Reading large unsorted file

[anda_skoa]

I'll give a try to the the QtConcurrent.

No point in doing that, QtConcurrent is for parallelizing multiple tasks, you only have one task.

As I see You don't reset map2. So after next line map2 contains all lines from begin to current.
I think that first line in whie loop should be :

map2.clear();

map2.clear();

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No, every loop iteration is a single record.

The loop needs to work with map3 and only with map3.

Something like

map3[list1[0].insertMulti(list1[1], list1[2]);

map3[list1[0].insertMulti(list1[1], list1[2]);

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i.e. retrieve or create the inner map for key "list1[0]" then insert the current inner pair, allowing multiple values for key "list1[1]"

Since the result is written back into files again, one could probably even avoid the encoding and decoding and just work with QByteArray instead of QString.

Is the machine's physical RAM large enough to do that without swapping?
Such a long time sounds like as if the machine started swapping.

Cheers,
_