From 978f5114d82b51f2327c08faf2bf692181bf2408 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bla=C5=BE=20Hrastnik?= <blaz@mxxn.io>
Date: Fri, 9 Apr 2021 18:48:56 +0900
Subject: [PATCH] Horizontal scrolling! It only took a year to get around to
 it.

---
 helix-term/src/ui/editor.rs | 16 +++++++++-------
 helix-view/src/view.rs      | 26 +++++++++++++++++++++-----
 2 files changed, 30 insertions(+), 12 deletions(-)

diff --git a/helix-term/src/ui/editor.rs b/helix-term/src/ui/editor.rs
index 5426e014..d32f7864 100644
--- a/helix-term/src/ui/editor.rs
+++ b/helix-term/src/ui/editor.rs
@@ -189,18 +189,20 @@ impl EditorView {
                         } else if grapheme == "\t" {
                             visual_x += (tab_width as u16);
                         } else {
-                            if visual_x >= viewport.width {
-                                // if we're offscreen just keep going until we hit a new line
-                                // TODO: will need tweaking when we also take into account
-                                // horizontal scrolling
-                                continue;
-                            }
+                            let out_of_bounds = visual_x < view.first_col as u16
+                                || visual_x >= viewport.width + view.first_col as u16;
 
                             // Cow will prevent allocations if span contained in a single slice
                             // which should really be the majority case
                             let grapheme = Cow::from(grapheme);
                             let width = grapheme_width(&grapheme) as u16;
 
+                            if out_of_bounds {
+                                // if we're offscreen just keep going until we hit a new line
+                                visual_x += width;
+                                continue;
+                            }
+
                             // ugh,interleave highlight spans with diagnostic spans
                             let is_diagnostic = doc.diagnostics.iter().any(|diagnostic| {
                                 diagnostic.range.start <= char_index
@@ -214,7 +216,7 @@ impl EditorView {
                             };
 
                             surface.set_string(
-                                viewport.x + visual_x,
+                                viewport.x + visual_x - view.first_col as u16,
                                 viewport.y + line,
                                 grapheme,
                                 style,
diff --git a/helix-view/src/view.rs b/helix-view/src/view.rs
index 89caaf3e..93bb005e 100644
--- a/helix-view/src/view.rs
+++ b/helix-view/src/view.rs
@@ -4,6 +4,7 @@ use std::borrow::Cow;
 
 use crate::{Document, DocumentId, ViewId};
 use helix_core::{
+    coords_at_pos,
     graphemes::{grapheme_width, RopeGraphemes},
     Position, RopeSlice, Selection,
 };
@@ -56,6 +57,7 @@ pub struct View {
     pub id: ViewId,
     pub doc: DocumentId,
     pub first_line: usize,
+    pub first_col: usize,
     pub area: Rect,
     pub jumps: JumpList,
 }
@@ -66,6 +68,7 @@ impl View {
             id: ViewId::default(),
             doc,
             first_line: 0,
+            first_col: 0,
             area: Rect::default(), // will get calculated upon inserting into tree
             jumps: JumpList::new((doc, Selection::point(0))), // TODO: use actual sel
         };
@@ -75,18 +78,30 @@ impl View {
 
     pub fn ensure_cursor_in_view(&mut self, doc: &Document) {
         let cursor = doc.selection(self.id).cursor();
-        let line = doc.text().char_to_line(cursor);
-        let document_end = self.first_line + (self.area.height as usize).saturating_sub(2);
+        let pos = coords_at_pos(doc.text().slice(..), cursor);
+        let line = pos.row;
+        let col = pos.col;
+        let last_line = self.first_line + (self.area.height as usize).saturating_sub(2);
 
-        // TODO: side scroll
+        // TODO: not ideal
+        const OFFSET: usize = 7; // 1 diagnostic + 5 linenr + 1 gutter
+        let last_col = self.first_col + (self.area.width as usize - OFFSET);
 
-        if line > document_end.saturating_sub(PADDING) {
+        if line > last_line.saturating_sub(PADDING) {
             // scroll down
-            self.first_line += line - (document_end.saturating_sub(PADDING));
+            self.first_line += line - (last_line.saturating_sub(PADDING));
         } else if line < self.first_line + PADDING {
             // scroll up
             self.first_line = line.saturating_sub(PADDING);
         }
+
+        if col > last_col.saturating_sub(PADDING) {
+            // scroll right
+            self.first_col += col - (last_col.saturating_sub(PADDING));
+        } else if col < self.first_col + PADDING {
+            // scroll left
+            self.first_col = col.saturating_sub(PADDING);
+        }
     }
 
     /// Calculates the last visible line on screen
@@ -130,6 +145,7 @@ impl View {
         }
 
         let row = line - self.first_line as usize;
+        let col = col - self.first_col as usize;
 
         Some(Position::new(row, col))
     }