[Issue 13877] Problem with map+join

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=13877

safety0ff.bugz <safety0ff.bugz at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |safety0ff.bugz at gmail.com
           Hardware|x86                         |All
                 OS|Windows                     |All

--- Comment #3 from safety0ff.bugz <safety0ff.bugz at gmail.com> ---
IMO the problem is that join assumes that traversal of the range of ranges is
cheap.

All the optimization is doing is avoiding reallocation costs while building the
array of results (by computing length then allocating appropriately sized array
for the results.)

As shown by bearophile, this shouldn't be assumed in the general case.

I ran into an issue where std.parallelism assumed that opIndex was as cheap as
popFront, but for my specific range it was easier to compute the next front
knowing the previous one than it was to find the element given by the index.

If these type of optimizations are important, perhaps there should be some
"performance hint tags" for range operations. e.g. popFront on an array range
is "cheap" whereas popFront on a MapResult may vary arbitrarily.

--