gbfs-web was developed for and tested with 4-e, a Game Boy Advance homebrew application that can emulate an e-Reader for Super Mario Advance 4.
By following the simple steps in the app, you can take any ROM that supports GBFS, select files to attach, and save a new ROM with those files attached.
Everything is done locally in your browser. No ROM or file data is sent to or received from any server.
Go to https://mattiebee.app/gbfs-web.
Or, open index.html directly.
One key difference between GBFS files made by gbfs-web and the original gbfs tool is that gbfs-web does not bother to pad the objects in the GBFS file. I did not see a use for it, and 4-e (which uses libgbfs) does not appear to have any issues with it, even when loaded with lots of files.
gbfs-web does not currently pad the base ROM. The original tool doesn't either, but the original distribution did include a tool called padbin which does do this. 4-e's build process includes a step to pad to 256 bytes, libgbfs' default stride, so it did not seem necessary here, but it could be added.
If folks encounter issues due to these differences, gbfs-web can be changed accordingly. Please check to see if using the original tools solves the issue before filing bugs or submitting changes.
gbfs-web wouldn't exist without:
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GBFS (obviously!)