Implement cursorline (#2170)

* Implement cursorline

* Binary search possible lines

3 files changed, 45 insertions, 0 deletions

diff --git a/book/src/configuration.md b/book/src/configuration.md
index 3fa9b307..af85379f 100644
--- a/book/src/configuration.md
+++ b/book/src/configuration.md
@@ -37,6 +37,7 @@ hidden = false
 | `scroll-lines` | Number of lines to scroll per scroll wheel step. | `3` |
 | `shell` | Shell to use when running external commands. | Unix: `["sh", "-c"]`<br/>Windows: `["cmd", "/C"]` |
 | `line-number` | Line number display: `absolute` simply shows each line's number, while `relative` shows the distance from the current line. When unfocused or in insert mode, `relative` will still show absolute line numbers. | `absolute` |
+| `cursorline` | Highlight all lines with a cursor. | `false` |
 | `gutters` | Gutters to display: Available are `diagnostics` and `line-numbers`, note that `diagnostics` also includes other features like breakpoints | `["diagnostics", "line-numbers"]` |
 | `auto-completion` | Enable automatic pop up of auto-completion. | `true` |
 | `auto-format` | Enable automatic formatting on save. | `true` |
diff --git a/helix-term/src/ui/editor.rs b/helix-term/src/ui/editor.rs
index 70f60070..2f44dae9 100644
--- a/helix-term/src/ui/editor.rs
+++ b/helix-term/src/ui/editor.rs
@@ -112,6 +112,10 @@ impl EditorView {
             }
         }
 
+        if editor.config().cursorline {
+            Self::highlight_cursorline(doc, view, surface, theme);
+        }
+
         let highlights = Self::doc_syntax_highlights(doc, view.offset, inner.height, theme);
         let highlights = syntax::merge(highlights, Self::doc_diagnostics_highlights(doc, theme));
         let highlights: Box<dyn Iterator<Item = HighlightEvent>> = if is_focused {
@@ -686,6 +690,43 @@ impl EditorView {
         );
     }
 
+    /// Apply the highlighting on the lines where a cursor is active
+    pub fn highlight_cursorline(doc: &Document, view: &View, surface: &mut Surface, theme: &Theme) {
+        let text = doc.text().slice(..);
+        let last_line = view.last_line(doc);
+
+        let primary_line = doc.selection(view.id).primary().cursor_line(text);
+
+        // The secondary_lines do contain the primary_line, it doesn't matter
+        // as the else-if clause in the loop later won't test for the
+        // secondary_lines if primary_line == line.
+        // It's used inside a loop so the collect isn't needless:
+        // https://github.com/rust-lang/rust-clippy/issues/6164
+        #[allow(clippy::needless_collect)]
+        let secondary_lines: Vec<_> = doc
+            .selection(view.id)
+            .iter()
+            .map(|range| range.cursor_line(text))
+            .collect();
+
+        let primary_style = theme.get("ui.cursorline.primary");
+        let secondary_style = theme.get("ui.cursorline.secondary");
+
+        for line in view.offset.row..(last_line + 1) {
+            let area = Rect::new(
+                view.area.x,
+                view.area.y + (line - view.offset.row) as u16,
+                view.area.width,
+                1,
+            );
+            if primary_line == line {
+                surface.set_style(area, primary_style);
+            } else if secondary_lines.binary_search(&line).is_ok() {
+                surface.set_style(area, secondary_style);
+            }
+        }
+    }
+
     pub fn render_statusline(
         &self,
         doc: &Document,
diff --git a/helix-view/src/editor.rs b/helix-view/src/editor.rs
index c5a458d7..a9741a33 100644
--- a/helix-view/src/editor.rs
+++ b/helix-view/src/editor.rs
@@ -121,6 +121,8 @@ pub struct Config {
     pub shell: Vec<String>,
     /// Line number mode.
     pub line_number: LineNumber,
+    /// Highlight the lines cursors are currently on. Defaults to false.
+    pub cursorline: bool,
     /// Gutters. Default ["diagnostics", "line-numbers"]
     pub gutters: Vec<GutterType>,
     /// Middle click paste support. Defaults to true.
@@ -394,6 +396,7 @@ impl Default for Config {
                 vec!["sh".to_owned(), "-c".to_owned()]
             },
             line_number: LineNumber::Absolute,
+            cursorline: false,
             gutters: vec![GutterType::Diagnostics, GutterType::LineNumbers],
             middle_click_paste: true,
             auto_pairs: AutoPairConfig::default(),