fix(examples): changed user_input example to work with multi-byte unicode chars (#1069)

This is the proposed solution for issue #1068. It solves the bug in the
user_input example with multi-byte UTF-8 characters as input.

Fixes: #1068

---------

Co-authored-by: Josh McKinney <joshka@users.noreply.github.com>

examples/user_input.rs

@@ -49,7 +49,7 @@ struct App {
     /// Current value of the input box
     input: String,
     /// Position of cursor in the editor area.
-    cursor_position: usize,
+    character_index: usize,
     /// Current input mode
     input_mode: InputMode,
     /// History of recorded messages
@@ -62,34 +62,46 @@ impl App {
             input: String::new(),
             input_mode: InputMode::Normal,
             messages: Vec::new(),
-            cursor_position: 0,
+            character_index: 0,
         }
     }
 
     fn move_cursor_left(&mut self) {
-        let cursor_moved_left = self.cursor_position.saturating_sub(1);
-        self.cursor_position = self.clamp_cursor(cursor_moved_left);
+        let cursor_moved_left = self.character_index.saturating_sub(1);
+        self.character_index = self.clamp_cursor(cursor_moved_left);
     }
 
     fn move_cursor_right(&mut self) {
-        let cursor_moved_right = self.cursor_position.saturating_add(1);
-        self.cursor_position = self.clamp_cursor(cursor_moved_right);
+        let cursor_moved_right = self.character_index.saturating_add(1);
+        self.character_index = self.clamp_cursor(cursor_moved_right);
     }
 
     fn enter_char(&mut self, new_char: char) {
-        self.input.insert(self.cursor_position, new_char);
-
+        let index = self.byte_index();
+        self.input.insert(index, new_char);
         self.move_cursor_right();
     }
 
+    /// Returns the byte index based on the character position.
+    ///
+    /// Since each character in a string can be contain multiple bytes, it's necessary to calculate
+    /// the byte index based on the index of the character.
+    fn byte_index(&mut self) -> usize {
+        self.input
+            .char_indices()
+            .map(|(i, _)| i)
+            .nth(self.character_index)
+            .unwrap_or(self.input.len())
+    }
+
     fn delete_char(&mut self) {
-        let is_not_cursor_leftmost = self.cursor_position != 0;
+        let is_not_cursor_leftmost = self.character_index != 0;
         if is_not_cursor_leftmost {
             // Method "remove" is not used on the saved text for deleting the selected char.
             // Reason: Using remove on String works on bytes instead of the chars.
             // Using remove would require special care because of char boundaries.
 
-            let current_index = self.cursor_position;
+            let current_index = self.character_index;
             let from_left_to_current_index = current_index - 1;
 
             // Getting all characters before the selected character.
@@ -105,11 +117,11 @@ impl App {
     }
 
     fn clamp_cursor(&self, new_cursor_pos: usize) -> usize {
-        new_cursor_pos.clamp(0, self.input.len())
+        new_cursor_pos.clamp(0, self.input.chars().count())
     }
 
     fn reset_cursor(&mut self) {
-        self.cursor_position = 0;
+        self.character_index = 0;
     }
 
     fn submit_message(&mut self) {
@@ -240,7 +252,7 @@ fn ui(f: &mut Frame, app: &App) {
             f.set_cursor(
                 // Draw the cursor at the current position in the input field.
                 // This position is can be controlled via the left and right arrow key
-                input_area.x + app.cursor_position as u16 + 1,
+                input_area.x + app.character_index as u16 + 1,
                 // Move one line down, from the border to the input line
                 input_area.y + 1,
             );