bug#24603: [RFC 03/18] Don’t assume character can be either upper- or lower-case when casing

[Michal Nazarewicz <mina86@mina86.com>]

A compatibility digraph characters, such as Dž, are neither upper- nor
lower-case.  At the moment however, those are reported as upper-case¹
despite the fact that they change when upper-cased.

Stop checking if a character is upper-case before trying to up-case it
so that title-case characters are handled correctly.

¹ Because they change when converted to lower-case.  Notice an asymmetry
  in that for a character to be considered lower-case it must not be
  upper-case (plus the usual condition of changing when upper-cased).

* src/buffer.h (upcase1): Delete.
(upcase): Change to upcase character unconditionally just like downcase
does it.  This is what upcase1 was.

* src/casefiddle.c (casify_object, casify_region): Use upcase instead
of upcase1 and don’t check !uppercasep(x) before calling upcase.

* src/keyboard.c (read_key_sequence): Don’t check if uppercase(x), just
downcase(x) and see if it changed.

* test/src/casefiddle-tests.el (casefiddle-tests--characters,
casefiddle-tests-casing): Update test cases which are now passing.
---
 etc/NEWS                     |  8 +++++++-
 src/buffer.h                 | 18 +++++++++---------
 src/casefiddle.c             | 20 +++++++-------------
 src/keyboard.c               | 25 +++++++++++++++----------
 test/src/casefiddle-tests.el |  8 ++++----
 5 files changed, 42 insertions(+), 37 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index bd94c94..61afcc6 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -235,6 +235,12 @@ same as in modes where the character is not whitespace.
 Instead of only checking the modification time, Emacs now also checks
 the file's actual content before prompting the user.
 
+** Title case characters are properly converted to upper case.
+'upcase', 'upcase-region' et al. convert title case characters (such
+as Dz) into their upper case form (such as DZ).  As a downside,
+'capitalize' and 'upcase-initials' produce awkward words where first
+two letters are upper case, e.g. DŽungla (instead of Džungla).
+
 \f
 * Changes in Specialized Modes and Packages in Emacs 26.1
 
@@ -662,7 +668,7 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
 
 \f
 Local variables:
-coding: us-ascii
+coding: utf-8
 mode: outline
 paragraph-separate: "[ 	\f]*$"
 end:
diff --git a/src/buffer.h b/src/buffer.h
index 6ac161c..1543f67 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -1349,28 +1349,28 @@ downcase (int c)
   return NATNUMP (down) ? XFASTINT (down) : c;
 }
 
-/* True if C is upper case.  */
-INLINE bool uppercasep (int c) { return downcase (c) != c; }
-
-/* Upcase a character C known to be not upper case.  */
+/* Upcase a character C, or make no change if that cannot be done. */
 INLINE int
-upcase1 (int c)
+upcase (int c)
 {
   Lisp_Object upcase_table = BVAR (current_buffer, upcase_table);
   Lisp_Object up = CHAR_TABLE_REF (upcase_table, c);
   return NATNUMP (up) ? XFASTINT (up) : c;
 }
 
+/* True if C is upper case.  */
+INLINE bool uppercasep (int c)
+{
+  return downcase (c) != c;
+}
+
 /* True if C is lower case.  */
 INLINE bool
 lowercasep (int c)
 {
-  return !uppercasep (c) && upcase1 (c) != c;
+  return !uppercasep (c) && upcase (c) != c;
 }
 
-/* Upcase a character C, or make no change if that cannot be done.  */
-INLINE int upcase (int c) { return uppercasep (c) ? c : upcase1 (c); }
-
 INLINE_HEADER_END
 
