On Tue, 8 Aug 2017, Jochen Topf wrote:

When JOSM saves OSM files it uses a particular order: First nodes, then
ways, then relations as usual. For each object type it first writes out
objects with negative IDs (ie objects that are not uploaded yet), then
objects with positive IDs, both are ordered by absolute value.

Is this something I can rely on or is this just something that happened
accidentally with my version of JOSM when I tried this?

The reason I am asking: I sometimes get requests for Osmium features
from people who want to do something with files saved from JOSM, like
renumber them to have only small positive IDs, or convert them into
other formats. Osmium can read JOSM files and handle negative IDs, so
these things mostly work, but in some cases having a known order helps
(or is even necessary for correct functioning). I am currently working
on some things there but if JOSM would not keep to this order in the
future they would break again.

I would not rely on the order of the individual elements. There are ideas to rework the data storage to prevent changing IDs for the new objects (allows better diffs). That may have other side effects as well. I would expect the only thing you can rely on is the nodes, ways, relation order.

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