[OT] n-way union

[Andrei Alexandrescu]

This is somewhat OT but I think it's an interesting problem. Consider 
the following data:

double[][] a =
[
     [ 1, 4, 7, 8 ],
     [ 1, 7 ],
     [ 1, 7, 8],
     [ 4 ],
     [ 7 ],
];

We want to compute an n-way union, i.e., efficiently span all elements 
in all arrays in a, in sorted order. You can assume that each individual 
array in a is sorted. The output of n-way union should be:

auto witness = [
     1, 1, 1, 4, 4, 7, 7, 7, 7, 8, 8
];
assert(equal(nWayUnion(a), witness[]));

The STL and std.algorithm have set_union that does that for two sets: 
for example, set_union(a[0], a[1]) outputs [ 1, 1, 4, 7, 7, 8 ]. But 
n-way unions poses additional challenges. What would be a fast 
algorithm? (Imagine a could be a very large range of ranges).

Needless to say, nWayUnion is a range :o).

Finally, why would anyone care for something like this?


Andrei