Julia: `sort(collection; by=transformation)` should invoke `transformation` only once per element

[lassepe]

Currently, when sorting a collection with sort(collection; by=transformation), the by callable can be called multiple times on the same element (essentially whenever it is compared to another element within the sort algorithm).

MWE

using Random
struct Counter
    d::Dict
end
Counter() = Counter(Dict{Any, Int}())
updatecount(counter::Counter, x) = counter.d[x] = get(counter.d, x, 0) + 1
getcount(counter::Counter, x) = get(counter.d, x, 0)

function counted_transformation(x; counter)
    updatecount(counter, x)
    return x
end

@show collection = shuffle(1:10)
counter = Counter()
sort(collection; by=x->counted_transformation(x; counter=counter))
@show [getcount(counter, x) for x in collection]

Output:

collection = shuffle(1:10) = [2, 6, 9, 10, 3, 5, 7, 8, 1, 4]
[getcount(counter, x) for x = collection] = [3, 7, 8, 7, 7, 6, 6, 5, 8, 7]

As you can see, for some elements by has been called 7 or 8 times.
This can be quite expensive if the by transformation involves some non-trivial computation.
IMHO, it would be preferable if sort transforms these values only once before sorting.

From the high-level, this could look like this (this allocates a bunch, so this is not a good workaround)

# single transformation sort
function sort_custom(collection; by=identity, kwargs...)
    collection_with_transformation = map(x->x=>by(x), collection)
    sort!(collection_with_transformation; by=x->last(x))
    return map(x -> first(x), collection_with_transformation)
end

[JeffBezanson]

This is just a space vs. time tradeoff. Often by is a very simple function. I think the right way to do this is sortperm, or a variation that sorts two arrays together using one for the keys. I don't believe we have that function in Base but it might be in a package.

[lassepe]

Okay. I agree that this case may be rare enough that it should maybe not be provided by Base. Though, I believe, the fact that by is called more than once per element should be documented (i.e. by should not have any side-effects). This first bit when passing a method to by that involved sampling from a distribution parameterized by the element (i.e. non-deterministic transformation from element to a value). In that case, all sorts of other things break in the sort method (which is okay because non-deterministic transformations are a very esoteric use case).