[Tile-serving] [openstreetmap/osm2pgsql] Rewrite tile expiry (management of expired tiles) (#709)

[Michael Reichert]

@nvkelso wrote:
> 1. Can you please add more detail on how the lists are different, please?

The format is the same.

The only important difference is relevant if you produce tile expiry lists for multiple zoom levels. If you produce tile expiry lists for zoom levels 10 to 11, the tile expiry will only output tile 10/612/321 if all its four subtiles (11/1224/642, 11/1224/643, 11/1225/642, 11/1225/643) have changed. If there only changes in 11/1224/642, only this tile will be written to the output file. If you produce to the tile expiry list for zoom levels 10 to 12 and there are only changes in 12/2449/1285 but no other sub-sub-tiles of 10/612/321, only 12/2449/1285 will be written to the output file.

My code will output 10/612/321 and 11/1224/642 if you requested the zoom levels 10 to 11 and the only map change happened in 12/2449/1285. If you requested zoom level 10 to 12 and the only map change happened in 12/2449/1285, it will output 10/612/321, 11/1224/642 and 12/2449/1285.

Use the [Tile Calculator of Geofabrik](http://tools.geofabrik.de/calc/#type=geofabrik_standard&bbox=34.475487,55.255325,36.138542,55.918888&grid=1), if you need a map showing you the tile IDs on top of the map.

Have you had a look at the shape files (and the raw tile expiry lists) I [linked](https://michreichert.de/share/public/osm2pgsql/expiry-example-shapes.zip) in my [initial posting](https://github.com/openstreetmap/osm2pgsql/issues/709#issue-213343500)? They are the source of the images I posted.

> 2. Can you confirm we could still get a list of just the changed tiles at a set zoom (say 15 or 16)?

Yes.

> 3. Reading the comments it's not clear to me why this image is so different between the blue and red tiles, can you please elaborate?

It seems that the current implementation of a tree drops some tiles. I did not investigate further because it is hidden in the old, ugly implementation. Red tiles were written to the output file by both implementations. Blue tiles were only written by my implementation.

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