On Tuesday, 15 June 2021 at 04:24:09 UTC, surlymoor wrote:
All my custom range types perform all their meaningful work in
their respective popFront methods, in addition to its expected
source data iteration duties. The reason I do this is because I
swear I read in a github discussion that front is expected to
be O(1), and the only way I can think to achieve this is to
stash the front element of a range in a private field; popFront
would thus also set this field to a new value upon every call,
and front would forward to it. (Or front would be the cache
itself.)
At the moment, I feel that as long as the stashed front element
isn't too "big" (For some definition of big, I guess.), that
built-in caching should be fine. But is this acceptable? What's
the best practice for determining which range member should
perform what work? (Other than iterating, of course.)
Well, consider this program:
```d
import std;
struct Noisy {
int[] source;
int pops, fronts;
bool empty() { return source.empty; }
void popFront() { writeln("popFront #", ++pops);
source.popFront; }
int front() { writeln("front #", ++fronts); return
source.front; }
}
void main() {
iota(5).array
.Noisy
.filter!"a%2"
.each!writeln;
}
```
Out of [0,1,2,3,4], only 1,3 pass the filter.
Noisy's front is called seven times, 2x for each filter success.
Noisy's popFront is called five times, 1x for each source member.
But if you slap a .cache (from std.algorithm.cache) before the
.filter then these counts are the same.
