[bug #66919] [troff] behavior change in some .hcode calls when a special character is the first argument

[G. Branden Robinson]

Follow-up Comment #13, bug #66919 (group groff):

Bear in mind that "bad" is merely a label.  In this case it refers to the
change from _groff_ 1.23.0 and earlier behavior.


a52141ac46eef95dd1f85e4c2e0a336affa9bcc9 is the first bad commit
commit a52141ac46eef95dd1f85e4c2e0a336affa9bcc9
Author: G. Branden Robinson <g.branden.robin...@gmail.com>
Date:   Thu Aug 8 06:53:23 2024 -0500

    [troff]: Continue fixing Savannah #66054.
    
    * src/roff/troff/input.cpp (set_hyphenation_codes): Handle the common
      case (copying the existing hyphenation code of one character to
      another) first.  Support clearing a character's hyphenation code by
      copying that of a character that lacks one.  Reorganize for clarity,
      and add comments.
    
    Continues fixing <https://savannah.gnu.org/bugs/?66054>.

 ChangeLog                | 10 ++++++++++
 src/roff/troff/input.cpp | 23 ++++++++++++-----------
 2 files changed, 22 insertions(+), 11 deletions(-)


In this case, the change is deliberate:

"Support clearing a character's hyphenation code by copying that of a
character that lacks one."

Here's the entire diff (of "input.cpp").


diff --git a/src/roff/troff/input.cpp b/src/roff/troff/input.cpp
index cc7d9dd71..946b93570 100644
--- a/src/roff/troff/input.cpp
+++ b/src/roff/troff/input.cpp
@@ -7309,25 +7309,26 @@ static void set_hyphenation_codes()
       error("cannot use the hyphenation code of a numeral");
       break;
     }
-    unsigned char new_code = 0; // TODO: int
+    unsigned char new_code = 0;
     charinfo *cisrc = tok.get_char();
-    if (csrc != 0)
-      new_code = csrc;
-    else {
+    if (cisrc != 0 /* nullptr */)
+      // Common case: assign destination character the hyphenation code
+      // of the source character.
+      new_code = cisrc->get_hyphenation_code();
+    if (0 == csrc) {
       if (0 /* nullptr */ == cisrc) {
        error("expected ordinary or special character, got %1",
              tok.description());
        break;
       }
-      // source character is special
-      if (0 == cisrc->get_hyphenation_code()) {
-       error("second member of hyphenation code pair must be an"
-             " ordinary character, or a special character already"
-             " assigned a hyphenation code");
-       break;
-      }
       new_code = cisrc->get_hyphenation_code();
     }
+    else {
+      // If assigning a ordinary character's hyphenation code to itself,
+      // use its character code point as the value.
+      if (csrc == cdst)
+       new_code = tok.ch();
+    }
     cidst->set_hyphenation_code(new_code);
     if (cidst->get_translation()
        && cidst->get_translation()->get_translation_input())




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