Truncate the starts of file paths instead of the ends in picker (#951)

* Truncate the starts of file paths in picker

* Simplify the truncate implementation

* Break loop at appropriate point

* Fix alignment and ellipsis presence

* Remove extraneous usage of `x_offset`

Co-authored-by: Blaž Hrastnik <blaz@mxxn.io>

helix-term/src/ui/picker.rs

@@ -483,6 +483,7 @@ impl<T: 'static> Component for Picker<T> {
                     text_style
                 },
                 true,
+                true,
             );
         }
     }

helix-tui/src/buffer.rs

@@ -266,12 +266,14 @@ impl Buffer {
     where
         S: AsRef<str>,
     {
-        self.set_string_truncated(x, y, string, width, style, false)
+        self.set_string_truncated(x, y, string, width, style, false, false)
     }
 
     /// Print at most the first `width` characters of a string if enough space is available
-    /// until the end of the line. If `markend` is true appends a `…` at the end of
+    /// until the end of the line. If `ellipsis` is true appends a `…` at the end of
-    /// truncated lines.
+    /// truncated lines. If `truncate_start` is `true`, truncate the beginning of the string
+    /// instead of the end.
+    #[allow(clippy::too_many_arguments)]
     pub fn set_string_truncated<S>(
         &mut self,
         x: u16,
@@ -280,6 +282,7 @@ impl Buffer {
         width: usize,
         style: Style,
         ellipsis: bool,
+        truncate_start: bool,
     ) -> (u16, u16)
     where
         S: AsRef<str>,
@@ -289,6 +292,7 @@ impl Buffer {
         let width = if ellipsis { width - 1 } else { width };
         let graphemes = UnicodeSegmentation::graphemes(string.as_ref(), true);
         let max_offset = min(self.area.right() as usize, width.saturating_add(x as usize));
+        if !truncate_start {
             for s in graphemes {
                 let width = s.width();
                 if width == 0 {
@@ -312,6 +316,36 @@ impl Buffer {
             if ellipsis && x_offset - (x as usize) < string.as_ref().width() {
                 self.content[index].set_symbol("…");
             }
+        } else {
+            let mut start_index = self.index_of(x, y);
+            let mut index = self.index_of(max_offset as u16, y);
+
+            let total_width = string.as_ref().width();
+            let truncated = total_width > width;
+            if ellipsis && truncated {
+                self.content[start_index].set_symbol("…");
+                start_index += 1;
+            }
+            if !truncated {
+                index -= width - total_width;
+            }
+            for s in graphemes.rev() {
+                let width = s.width();
+                if width == 0 {
+                    continue;
+                }
+                let start = index - width;
+                if start < start_index {
+                    break;
+                }
+                self.content[start].set_symbol(s);
+                self.content[start].set_style(style);
+                for i in start + 1..index {
+                    self.content[i].reset();
+                }
+                index -= width;
+            }
+        }
         (x_offset as u16, y)
     }