Fix image collection id gaps #3864

Nitpick: You're talking about local tile IDs, not GIDs - the gaps in tile IDs just indirectly create gaps in GIDs as well. You would generally never use a GID as an index into an array of tiles (i.e. anywhere where the gaps would matter), because while multiple Maps may use the same Tileset, they may use different GIDs to refer to those tiles.

Why do you "need to" remove the gaps? Is it something to do with how you're storing and accessing the tiles in your code?

Regardless of need, I agree that it would be nice to be able to renumber the tiles in "image collection" Tilesets to remove gaps and "bake in" any tile reordering that's been done with the Rearrange Tiles tool. But Tiled would then need to provide some method to update all the maps using those tilesets, which is not as simple as it sounds, because Tiled can't know about the state of any maps that aren't currently open in the editor. Since Tiled has Projects now, perhaps it can prompt whether you'd like to adjust all the maps in the current Project when you choose to renumber tiles. Some discussion related to this is in #2866, which is about "based on tileset image" Tilesets, but perhaps a similar approach could be used for this - both involve some form of oldTile->newTile mapping that is only valid for a single moment in time, after all.

In the interim, I'd like to suggest a couple of workarounds you didn't mention in your post:

1. If you need this due to your code, the most obvious is for you to use a data structure that can deal with gaps, such as a map. In practice, accessing these data structures should be so infrequent that the performance impact of using a map instead of an array should be negligible. In some languages, sparse arrays are maps under the hood, so if you're using one of those, you could even just treat it as an array.
2. A Tiled script. Rather than doing the renumbering as a build step after exporting the TMJ file, you could add a scripted Action to Tiled that removes the gaps and updates all the maps in your project. Since you can't renumber existing tiles, you'd have to create a new Tileset with the renumbered tiles, but this isn't complicated to do (and you can save it over the old Tileset while keeping the old Tileset open in Tiled until you're done using it). After that, you'd iterate all the maps in your Project, performing the replacement, and removing the old Tileset; for these steps, my Mass Replace Tiles script may be a useful reference or could even be used directly if you also build the old-new mapping it expects using your script.

Why do you "need to" remove the gaps? Is it something to do with how you're storing and accessing the tiles in your code? [...] If you need this due to your code, the most obvious is for you to use a data structure that can deal with gaps, such as a map. In practice, accessing these data structures should be so infrequent that the performance impact of using a map instead of an array should be negligible.

Yes, the performance is not negligible since I'm rendering a huge chunked tilemap, even so it's wasted perfomance since it can be done offline, I need to access the array for every tile, so 1. is out for me. It is still a solution of course.

2. A Tiled script.

This seems like a good solution, I saw your mass replace tiles but it's too manual, as I'm frequently removing and adding tiles to the collection, I find it much easier to just edit the exported tmj. And I don't want to mess with tiled scripts and explode everything.

The script as linked is indeed "too manual", but the action or the code can easily be incorporated into another script that automates the entire process, and the creation of the mapping from one group of tiles to another is simple to automate. Any action built into Tiled would also have to be manually triggered as well, this sort of update could not be done automatically - opening all those maps to update them is just too heavy to perform every time you remove tiles, even if it's done on the C++ side rather than in a script. In practice, there would be very little difference between a script that does what you need, and that being built into Tiled - either way, it would be an action you manually trigger. It can be a little faster if done natively, but that's the only difference. Edit: You would also be able to trigger either one automatically via script, e.g. when the image collection is saved.