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Re: Converting compositions to strings

From: Clément Pit-Claudel
Subject: Re: Converting compositions to strings
Date: Mon, 30 Jan 2017 11:39:09 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

On 2017-01-30 10:18, Eli Zaretskii wrote:
> The information about this can be found in the doc strong of 
> compose-region, under COMPONENTS.

Thanks.  This covers only "relative" compositions, right? Is there more 
information on non-relative compositions somewhere?

>> Is there a recipe that I can use to construct a string 
>> representation of a composition? (I realize that many compositions 
>> can't adequately be represented as plain strings; I'm only 
>> interested in those that can).
> The ones that seem to be relevant to what you are looking for (I'm 
> guessing) are a small class, and I don't think you can discern them 
> from the others unless you look at the "rules" parts of a 
> composition.

Looking at the rules sounds fine; thanks! I think these compositions form a 
small class in terms of what Emacs supports, but that class seems to most if 
not all uses of composition that I see in my daily use of Emacs (where are 
other compositions used, beyond character spacing in what-cursor-position?).

Based on your pointers, the following seems to work OK for me (I guess it could 
be extended to cover the (Bl . Br) case, but I haven't seen that pop up yet in 

(defun esh--parse-composition (components)
  "Translate composition COMPONENTS into a string."
  (let ((chars (list (aref components 0)))
        (nrules (/ (length components) 2)))
    (dotimes (nrule nrules)
      (let* ((rule (aref components (+ 1 (* 2 nrule))))
             (char (aref components (+ 2 (* 2 nrule)))))
        (pcase rule
          (`(Br . Bl) (push char chars))
          (_ (error "Unsupported composition COMPONENTS")))))
    (concat chars)))


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