From 90fb28b4b6146ca8eace35d33102cca202d5d544 Mon Sep 17 00:00:00 2001
From: Laurenz <laurmaedje@gmail.com>
Date: Fri, 4 Feb 2022 21:32:14 +0100
Subject: [PATCH] Add `group` in addition to `group_by_key`

---
 src/library/par.rs |  4 ++--
 src/util/mod.rs    | 54 +++++++++++++++++++++++++++++++---------------
 2 files changed, 39 insertions(+), 19 deletions(-)

diff --git a/src/library/par.rs b/src/library/par.rs
index e87d5f671..4568a2144 100644
--- a/src/library/par.rs
+++ b/src/library/par.rs
@@ -239,9 +239,9 @@ impl<'a> ParLayouter<'a> {
                 ParChild::Text(_) => {
                     // TODO: Also split by language and script.
                     let mut cursor = range.start;
-                    for (level, group) in bidi.levels[range].group_by_key(|&lvl| lvl) {
+                    for (level, count) in bidi.levels[range].group() {
                         let start = cursor;
-                        cursor += group.len();
+                        cursor += count;
                         let subrange = start .. cursor;
                         let text = &bidi.text[subrange.clone()];
                         let shaped = shape(ctx.fonts, text, styles, level.dir());
diff --git a/src/util/mod.rs b/src/util/mod.rs
index de37354e7..2f91100f0 100644
--- a/src/util/mod.rs
+++ b/src/util/mod.rs
@@ -82,9 +82,14 @@ where
 
 /// Additional methods for slices.
 pub trait SliceExt<T> {
-    /// Split a slice into consecutive groups with the same key.
-    ///
-    /// Returns an iterator of pairs of a key and the group with that key.
+    /// Find consecutive runs of the same elements in a slice and yield for
+    /// each such run the element and number of times it appears.
+    fn group(&self) -> Group<'_, T>
+    where
+        T: PartialEq;
+
+    /// Split a slice into consecutive runs with the same key and yield for
+    /// each such run the key and the slice of elements with that key.
     fn group_by_key<K, F>(&self, f: F) -> GroupByKey<'_, T, F>
     where
         F: FnMut(&T) -> K,
@@ -92,15 +97,35 @@ pub trait SliceExt<T> {
 }
 
 impl<T> SliceExt<T> for [T] {
-    fn group_by_key<K, F>(&self, f: F) -> GroupByKey<'_, T, F>
-    where
-        F: FnMut(&T) -> K,
-        K: PartialEq,
-    {
+    fn group(&self) -> Group<'_, T> {
+        Group { slice: self }
+    }
+
+    fn group_by_key<K, F>(&self, f: F) -> GroupByKey<'_, T, F> {
         GroupByKey { slice: self, f }
     }
 }
 
+/// This struct is created by [`SliceExt::group`].
+pub struct Group<'a, T> {
+    slice: &'a [T],
+}
+
+impl<'a, T> Iterator for Group<'a, T>
+where
+    T: PartialEq,
+{
+    type Item = (&'a T, usize);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let mut iter = self.slice.iter();
+        let first = iter.next()?;
+        let count = 1 + iter.take_while(|&t| t == first).count();
+        self.slice = &self.slice[count ..];
+        Some((first, count))
+    }
+}
+
 /// This struct is created by [`SliceExt::group_by_key`].
 pub struct GroupByKey<'a, T, F> {
     slice: &'a [T],
@@ -115,15 +140,10 @@ where
     type Item = (K, &'a [T]);
 
     fn next(&mut self) -> Option<Self::Item> {
-        let first = self.slice.first()?;
-        let key = (self.f)(first);
-
-        let mut i = 1;
-        while self.slice.get(i).map_or(false, |t| (self.f)(t) == key) {
-            i += 1;
-        }
-
-        let (head, tail) = self.slice.split_at(i);
+        let mut iter = self.slice.iter();
+        let key = (self.f)(iter.next()?);
+        let count = 1 + iter.take_while(|t| (self.f)(t) == key).count();
+        let (head, tail) = self.slice.split_at(count);
         self.slice = tail;
         Some((key, head))
     }