perf(reflow): eliminate most WordWrapper allocations (#1239)

On large paragraphs (~1MB), this saves hundreds of thousands of
allocations.

TL;DR: reuse as much memory as possible across `next_line` calls.
Instead of allocating new buffers each time, allocate the buffers once
and clear them before reuse.

Signed-off-by: Alex Saveau <saveau.alexandre@gmail.com>

src/widgets/reflow.rs

@@ -1,4 +1,4 @@
-use std::{collections::VecDeque, vec::IntoIter};
+use std::{collections::VecDeque, mem};
 
 use unicode_segmentation::UnicodeSegmentation;
 use unicode_width::UnicodeWidthStr;
@@ -34,11 +34,16 @@ where
     /// The given, unprocessed lines
     input_lines: O,
     max_line_width: u16,
-    wrapped_lines: Option<IntoIter<Vec<StyledGrapheme<'a>>>>,
+    wrapped_lines: VecDeque<Vec<StyledGrapheme<'a>>>,
     current_alignment: Alignment,
     current_line: Vec<StyledGrapheme<'a>>,
     /// Removes the leading whitespace from lines
     trim: bool,
+
+    // These are cached allocations that hold no state across next_line invocations
+    pending_word: Vec<StyledGrapheme<'a>>,
+    pending_whitespace: VecDeque<StyledGrapheme<'a>>,
+    pending_line_pool: Vec<Vec<StyledGrapheme<'a>>>,
 }
 
 impl<'a, O, I> WordWrapper<'a, O, I>
@@ -50,29 +55,30 @@ where
         Self {
             input_lines: lines,
             max_line_width,
-            wrapped_lines: None,
+            wrapped_lines: VecDeque::new(),
             current_alignment: Alignment::Left,
             current_line: vec![],
             trim,
-        }
-    }
 
-    fn next_cached_line(&mut self) -> Option<Vec<StyledGrapheme<'a>>> {
+            pending_word: Vec::new(),
-        self.wrapped_lines.as_mut()?.next()
+            pending_line_pool: Vec::new(),
+            pending_whitespace: VecDeque::new(),
+        }
     }
 
     /// Split an input line (`line_symbols`) into wrapped lines
     /// and cache them to be emitted later
     fn process_input(&mut self, line_symbols: impl IntoIterator<Item = StyledGrapheme<'a>>) {
-        let mut result_lines = vec![];
+        let mut pending_line = self.pending_line_pool.pop().unwrap_or_default();
-        let mut pending_line = vec![];
         let mut line_width = 0;
-        let mut pending_word = vec![];
         let mut word_width = 0;
-        let mut pending_whitespace: VecDeque<StyledGrapheme> = VecDeque::new();
         let mut whitespace_width = 0;
         let mut non_whitespace_previous = false;
 
+        self.pending_word.clear();
+        self.pending_whitespace.clear();
+        pending_line.clear();
+
         for grapheme in line_symbols {
             let is_whitespace = grapheme.is_whitespace();
             let symbol_width = grapheme.symbol.width() as u16;
@@ -99,14 +105,14 @@ where
             // append finished segment to current line
             if word_found || trimmed_overflow || whitespace_overflow || untrimmed_overflow {
                 if !pending_line.is_empty() || !self.trim {
-                    pending_line.extend(pending_whitespace.drain(..));
+                    pending_line.extend(self.pending_whitespace.drain(..));
                     line_width += whitespace_width;
                 }
 
-                pending_line.append(&mut pending_word);
+                pending_line.append(&mut self.pending_word);
                 line_width += word_width;
 
-                pending_whitespace.clear();
+                self.pending_whitespace.clear();
                 whitespace_width = 0;
                 word_width = 0;
             }
@@ -121,11 +127,11 @@ where
             if line_full || pending_word_overflow {
                 let mut remaining_width = u16::saturating_sub(self.max_line_width, line_width);
 
-                result_lines.push(std::mem::take(&mut pending_line));
+                self.wrapped_lines.push_back(mem::take(&mut pending_line));
                 line_width = 0;
 
                 // remove whitespace up to the end of line
-                while let Some(grapheme) = pending_whitespace.front() {
+                while let Some(grapheme) = self.pending_whitespace.front() {
                     let width = grapheme.symbol.width() as u16;
 
                     if width > remaining_width {
@@ -134,11 +140,11 @@ where
 
                     whitespace_width -= width;
                     remaining_width -= width;
-                    pending_whitespace.pop_front();
+                    self.pending_whitespace.pop_front();
                 }
 
                 // don't count first whitespace toward next word
-                if is_whitespace && pending_whitespace.is_empty() {
+                if is_whitespace && self.pending_whitespace.is_empty() {
                     continue;
                 }
             }
@@ -146,33 +152,43 @@ where
             // append symbol to a pending buffer
             if is_whitespace {
                 whitespace_width += symbol_width;
-                pending_whitespace.push_back(grapheme);
+                self.pending_whitespace.push_back(grapheme);
             } else {
                 word_width += symbol_width;
-                pending_word.push(grapheme);
+                self.pending_word.push(grapheme);
             }
 
             non_whitespace_previous = !is_whitespace;
         }
 
         // append remaining text parts
-        if pending_line.is_empty() && pending_word.is_empty() && !pending_whitespace.is_empty() {
+        if pending_line.is_empty()
-            result_lines.push(vec![]);
+            && self.pending_word.is_empty()
+            && !self.pending_whitespace.is_empty()
+        {
+            self.wrapped_lines.push_back(vec![]);
         }
         if !pending_line.is_empty() || !self.trim {
-            pending_line.extend(pending_whitespace);
+            pending_line.extend(self.pending_whitespace.drain(..));
         }
-        pending_line.extend(pending_word);
+        pending_line.append(&mut self.pending_word);
 
+        #[allow(clippy::else_if_without_else)]
         if !pending_line.is_empty() {
-            result_lines.push(pending_line);
+            self.wrapped_lines.push_back(pending_line);
+        } else if pending_line.capacity() > 0 {
+            self.pending_line_pool.push(pending_line);
         }
-        if result_lines.is_empty() {
+        if self.wrapped_lines.is_empty() {
-            result_lines.push(vec![]);
+            self.wrapped_lines.push_back(vec![]);
         }
+    }
 
-        // save processed lines for emitting later
+    fn replace_current_line(&mut self, line: Vec<StyledGrapheme<'a>>) {
-        self.wrapped_lines = Some(result_lines.into_iter());
+        let cache = mem::replace(&mut self.current_line, line);
+        if cache.capacity() > 0 {
+            self.pending_line_pool.push(cache);
+        }
     }
 }
 
@@ -189,13 +205,13 @@ where
 
         loop {
             // emit next cached line if present
-            if let Some(line) = self.next_cached_line() {
+            if let Some(line) = self.wrapped_lines.pop_front() {
                 let line_width = line
                     .iter()
                     .map(|grapheme| grapheme.symbol.width() as u16)
                     .sum();
 
-                self.current_line = line;
+                self.replace_current_line(line);
                 return Some(WrappedLine {
                     line: &self.current_line,
                     width: line_width,