From 561b83dbc9810ec09267c779e539a7d913ae3332 Mon Sep 17 00:00:00 2001
From: Chris Vest <mr.chrisvest@gmail.com>
Date: Tue, 12 Mar 2019 21:30:46 +0100
Subject: [PATCH] Avoid iterating graphemes unless we have to.

Iterating graphemes with `text.graphemes(true)` is somewhat expensive due to the complexity of unicode.
However, we were doing it _twice_ in the `Printer::print` method in order to truncate the input text to fit the content area.
This changes that method to track the width of the intput text, and only do the grapheme iteration when truncation is actually necessary.
---
 src/printer.rs | 59 +++++++++++++++++++++++++++++---------------------
 1 file changed, 34 insertions(+), 25 deletions(-)

diff --git a/src/printer.rs b/src/printer.rs
index 282e081..4dc8f82 100644
--- a/src/printer.rs
+++ b/src/printer.rs
@@ -129,46 +129,55 @@ impl<'a, 'b> Printer<'a, 'b> {
             return;
         }
 
-        let text_width = text.width();
+        let mut text_width = text.width();
 
         // If we're waaaay too far left, just give up.
         if hidden_part.x > text_width {
             return;
         }
+        
+        let mut text = text;
+        let mut start = start;
+        
+        if hidden_part.x > 0 {
+            // We have to drop hidden_part.x width from the start of the string.
+            // prefix() may be too short if there's a double-width character.
+            // So instead, keep the suffix and drop the prefix.
 
-        // We have to drop hidden_part.x width from the start of the string.
-        // prefix() may be too short if there's a double-width character.
-        // So instead, keep the suffix and drop the prefix.
+            // TODO: use a different prefix method that is *at least* the width
+            // (and not *at most*)
+            let tail =
+                suffix(text.graphemes(true), text_width - hidden_part.x, "");
+            let skipped_len = text.len() - tail.length;
+            let skipped_width = text_width - tail.width;
+            assert_eq!(text[..skipped_len].width(), skipped_width);
 
-        // TODO: use a different prefix method that is *at least* the width
-        // (and not *at most*)
-        let tail =
-            suffix(text.graphemes(true), text_width - hidden_part.x, "");
-        let skipped_len = text.len() - tail.length;
-        let skipped_width = text_width - tail.width;
-        assert_eq!(text[..skipped_len].width(), skipped_width);
+            // This should be equal most of the time, except when there's a double
+            // character preventing us from splitting perfectly.
+            assert!(skipped_width >= hidden_part.x);
 
-        // This should be equal most of the time, except when there's a double
-        // character preventing us from splitting perfectly.
-        assert!(skipped_width >= hidden_part.x);
-
-        // Drop part of the text, and move the cursor correspondingly.
-        let text = &text[skipped_len..];
-        let start = start + (skipped_width, 0);
+            // Drop part of the text, and move the cursor correspondingly.
+            text = &text[skipped_len..];
+            start = start + (skipped_width, 0);
+            text_width -= skipped_width;
+        }
+        
         assert!(start.fits(self.content_offset));
 
         // What we did before should guarantee that this won't overflow.
-        let start = start - self.content_offset;
+        start = start - self.content_offset;
 
         // Do we have enough room for the entire line?
         let room = self.output_size.x - start.x;
 
-        // Drop the end of the text if it's too long
-        // We want the number of CHARACTERS, not bytes.
-        // (Actually we want the "width" of the string, see unicode-width)
-        let prefix_len = prefix(text.graphemes(true), room, "").length;
-        let text = &text[..prefix_len];
-        assert!(text.width() <= room);
+        if room < text_width {
+            // Drop the end of the text if it's too long
+            // We want the number of CHARACTERS, not bytes.
+            // (Actually we want the "width" of the string, see unicode-width)
+            let prefix_len = prefix(text.graphemes(true), room, "").length;
+            text = &text[..prefix_len];
+            assert!(text.width() <= room);
+        }
 
         let start = start + self.offset;
         self.backend.print_at(start, text);