On Fri, 19 Dec 2014 12:20:34 +0000 bearophile via Digitalmars-d-learn <digitalmars-d-learn@puremagic.com> wrote:
> anon: > > > Changed to > > return data.map!(n => foo2(data, i + 1, > > max)).cache.joiner.array; > > then it produced the same result as array version. > > `map.cache.join` resulted in 597871. > > This is close to tell what the cause is :-) easy deal indeed: this is due to how `.save` done in lazy mapped range, plus some range properties. as source range for mapping is forward range (i.e. indicating that it has `.save`), `.join` assumes that `.save` is cheap and goes by the route where it iterates the range twice: first calculating range length and then generating values for preallocated array. but `.save` is not cheap in our case, so `.join` is effectively calculating EVERYTHING TWICE. oops. and as we have no memoization here, the number of calls is exploding. thank you for the riddle, it took me some time to see what's going on here. ;-)
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