 #endif /* EMACS_BUFFER_H */
diff --git a/src/casefiddle.c b/src/casefiddle.c
index 2d32f49..b86f485 100644
--- a/src/casefiddle.c
+++ b/src/casefiddle.c
@@ -64,13 +64,9 @@ casify_object (enum case_action flag, Lisp_Object obj)
 	multibyte = 1;
       if (! multibyte)
 	MAKE_CHAR_MULTIBYTE (c1);
-      c = downcase (c1);
-      if (inword)
-	XSETFASTINT (obj, c | flags);
-      else if (c == (XFASTINT (obj) & ~flagbits))
+      c = flag == CASE_DOWN ? downcase (c1) : upcase (c1);
+      if (c != c1)
 	{
-	  if (! inword)
-	    c = upcase1 (c1);
 	  if (! multibyte)
 	    MAKE_CHAR_UNIBYTE (c);
 	  XSETFASTINT (obj, c | flags);
@@ -95,7 +91,7 @@ casify_object (enum case_action flag, Lisp_Object obj)
 	    c = downcase (c);
 	  else if (!uppercasep (c)
 		   && (!inword || flag != CASE_CAPITALIZE_UP))
-	    c = upcase1 (c1);
+	    c = upcase (c1);
 	  if ((int) flag >= (int) CASE_CAPITALIZE)
 	    inword = (SYNTAX (c) == Sword);
 	  if (c != c1)
@@ -127,9 +123,8 @@ casify_object (enum case_action flag, Lisp_Object obj)
 	  c = STRING_CHAR_AND_LENGTH (SDATA (obj) + i_byte, len);
 	  if (inword && flag != CASE_CAPITALIZE_UP)
 	    c = downcase (c);
-	  else if (!uppercasep (c)
-		   && (!inword || flag != CASE_CAPITALIZE_UP))
-	    c = upcase1 (c);
+	  else if (!inword || flag != CASE_CAPITALIZE_UP)
+	    c = upcase (c);
 	  if ((int) flag >= (int) CASE_CAPITALIZE)
 	    inword = (SYNTAX (c) == Sword);
 	  o += CHAR_STRING (c, o);
@@ -236,9 +231,8 @@ casify_region (enum case_action flag, Lisp_Object b, Lisp_Object e)
       c2 = c;
       if (inword && flag != CASE_CAPITALIZE_UP)
 	c = downcase (c);
-      else if (!uppercasep (c)
-	       && (!inword || flag != CASE_CAPITALIZE_UP))
-	c = upcase1 (c);
+      else if (!inword || flag != CASE_CAPITALIZE_UP)
+	c = upcase (c);
       if ((int) flag >= (int) CASE_CAPITALIZE)
 	inword = ((SYNTAX (c) == Sword)
 		  && (inword || !syntax_prefix_flag_p (c)));
diff --git a/src/keyboard.c b/src/keyboard.c
index ca40c6e..2115fc9 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -9633,22 +9633,26 @@ read_key_sequence (Lisp_Object *keybuf, int bufsize, Lisp_Object prompt,
 	 use the corresponding lower-case letter instead.  */
       if (NILP (current_binding)
 	  && /* indec.start >= t && fkey.start >= t && */ keytran.start >= t
-	  && INTEGERP (key)
-	  && ((CHARACTERP (make_number (XINT (key) & ~CHAR_MODIFIER_MASK))
-	       && uppercasep (XINT (key) & ~CHAR_MODIFIER_MASK))
-	      || (XINT (key) & shift_modifier)))
+	  && INTEGERP (key))
 	{
 	  Lisp_Object new_key;
+	  int k = XINT (key);
+
+	  if (k & shift_modifier)
+	    XSETINT (new_key, k & ~shift_modifier);
+	  else if (CHARACTERP (make_number (k & ~CHAR_MODIFIER_MASK)))
+	    {
+	      int dc = downcase(k & ~CHAR_MODIFIER_MASK);
+	      if (dc == (k & ~CHAR_MODIFIER_MASK))
+		goto not_upcase;
+	      XSETINT (new_key, dc | (k & CHAR_MODIFIER_MASK));
+	    }
+	  else
+	    goto not_upcase;
 
 	  original_uppercase = key;
 	  original_uppercase_position = t - 1;
 
-	  if (XINT (key) & shift_modifier)
-	    XSETINT (new_key, XINT (key) & ~shift_modifier);
-	  else
-	    XSETINT (new_key, (downcase (XINT (key) & ~CHAR_MODIFIER_MASK)
-			       | (XINT (key) & CHAR_MODIFIER_MASK)));
-
 	  /* We have to do this unconditionally, regardless of whether
 	     the lower-case char is defined in the keymaps, because they
 	     might get translated through function-key-map.  */
@@ -9659,6 +9663,7 @@ read_key_sequence (Lisp_Object *keybuf, int bufsize, Lisp_Object prompt,
 	  goto replay_sequence;
 	}
 
+    not_upcase:
       if (NILP (current_binding)
 	  && help_char_p (EVENT_HEAD (key)) && t > 1)
 	    {
diff --git a/test/src/casefiddle-tests.el b/test/src/casefiddle-tests.el
index ca3657d..8d9bf01 100644
--- a/test/src/casefiddle-tests.el
+++ b/test/src/casefiddle-tests.el
@@ -62,13 +62,13 @@ casefiddle-tests--characters
     (?Ł ?Ł ?ł ?Ł)
     (?ł ?Ł ?ł ?Ł)
 
-    ;; FIXME: We should have:
+    ;; FIXME: Commented one is what we want.
     ;;(?DŽ ?DŽ ?dž ?Dž)
-    ;; but instead we have:
     (?DŽ ?DŽ ?dž ?DŽ)
-    ;; FIXME: Those two are broken at the moment:
     ;;(?Dž ?DŽ ?dž ?Dž)
+    (?Dž ?DŽ ?dž ?DŽ)
     ;;(?dž ?DŽ ?dž ?Dž)
+    (?dž ?DŽ ?dž ?DŽ)
 
     (?Σ ?Σ ?σ ?Σ)
     (?σ ?Σ ?σ ?Σ)
@@ -152,7 +152,7 @@ casefiddle-tests--characters
               ;;("ΌΣΟΣ" "ΌΣΟΣ" "όσος" "Όσος" "Όσος")
               ;; And here’s what is actually happening:
               ("DŽUNGLA" "DŽUNGLA" "džungla" "DŽungla" "DŽUNGLA")
-              ("Džungla" "DžUNGLA" "džungla" "Džungla" "Džungla")
+              ("Džungla" "DŽUNGLA" "džungla" "DŽungla" "DŽungla")
               ("džungla" "DŽUNGLA" "džungla" "DŽungla" "DŽungla")
               ("define" "DEfiNE" "define" "Define" "Define")
               ("fish" "fiSH" "fish" "fish" "fish")
-- 
2.8.0.rc3.226.g39d4020