Horizontal scrolling and line end snapping

editor/draw.go

@@ -20,7 +20,7 @@ func (e *editor) RefreshScreen() {
 	ab.append(terminal.ResetCursor)
 
 	e.drawRows(ab)
-	ab.append(terminal.MoveCursor(e.cursor.x, e.cursor.y))
+	ab.append(terminal.MoveCursor(e.cursor.x - e.offset.x, e.cursor.y - e.offset.y))
 
 	ab.append(terminal.ShowCursor)
 
@@ -35,6 +35,14 @@ func (e *editor) scroll() {
 	if e.cursor.y >= e.offset.y + e.screen.Rows {
 		e.offset.y = e.cursor.y - e.screen.Rows + 1
 	}
+
+	if e.cursor.x < e.offset.x {
+		e.offset.x = e.cursor.x
+	}
+
+	if e.cursor.x >= e.offset.x + e.screen.Cols {
+		e.offset.x = e.cursor.x - e.screen.Cols
+	}
 }
 
 func (e *editor) drawRows(ab *buffer) {
@@ -44,7 +52,17 @@ func (e *editor) drawRows(ab *buffer) {
 		if fileRow >= len(e.rows) {
 			e.drawPlaceholderRow(y, ab)
 		} else {
-			row := truncateString(string(e.rows[fileRow].chars), e.screen.Cols)
+			rawRow := e.rows[fileRow]
+
+			// If the column offset is greater than the length of the row,
+			// just display an empty row
+			if e.offset.x > rawRow.Size() {
+				ab.append("")
+
+				continue
+			}
+
+			row := truncateString(string(e.rows[fileRow].chars[e.offset.x:]), e.screen.Cols)
 			ab.append(row)
 		}
 

editor/editor.go

@@ -85,15 +85,34 @@ func (e *editor) ProcessKeypress() bool {
 }
 
 func (e *editor) moveCursor (key string) {
+	var row *row
+	if e.cursor.y > len(e.rows) {
+		row = nil
+	} else {
+		row = e.rows[e.cursor.y]
+	}
+
 	switch key {
 	case keyLeft:
 		if e.cursor.x != 0 {
 			e.cursor.x -= 1
 		}
+
+		// Move from beginning of current row to end of previous row
+		if e.cursor.y > 0 {
+			e.cursor.y -= 1
+			e.cursor.x = e.rows[e.cursor.y].Size()
+		}
 	case keyRight:
-		if e.cursor.x != e.screen.Cols-1 {
+		if row != nil && e.cursor.x < row.Size() {
 			e.cursor.x += 1
 		}
+
+		// Move from end of current line to beginning of next line
+		if row != nil && e.cursor.x == row.Size() {
+			e.cursor.y += 1
+			e.cursor.x = 0
+		}
 	case keyUp:
 		if e.cursor.y != 0 {
 			e.cursor.y -= 1
@@ -109,7 +128,21 @@ func (e *editor) moveCursor (key string) {
 	case keyHome:
 		e.cursor.x = 0
 	case keyEnd:
-		e.cursor.x = e.screen.Cols
+		if row != nil {
+			e.cursor.x = row.Size()
+		}
+	}
+
+	if e.cursor.y > len(e.rows) {
+		row = nil
+	} else {
+		row = e.rows[e.cursor.y]
+		rowLen := row.Size()
+
+		// Snap to the end of a shorter line from a longer one
+		if e.cursor.x > rowLen {
+			e.cursor.x = rowLen
+		}
 	}
 }