[buffer] Expose more glyph flags

Fixes #163

Author: behdad

src/hb/buffer.rs

@@ -193,6 +193,49 @@ impl hb_glyph_info_t {
         self.mask & glyph_flag::UNSAFE_TO_BREAK != 0
     }
 
+    /// Indicates that if input text is changed on one side of the beginning of the cluster
+    /// this glyph is part of, then the shaping results for the other side might change.
+    /// Note that the absence of this flag will NOT by itself mean that it IS safe to concat
+    /// text. Only two pieces of text both of which clear of this flag can be concatenated
+    /// safely.
+    ///
+    /// This can be used to optimize paragraph layout, by avoiding re-shaping of each line after
+    /// line-breaking, by limiting the reshaping to a small piece around the breaking position
+    /// only, even if the breaking position carries the unsafe-to-break flag or when hyphenation
+    /// or other text transformation happens at line-break position, in the following way:
+    /// 1. Iterate back from the line-break position until the first cluster start position
+    /// that is NOT unsafe-to-concat.
+    /// 2. Shape the segment from there till the end of line.
+    /// 3. Check whether the resulting glyph-run also is clear of the unsafe-to-concat at its
+    /// start-of-text position; if it is, just splice it into place and the line is shaped;
+    /// If not, move on to a position further back that is clear of unsafe-to-concat and retry
+    /// from there, and repeat.
+    ///
+    /// At the start of next line a similar algorithm can be implemented. That is:
+    /// 1. Iterate forward from the line-break position until the first cluster start position that is
+    /// NOT unsafe-to-concat.
+    /// 2. Shape the segment from beginning of the line to that position.
+    /// 3. Check whether the resulting glyph-run also is clear of the unsafe-to-concat at its end-of-text
+    /// position; if it is, just splice it into place and the beginning is shaped; If not, move on to a
+    /// position further forward that is clear of unsafe-to-concat and retry up to there, and repeat. A
+    /// slight complication will arise in the implementation of the algorithm above, because while our
+    /// buffer API has a way to return flags for position corresponding to start-of-text, there is currently
+    /// no position corresponding to end-of-text. This limitation can be alleviated by shaping more text
+    /// than needed and looking for unsafe-to-concat flag within text clusters. The unsafe-to-break flag will
+    /// always imply this flag. To use this flag, you must enable the buffer flag [`BufferFlags::PRODUCE_UNSAFE_TO_CONCAT`]
+    /// during shaping, otherwise the buffer flag will not be reliably produced.
+    pub fn unsafe_to_concat(&self) -> bool {
+        self.mask & glyph_flag::UNSAFE_TO_CONCAT != 0
+    }
+
+    /// In scripts that use elongation (Arabic, Mongolian, Syriac, etc.), this flag signifies that it is
+    /// safe to insert a U+0640 TATWEEL character before this cluster for elongation. This flag does not
+    /// determine the script-specific elongation places, but only when it is safe to do the elongation
+    /// without interrupting text shaping.
+    pub fn safe_to_insert_tatweel(&self) -> bool {
+        self.mask & glyph_flag::SAFE_TO_INSERT_TATWEEL != 0
+    }
+
     #[inline]
     pub(crate) fn as_char(&self) -> char {
         char::try_from(self.glyph_id).unwrap()