[latex3-commits] [latex3/latex3] main: Speed up tl_if_head_is_N_type as suggested by Skillmon (closes #1127) (71e9705ea)

[github at latex-project.org]

Repository : https://github.com/latex3/latex3
On branch  : main
Link       : https://github.com/latex3/latex3/commit/71e9705ea0cd6c4c6347edf1765a44c7c3df4669

>---------------------------------------------------------------

commit 71e9705ea0cd6c4c6347edf1765a44c7c3df4669
Author: Bruno Le Floch <blflatex at gmail.com>
Date:   Fri Jan 12 22:57:22 2024 +0100

    Speed up tl_if_head_is_N_type as suggested by Skillmon (closes #1127)
    
    Doing this as a single commit on main seems much simpler than revising
    this old pull request a bit, squashing etc.  I've clarified our
    explanations of what is going on in the code.


>---------------------------------------------------------------

71e9705ea0cd6c4c6347edf1765a44c7c3df4669
 l3kernel/l3tl.dtx | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/l3kernel/l3tl.dtx b/l3kernel/l3tl.dtx
index 105cc7581..787713045 100644
--- a/l3kernel/l3tl.dtx
+++ b/l3kernel/l3tl.dtx
@@ -3208,17 +3208,22 @@
 % \begin{macro}[EXP]
 %   {
 %     \@@_if_head_is_N_type_auxi:w   ,
-%     \@@_if_head_is_N_type_auxii:nn ,
-%     \@@_if_head_is_N_type_auxiii:n
+%     \@@_if_head_is_N_type_auxii:n ,
 %   }
 %   A token list can be empty, can start with an explicit space
 %   character (catcode 10 and charcode 32), can start with a begin-group
 %   token (catcode 1), or start with an \texttt{N}-type argument.  In
-%   the first two cases, the line involving \cs{@@_if_head_is_N_type_auxi:w}
-%   produces~|f| (and otherwise nothing).  In the third case
-%   (begin-group token), the lines involving \cs{token_to_str:N} produce a
-%   single closing brace.  The category code test is thus true exactly
-%   in the fourth case, which is what we want.  One cannot optimize by
+%   the first two cases, and when |#1~| starts with |{}~|,
+%   \cs{@@_if_head_is_N_type_auxi:w} receives an empty argument hence
+%   produces |f| and removes everything before the first
+%   \cs{scan_stop:}.  In the third case (except when |#1~| starts with
+%   |{}~|), the second auxiliary removes the first copy of~|#1| that was
+%   used for the space test, then expands \cs{token_to_str:N} which hits
+%   the leading begin-group token, leaving a single closing brace to be
+%   compared with \cs{scan_stop:}.  In the last case,
+%   \cs{token_to_str:N} does not change the brace balance so that only
+%   \cs{scan_stop:} \cs{scan_stop:} remain, making the character code
+%   test true.  One cannot optimize by
 %   moving one of the \cs{scan_stop:} to the beginning: if |#1| contains
 %   primitive conditionals, all of its occurrences must be dealt with before
 %   the \cs{if:w} tries to skip the \texttt{true} branch of the
@@ -3227,7 +3232,7 @@
 \prg_new_conditional:Npnn \tl_if_head_is_N_type:n #1 { p , T , F , TF }
   {
     \if:w
-        \if_false: { \fi: \@@_if_head_is_N_type_auxi:w \prg_do_nothing: #1 ~ }
+        \if_false: { \fi: \@@_if_head_is_N_type_auxi:w #1 ~ }
         { \exp_after:wN { \token_to_str:N #1 } }
         \scan_stop: \scan_stop:
       \prg_return_true:
@@ -3237,7 +3242,7 @@
   }
 \exp_args:Nno \use:n { \cs_new:Npn \@@_if_head_is_N_type_auxi:w #1 ~ }
   {
-    \tl_if_empty:oTF { #1 }
+    \tl_if_empty:nTF {#1}
       { f \exp_after:wN \use_none:nn }
       { \exp_after:wN \@@_if_head_is_N_type_auxii:n }
     \exp_after:wN { \if_false: } \fi: