How to (efficiently) find the set of maxima of an array?

General Usage Performance
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2

If you can give a upper bound of output length, and it’s not large (<= 20 for example), you might try

findmaxs(A::AbstractArray) = begin
    inds = Vector{eltype(keys(A))}(undef, 20)
    maxval = typemin(eltype(A))
    n = 0
    @inbounds for i in keys(A)
        Ai = A[i]
        Ai < maxval && continue
        if Ai > maxval
            maxval = Ai
            n = 1
            inds[n] = i
        else
            inds[n+=1] = i
        end
    end
    resize!(inds, n)
end

For inputs like A = rand(1:1000, 100, 100), this save about 18% time cost on my desktop. (I have to say the space for optimization is quite small). If you accept linear index, replacing keys with eachindex seems better.

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