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Re: search-default-mode char-fold-to-regexp and Greek Extended block cha

From: Robert Pluim
Subject: Re: search-default-mode char-fold-to-regexp and Greek Extended block characters
Date: Mon, 22 Jul 2019 20:39:22 +0200

>>>>> On Sun, 21 Jul 2019 13:03:37 +0200, Robert Pluim <address@hidden> said:

>>>>> On Fri, 19 Jul 2019 21:13:02 +0300, Eli Zaretskii <address@hidden> said:
    Eli> (get-char-code-property ?ί 'decomposition) => (943) ; (#x03af) i.e (?ί)
    Eli> (get-char-code-property ?ί 'decomposition) => (953 769) ; (#x03b9 

    Eli> Do we expand the decomposition property recursively?  It sounds like
    Eli> we don't, but maybe we should.

    Robert> We donʼt. The following patch allows searching for ι (0x3b9) to 
    Robert> both ί (0x3af) and ί (1f77). It doesnʼt recurse, but I have no idea 
    Robert> there are longer chains of decompositions.

The answer to that, empirically, is 'yes', since with the following
patch the number of characters equivalent to ι increases, ie:

Standard =>
(aref char-fold-table ?ι)"\\(?:ι[̀́̄̆̈̓̔͂]\\|[ίιϊἰἱὶιῐῑῖ𝛊𝜄𝜾𝝸𝞲]\\)"

2 level decomposition =>
(aref char-fold-table ?ι)"\\(?:ι[̀́̄̆̈̓̔͂]\\|[ΐίιϊἰ-ἷὶίιῐῑῒῖῗ𝛊𝜄𝜾𝝸𝞲]\\)"

n level decomposition =>
(aref char-fold-table ?ι)"\\(?:ι[̀́̄̆̈̓̔͂]\\|[ΐίιϊἰ-ἷὶίιῐ-ΐῖῗ𝛊𝜄𝜾𝝸𝞲]\\)"

>From 3628379cf461805008b34e01dba751183c0b857c Mon Sep 17 00:00:00 2001
From: Robert Pluim <address@hidden>
Date: Mon, 22 Jul 2019 20:27:59 +0200
Subject: [PATCH] Follow decomposition chains when constructing char-fold-table
To: address@hidden

* lisp/char-fold.el (char-fold-make-table): Decompose the
decomposition of each character, adding equivalences to the original
character, until no more decompositions are left.
 etc/NEWS          |  8 ++++++++
 lisp/char-fold.el | 21 +++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/etc/NEWS b/etc/NEWS
index e9ec21bb4c..33fe7075ec 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1169,6 +1169,14 @@ and case-sensitivity together with search strings in the 
search ring.
 *** 'flush-lines' prints and returns the number of deleted matching lines.
+*** 'char-fold-to-regexp' now matches more variants of a base character.
+The table used to check for equivalence of characters is now built
+using the complete chain of unicode decompositions of a character,
+rather than stopping after one level, such that searching for
 ** Debugger
diff --git a/lisp/char-fold.el b/lisp/char-fold.el
index 9d3ea17b41..6842d38a62 100644
--- a/lisp/char-fold.el
+++ b/lisp/char-fold.el
@@ -78,6 +78,27 @@
                               (cons (char-to-string char)
                                     (aref equiv (car decomp))))))))
                (funcall make-decomp-match-char decomp char)
+               ;; Check to see if the first char of the decomposition
+               ;; has a further decomposition.  If so, add a mapping
+               ;; back from that second decomposition to the original
+               ;; character.  This allows e.g. 'ι' (GREEK SMALL LETTER
+               ;; IOTA) to match both the Basic Greek block and
+               ;; Extended Greek block variants of IOTA +
+               ;; diacritical(s).  Repeat until there are no more
+               ;; decompositions.
+               (let ((dec decomp)
+                     next-decomp)
+                 (catch 'done
+                   (while dec
+                     (setq next-decomp (char-table-range table (car dec)))
+                     (when (consp next-decomp)
+                       (when (symbolp (car next-decomp))
+                         (setq next-decomp (cdr next-decomp)))
+                       (if (not (eq (car dec)
+                                    (car next-decomp)))
+                           (funcall make-decomp-match-char (list (car 
next-decomp)) char)
+                         (throw 'done t)))
+                     (setq dec next-decomp))))
                ;; Do it again, without the non-spacing characters.
                ;; This allows 'a' to match 'ä'.
                (let ((simpler-decomp nil)

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