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An automaton for indenting the lisp code in linear time

From: A Soare
Subject: An automaton for indenting the lisp code in linear time
Date: Mon, 30 Jun 2008 02:41:36 +0200 (CEST)

Here I send the old promised automaton for indenting in linear time...

I looked in the code of emacs to the function scan_sexps_forward,
because I want to write for myself a little web browser, and I saw
that its automaton of changing the classes of syntax looks very
closely to the automaton that I wrote about 1 year ago to indent the
lisp code.

That is the reason for which I decided to open the archives and
to remember.

This code indents a line of code and a whole region in O(n) time
complexity; oups! no, sorry, in O(n+l) (the automaton in O(n) + O(l)
to use its output to indent), where n is the number of bytes from the
beginning of the function to the ending point, and l is the number of
lines from the beginning of the function to the ending point. (in the
code it's true that (current-column) is called very often, but I can
insert a variable `column' that keeps the current column, and it is
reset when we meet newline, and grows by 1 when the automaton passes
to the next state, and I think that I will modify it to keep the idea
of O(n)).

This code is very compact; installing it will cut thousands of lines
of old code from Emacs.

And, besides, it is imcomplete! Here are just the most basics. I can
complete it just after I speak with the emacs developers, in order to
agree on its behaviour. I do not like to work on in vain.

In this period I wish to work for this code, in order to finish it. If
I do not work now, I do not know when I can work on.

Besides, looking these days to write my web browser, I realised that
it would be a good idea embedding this automaton in C. But, again, I
do not want to work on it before asking you, because I want this job
to have a practical utility, not just to spend time working on this.

Here I give you just 2 very samples examples of how this automaton
works. It's evident how it works, and these 2 examples are too much!

There are a few new things in this code: for example, note the
easyness to add indent for comments, constant symbols, and look at the
easyness with which I made the indentation of a parameter of a
function before the first parameter, not under the second (i.e. one
makes now the difference between many kinds of list : not all the
lists have the first parameter a function):

(defun a (x y
          z t)

and not the old behaviour:

(defun a (x y
            z t)

The problem of indentig of lisp code is a problem of... homotopy:
however I deform the code (the spaces at the beginning of the lines),
this automaton must always return the same result.

Before calling (lisp-indent-automaton), one must have in the upper
environment the variables `state' and `indents-lines' initialised with
nil. It will return the result in indents-lines. The mapcar of car of
indents-lines is a list that contains the correct answer to the
homotopy problem. This result must be the same however I deform the
code (spaces at the beginning of lines). The mapcar of the cdr is a
list with the current indentation.

Please, tell me your impressions and... give me to work in order to
finish it well :)

Alin Soare.


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