internal data structure

internal data structure

[Alexander Doe]

Good day Ladies and Gentleman,


which data structure is used by GNU/Emacs for storing and editing text
strings on low-level? List, balanced tree, gap buffer, something else?

Which language is used for programming this? C or ELisp?



with the best regards
Alexander Sidorov

Re: internal data structure

[tomas]

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On Sat, May 20, 2017 at 02:54:23PM +0200, Alexander Doe wrote:
> Good day Ladies and Gentleman,
> 
> 
> which data structure is used by GNU/Emacs for storing and editing text
> strings on low-level? List, balanced tree, gap buffer, something else?

It's a gap buffer.

> Which language is used for programming this? C or ELisp?

The machinery is C, but there are some Lisp introspection functions:

  https://www.gnu.org/software/emacs/manual/html_node/elisp/Buffer-Gap.html

If you have a source installation, then C-h f "describe function" on e.g.
"gap-position" will take you all the way to the C source (somewhere in
editfns.c).

Here's a link to an online resource, for your browsing convenience:

  https://git.savannah.gnu.org/cgit/emacs.git/tree/src/editfns.c?h=emacs-25#n1090

Cheers
- -- tomás
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Re: internal data structure

[Emanuel Berg]

Alexander Doe wrote:

> which data structure is used by GNU/Emacs for
> storing and editing text strings on
> low-level? List, balanced tree, gap buffer,
> something else?
>
> Which language is used for programming this?
> C or ELisp?

Emacs is Lisp on top of C.

The strings in Lisp are data objects that are
string-typed. This contrary to a language like
C where it is the *variable*, non the data
object, that is associated with a type.

That said, there isn't a string type in C.
In C what you do is either you have an array of
chars, or you have a pointer to a char and then
continue in memory a certain number of bytes,
which is the length of the text string, as
1 char is stored as 1 byte.

In practice, the array and pointer methods in
C amount to virtually the same.

By the way, "char" is pronounced "kar"!

In Lisp, or Elisp at least, because everything
is lists, it is tempting to think of a string
as a list of chars, just like the C array.
But technically, this is not the case:

    (stringp "Sail Ho!")              ; t
    (listp   "True sailing is dead.") ; nil

-- 
underground experts united
http://user.it.uu.se/~embe8573