Implement special sigma casing rule (bug#24603)

In Greek, a sigma character has two lower case forms which depend on
their position in the word.  Implement logic determining it.

* src/casefiddle.c (struct casing_context, case_character_impl): Don’t
assume inword is true when flag is CASE_UP and false when flag is
CASE_DOWN.  For final sigma detection we need this information tracked
reliably;.
(CAPITAL_SIGMA, SMALL_SIGMA, SMALL_FINAL_SIGMA): New macros defining
Unicode code point of different forms of sigma letter.
(case_character): Implement support for final sigma casing.
(do_casify_multibyte_string, do_casify_multibyte_region): Update after
changes to case_character.

* test/src/casefiddle-tests.el (casefiddle-tests-casing): Add test
cases for final sigma.

1 files changed, 9 insertions, 6 deletions

diff --git a/test/src/casefiddle-tests.el b/test/src/casefiddle-tests.el
index dd260633f4c..234d233c71a 100644
--- a/test/src/casefiddle-tests.el
+++ b/test/src/casefiddle-tests.el
@@ -195,13 +195,16 @@
         ("define" "DEFINE" "define" "Define" "Define")
         ("fish" "FISH" "fish" "Fish" "Fish")
         ("Straße" "STRASSE" "straße" "Straße" "Straße")
-        ;; FIXME(bug#24603): Everything below is broken at the moment.
-        ;; Here’s what should happen:
-        ;;("ΌΣΟΣ" "ΌΣΟΣ" "όσος" "Όσος" "Όσος")
-        ;; And here’s what is actually happening:
-        ("ΌΣΟΣ" "ΌΣΟΣ" "όσοσ" "Όσοσ" "ΌΣΟΣ")
 
-        ("όσος" "ΌΣΟΣ" "όσος" "Όσος" "Όσος"))))))
+        ;; The word repeated twice to test behaviour at the end of a word
+        ;; inside of an input string as well as at the end of the string.
+        ("ΌΣΟΣ ΌΣΟΣ" "ΌΣΟΣ ΌΣΟΣ" "όσος όσος" "Όσος Όσος" "ΌΣΟΣ ΌΣΟΣ")
+        ;; What should be done with sole sigma?  It is ‘final’ but on the
+        ;; other hand it does not form a word.  We’re using regular sigma.
+        ("Σ Σ" "Σ Σ" "σ σ" "Σ Σ" "Σ Σ")
+        ("όσος" "ΌΣΟΣ" "όσος" "Όσος" "Όσος")
+        ;; If sigma is already lower case, we don’t want to change it.
+        ("όσοσ" "ΌΣΟΣ" "όσοσ" "Όσοσ" "Όσοσ"))))))
 
 (ert-deftest casefiddle-tests-casing-byte8 ()
   (should-not