Template limits

[Max Samukha]

On Mon, 25 May 2009 14:14:17 -0400, bearophile
<bearophileHUGS at lycos.com> wrote:

>Walter Bright:
>> The problem is that enough recursion will blow up the stack in the 
>> compiler. I set a limit below that. But, naturally, for any limit I set 
>> someone will try to exceed it.
>
>I accept that some limits exist. For many situations a template nesting of 1000 is plenty. My problems were:
>1) The limit of D1 seems different of the limit of D2, so a program I have written for D1 doesn't work with D2.
>2) Of course I have first tried to write it with CTFE, but I have failed. First I have tried the simple solution:
>
>int[] genSquares(int n) {
>    int[] result;
>    for (int i = 0; i < n; i++) {
>        int k = i;
>        int m;
>        while (k) {
>            int digit = k % 10;
>            k /= 10;
>            m += digit * digit;
>        }
>        result ~= m;
>    }
>    return result;
>}
>
>Then seeing it fail (I cast it into a fixed sized array at compile time), I have tried creating a fixed sized array, but functions can't return them yet. So I have tried to wrap the static array with a struct, but Don has shown this doesn't currently work yet:
>
>struct S(int N) { int[N] a; }
>
>S!(N) genSquares(int N)() {
>    S!(N) s;
>    for (int i = 0; i < N; i++) {
>        int n = i;
>        int m = 0;
>        while (n) {
>            int digit = n % 10;
>            n /= 10;
>            m += digit * digit;
>        }
>        s.a[i] = m;
>    }
>
>    return s;
>}
>void main() {
>    const int CHUNK = 1000;
>    static const auto squares = genSquares!(CHUNK)().a;
>}
>
>So I have created the mixed template/ctfe version I have shown in my original post.
>So far I have written tons of (sometimes quite complex) templates in D :-)
>
>Max Samukha then has gently offered me a working solution, but it's not obvious, it uses a dynamic array, but builds it in a more functional way.
>
>Bye,
>bearophile


It looks like you can workaround the bug by taking a slice of the
result to "normalize" the array. This works with both compiler
versions:

int[] genSquares(int n) {
    int[] result;
    for (int i = 0; i < n; i++) {
        int k = i;
        int m;
        while (k) {
            int digit = k % 10;
            k /= 10;
            m += digit * digit;
        }
        result ~= m;
    }
    return result[0..n];
}

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

But I'd rather use a dynamic array instead until Don fixes the bug :)