help with prime pairs code

[Sergey]

On Sunday, 2 February 2025 at 03:22:00 UTC, Jabari Zakiya wrote:
> The D version is way slower, because of the array operations.
> For an input of 1000000 (1M): D: 12+ secs; Crystal: 0.036 secs.

First of fall make sure you are using LDC or GDC compiler. Second 
step is to add proper flags for optimizations (-O3).
Your initial version showed on my M1 laptop:
```bash
[1000000, 5402]
[17, 999983]
[499943, 500057]
19 secs, 367 ms, 923 μs, and 9 hnsecs
```
Then I propose some small changes:

1) You can initialize lhr with 1 row in similar logic as in 
Ruby/Crystal
```d
uint[] lhr = iota(3, ndiv2, 2).filter!(e => gcd(e, n) == 1).array;
```

2) The logic with shift and duplicated array could be simplified 
in D with simple index processing and slising (you even don't 
need to create duplicated array)
```d
foreach(i, r; lhr)
     {
         auto ri_max = rhi / r;   // ri can't multiply r with 
values > this
         if (lhr[i+1] > ri_max)
             break;               // exit if product of 
consecutive r’s > n-2
         foreach(ri; lhr[i+1..$])     // for each residue in 
reduced list
         {
             if (ri > ri_max)
                 break;           // exit for r if cross-product 
with ri > n-2
             lhr_mults ~= r * ri; // store value if < n-2
         }
     }                            // check cross-products of next 
lhr member
```

But these 2 updates don't make a lot of improvements in terms of 
performance.

> In Ruby|Crystal you can just do: lhr -= lhr_mult
> to remove elements of one array from another, and not one at a 
> time.
> I assume there's a D equivalent?!?

In D we have `setDifference`, so the code instead of filter + 
canFind (which will be very costly in terms of big-O notation) 
will become
```d
lhr_del.sort!("a < b"); // to use setDifference we need to have 
sorted arrays
lhr = setDifference(lhr, lhr_del).array;
```

So after those updates the time improved
```bash
[1000000, 5402]
[17, 999983]
[499943, 500057]
81 ms, 144 μs, and 7 hnsecs
```