Cache flag indicators during reverse iteration

Author: Manishearth

src/grapheme.rs

@@ -48,6 +48,7 @@ pub struct Graphemes<'a> {
     extended: bool,
     cat: Option<GraphemeCat>,
     catb: Option<GraphemeCat>,
+    regional_count_back: Option<usize>,
 }
 
 // state machine for cluster boundary rules
@@ -85,6 +86,11 @@ impl<'a> Iterator for Graphemes<'a> {
         let mut idx = 0;
         let mut state = Start;
         let mut cat = gr::GC_Any;
+
+        // caching used by next_back() should be invalidated
+        self.regional_count_back = None;
+        self.catb = None;
+
         for (curr, ch) in self.string.char_indices() {
             idx = curr;
 
@@ -292,12 +298,15 @@ impl<'a> DoubleEndedIterator for Graphemes<'a> {
                 Regional => {               // rule GB12/GB13
                     // Need to scan backward to find if this is preceded by an odd or even number
                     // of Regional_Indicator characters.
-                    //
-                    // TODO: Save this state to avoid O(n^2) re-scanning in long RI sequences?
-                    let prev_chars = self.string[..previdx].chars().rev();
-                    let count = prev_chars.take_while(|c| {
-                        gr::grapheme_category(*c) == gr::GC_Regional_Indicator
-                    }).count();
+                    let count = match self.regional_count_back {
+                        Some(count) => count,
+                        None => self.string[..previdx].chars().rev().take_while(|c| {
+                                    gr::grapheme_category(*c) == gr::GC_Regional_Indicator
+                                }).count()
+                    };
+                    // Cache the count to avoid re-scanning the same chars on the next iteration.
+                    self.regional_count_back = count.checked_sub(1);
+
                     if count % 2 == 0 {
                         take_curr = false;
                         break;
@@ -372,7 +381,13 @@ impl<'a> DoubleEndedIterator for Graphemes<'a> {
 
 #[inline]
 pub fn new_graphemes<'b>(s: &'b str, is_extended: bool) -> Graphemes<'b> {
-    Graphemes { string: s, extended: is_extended, cat: None, catb: None }
+    Graphemes {
+        string: s,
+        extended: is_extended,
+        cat: None,
+        catb: None,
+        regional_count_back: None
+    }
 }
 
 #[inline]