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My Lua isn't strong, but I think this is what you're looking for:

How insertion works here, which illuminates how the table is used: https://github.com/rxi/lite/blob/143f8867a13a35f5688ad7c9771...

Looks like an array of lines, which is a completely reasonable structure to use.

There's a good post on the Visual Studio Code blog about why and when they moved on from an array of lines to a new structure based on a piece table.

And if you want to take that a step further down the rabbit hole, here's how Lua allocates the table in memory: https://stackoverflow.com/a/29930168

It's two dynamic arrays, of hashes and values, whose sizes grow as powers of two.