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Message-ID:  <905C5ED0-6639-4DEB-95AC-A2FCB2C4491D@gmail.com>
Date:         Thu, 10 Mar 2011 12:11:43 +1030
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              <LATEX-L@listserv.uni-heidelberg.de>
Sender: Mailing list for the LaTeX3 project <LATEX-L@listserv.uni-heidelberg.de>
From: Will Robertson <wspr81@GMAIL.COM>
Subject: expl3's seq_pop_right etc.
To: LATEX-L@listserv.uni-heidelberg.de
Precedence: list
Status: R

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Dear all,

Quite some time ago Morten and I were discussing improvements to the =
functions offered by the "seq" module in expl3, namely to make symmetric =
the push and pop commands so you could access them on both sides if you =
wished. (Obviously, perhaps, it's more efficient to do things on the =
left, however.)

The idea is that where the seq module currently offers

PUT LEFT   \seq_put_left, \seq_push (synonyms)
PUT RIGHT  \seq_put_right
POP LEFT   \seq_pop:NN
POP RIGHT  <none>
GET LEFT   \seq_get:NN, \seq_top:NN (synonyms)
GET RIGHT  <none>

we would augment/rename these functions as follows:

PUT LEFT   \seq_put_left, \seq_push
PUT RIGHT  \seq_put_right
POP LEFT   \seq_pop_left:NN, \seq_pop:NN
POP RIGHT  \seq_pop_right:NN
GET LEFT   \seq_get_left:NN, \seq_get:NN
GET RIGHT  \seq_get_right:NN

I've finally taken some minutes to write out some functions that fill in =
these gaps.

There are a few design decisions that I've chosen to delay -- e.g., =
should we be aiming for more expandable functions for dealing with =
sequences? My thoughts are that it's probable that expandable functions =
are faster for shorter lists and slower for longer ones, although I =
haven't done any benchmarking. So my first reaction is to do everything =
expandably with comma lists but un-expandably with sequences. However, =
my implementation actually does use the mapping approach since we've got =
that conveniently set up with quarks already. Happy to discuss.

With all this in mind, I'd like to invite comments on the functions =
offered (perhaps an f-expandable \seq_get_(left/right):N would also be a =
good idea? Contributions welcome.) and the particular implementation =
I've chosen below. After any discussion here I'll add the functions =
(with tests) to expl3.

In lieu of code comments, a rough description: sequences are mapped two =
elements at a time until the end of the sequence is reached. For the =
pop_right:NN operations, a new sequence is put_right into until the =
final element is found and omitted but stored in a token list. For the =
get_right operations, we simply save the final element in a token list. =
There is also a pop_right:N function offered that does not store the =
popped element.

Best regards,
-- Will


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\documentclass{article}
\usepackage{expl3}
\begin{document}
\ExplSyntaxOn

%% POP (no assignment)

\cs_new:Npn \seq_pop_right:N #1 {
  \seq_pop_right_aux:NNNN #1 \l_seq_tmpa_tl \seq_set_eq:NN \use_none:nn
}
\cs_new:Npn \seq_gpop_right:N #1 {
  \seq_pop_right_aux:NNNN #1 \l_seq_tmpa_tl \seq_gset_eq:NN \use_none:nn
}

%% POP (assignment)

\cs_new:Npn \seq_pop_right:NN #1#2 {
  \seq_pop_right_aux:NNNN #1 #2 \seq_set_eq:NN \tl_set_eq:NN
}
\cs_new:Npn \seq_gpop_right:NN #1#2 {
  \seq_pop_right_aux:NNNN #1 #2 \seq_gset_eq:NN \tl_set_eq:NN
}

\cs_new:Npn \seq_pop_right_aux:NNNN #1#2#3#4 {
  \seq_clear:N \l_seq_tmpa_seq
  \exp_after:wN \seq_pop_right_aux:w #1
  \seq_elt:w \q_recursion_tail \seq_elt_end:
  \seq_elt:w \q_recursion_tail \seq_elt_end:
  \seq_elt:w \q_recursion_stop \seq_elt_end:
  #3 #1 \l_seq_tmpa_seq
  #4 #2 \l_seq_tmpa_tl
}

\cs_set:Npn \seq_pop_right_aux:w
    \seq_elt:w #1 \seq_elt_end: \seq_elt:w #2 \seq_elt_end:
{
  \quark_if_recursion_tail_stop_do:nn {#1}
  { \tl_clear:N \l_seq_tmpa_tl     \use_none:n }
  \quark_if_recursion_tail_stop_do:nn {#2}
  { \tl_set:Nn \l_seq_tmpa_tl {#1} \use_none:n }
  \seq_put_right:Nn \l_seq_tmpa_seq {#1}
  \seq_pop_right_aux:w \seq_elt:w #2 \seq_elt_end:
}

%% GET

\cs_new:Npn \seq_get_right:NN #1#2 {
  \seq_get_right_aux:NNN #1 #2 \tl_set_eq:NN
}
\cs_new:Npn \seq_gget_right:NN #1#2 {
  \seq_get_right_aux:NNN #1 #2 \tl_gset_eq:NN
}

\cs_new:Npn \seq_get_right_aux:NNN #1#2#3 {
  \exp_after:wN \seq_get_right_aux:w #1
  \seq_elt:w \q_recursion_tail \seq_elt_end:
  \seq_elt:w \q_recursion_tail \seq_elt_end:
  \seq_elt:w \q_recursion_stop \seq_elt_end:
  #3 #2 \l_seq_tmpa_tl
}

\cs_set:Npn \seq_get_right_aux:w
    \seq_elt:w #1 \seq_elt_end: \seq_elt:w #2 \seq_elt_end:
{
  \quark_if_recursion_tail_stop_do:nn {#1}
  { \tl_clear:N \l_seq_tmpa_tl \use_none:n }
  \quark_if_recursion_tail_stop_do:nn {#2}
  { \tl_set:Nn \l_seq_tmpa_tl {#1} \use_none:n }
  \seq_get_right_aux:w \seq_elt:w #2 \seq_elt_end:
}

%% TESTING

\seq_new:N \foo
\seq_push:Nn \foo {a}
\seq_push:Nn \foo {b}
\seq_push:Nn \foo {c}
\seq_push:Nn \foo {d}
\seq_push:Nn \foo {e}
\seq_display:N \foo

\seq_get_right:NN \foo \baz
\tl_show:N \baz

\seq_pop_right:N \foo
\seq_display:N \foo
\seq_pop_right:N \foo
\seq_display:N \foo
\seq_pop_right:N \foo
\seq_display:N \foo
\seq_pop_right:N \foo
\seq_display:N \foo
\seq_pop_right:N \foo
\seq_display:N \foo
\seq_pop_right:N \foo
\seq_display:N \foo

\seq_push:Nn \foo {a}
\seq_push:Nn \foo {b}
\seq_push:Nn \foo {c}
\seq_push:Nn \foo {d}
\seq_push:Nn \foo {e}

\seq_pop_right:NN \foo \baz
\tl_show:N \baz
\seq_pop_right:NN \foo \baz
\tl_show:N \baz
\seq_pop_right:NN \foo \baz
\tl_show:N \baz
\seq_pop_right:NN \foo \baz
\tl_show:N \baz
\seq_pop_right:NN \foo \baz
\tl_show:N \baz
\seq_pop_right:NN \foo \baz
\tl_show:N \baz

\ExplSyntaxOff
\end{document}

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