iterate through it. There's also faster algorithms to check for primality.

[adrianratnapala]

That code is making an array of primes and iterating through it. It's just that the array is stored in instruction memory, and there is no explicit fiddling with indices or pointers.

As for more advanced primality testing methods, I guess it depends on where the the fancier algorithms begin to pay off. It would not surprise me if this simple algorithm was the fastest for any input small enough to be stored in a single machine word.

[chickenfries]

I think in the second one, a simple list of values that correspond to true would be more reasonable than putting this in your source code, if you just need a look up table. The way the comments are structured though, I wonder if each row an option map, (annotated by the comments along the top)

[Houshalter]

The second example was a 15 line switch statement that they replaced with a lookup because they thought it would be faster.