Detecting Unicode text ligatures in Clojure/Java

Ligatures are the Unicode characters which are represented by more than one code points. For example, in Devanagari त्र is a ligature which consists of code points त + ् + र.

When seen in simple text file editors like Notepad, त्र is shown as त् + र and is stored as three Unicode characters. However when the same file is opened in Firefox, it is shown as a proper ligature.

So my question is, how to detect such ligatures programmatically while reading the file from my code. Since Firefox does it, there must exist a way to do it programmatically. Are there any Unicode properties which contain this information or do I need to have a map to all such ligatures?

SVG CSS property text-rendering when set to optimizeLegibility does the same thing (combine code points into proper ligature).

PS: I am using Java.

EDIT

The purpose of my code is to count the characters in the Unicode text assuming a ligature to be a single character. So I need a way to collapse multiple code points into a single ligature.

Solution

While Aaron's answer is not exactly correct, it pushed me in the right direction. After reading through the Java API docs of java.awt.font.GlyphVector and playing a lot on the Clojure REPL, I was able to write a function which does what I want.

The idea is to find the width of glyphs in the glyphVector and combine the glyphs with zero width with the last found non-zero width glyph. The solution is in Clojure but it should be translatable to Java if required.

(ns net.abhinavsarkar.unicode
  (:import [java.awt.font TextAttribute GlyphVector]
           [java.awt Font]
           [javax.swing JTextArea]))

(let [^java.util.Map text-attrs {
        TextAttribute/FAMILY "Arial Unicode MS"
        TextAttribute/SIZE 25
        TextAttribute/LIGATURES TextAttribute/LIGATURES_ON}
      font (Font/getFont text-attrs)
      ta (doto (JTextArea.) (.setFont font))
      frc (.getFontRenderContext (.getFontMetrics ta font))]
  (defn unicode-partition
    "takes an unicode string and returns a vector of strings by partitioning
    the input string in such a way that multiple code points of a single
    ligature are in same partition in the output vector"
    [^String text]
    (let [glyph-vector 
            (.layoutGlyphVector
              font, frc, (.toCharArray text),
              0, (.length text), Font/LAYOUT_LEFT_TO_RIGHT)
          glyph-num (.getNumGlyphs glyph-vector)
          glyph-positions
            (map first (partition 2
                          (.getGlyphPositions glyph-vector 0 glyph-num nil)))
          glyph-widths
            (map -
              (concat (next glyph-positions)
                      [(.. glyph-vector getLogicalBounds width)])
              glyph-positions)
          glyph-indices 
            (seq (.getGlyphCharIndices glyph-vector 0 glyph-num nil))
          glyph-index-width-map (zipmap glyph-indices glyph-widths)
          corrected-glyph-widths
            (vec (reduce
                    (fn [acc [k v]] (do (aset acc k v) acc))
                    (make-array Float (count glyph-index-width-map))
                    glyph-index-width-map))]
      (loop [idx 0 pidx 0 char-seq text acc []]
        (if (nil? char-seq)
          acc
          (if-not (zero? (nth corrected-glyph-widths idx))
            (recur (inc idx) (inc pidx) (next char-seq)
              (conj acc (str (first char-seq))))
            (recur (inc idx) pidx (next char-seq)
              (assoc acc (dec pidx)
                (str (nth acc (dec pidx)) (first char-seq))))))))))

Also posted on Gist.