Template limits

[Max Samukha]

On Mon, 25 May 2009 05:57:39 -0400, bearophile
<bearophileHUGS at lycos.com> wrote:

>This post is a partial copy of a post of mine from digitalmars.D.learn. Lot of people seem to ignore that place.
>
>This is the only way I have found to create a certain static array at compile time:
>
>int sumSqrt(int n) {
>    int result = 0;
>    while (n) {
>        int digit = n % 10;
>        n /= 10;
>        result += digit * digit;
>    }
>    return result;
>}
>template GenSquares(int n) {
>    static if (n < 0)
>        const int[] GenSquares = [];
>    else
>        const int[] GenSquares = GenSquares!(n-1) ~ [sumSqrt(n)];
>}
>void main() {
>    const int CHUNK = 1000;
>    static const auto squares = cast(int[CHUNK])GenSquares!(CHUNK-1);
>}
>
>That code works with D1, but gives a "recursive expansion" error with D2, this is bad. Can D2 be fixed to have capabilities similar to D1?
>
>I have also tried to write a nicer version of the code that uses just one compile-time function and no templates, and starts as:
>struct S(int N) { int[N] a; } 
>S!(N) genSquares(int N)() { ...
>
>But so far I haven't found a way. If such limit is real, then I think D2 has to be improved to allow such things, because creating a fixed-size array at compile time is one of the main purposes of compile-time functions.
>
>Bye,
>bearophile

Your example can be rewritten like this:

int[] genSquares(int n)
{
    int[] result;

    for(; n >= 0; --n)
    {
        int k = n;
        int m;
        while (k) {
            int digit = k % 10;
            k /= 10;
            m += digit * digit;
        }
        result = m ~ result;
    }

    return result;
}

void main() {
    const int CHUNK = 1000;
    static const auto squares = cast(int[CHUNK])genSquares(CHUNK - 1);
}

Works for both compiler versions.