Emacs buffers are implemented using an invisible gap to make
insertion and deletion faster. Insertion works by filling in part of
the gap, and deletion adds to the gap. Of course, this means that the
gap must first be moved to the locus of the insertion or deletion.
Emacs moves the gap only when you try to insert or delete. This is why
your first editing command in one part of a large buffer, after
previously editing in another far-away part, sometimes involves a
noticeable delay.
This mechanism works invisibly, and Lisp code should never be affected
by the gap's current location, but these functions are available for
getting information about the gap status.
Function:gap-position
This function returns the current gap position in the current buffer.
Function:gap-size
This function returns the current gap size of the current buffer.
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