fix: Fix prompt for multi-byte characters.

Before this contribution, entering characters that consists of multiple
bytes lead to undesired behaviour such as panics.
This results from the position previous position handling inside the
prompt.
The String::len() returns the size of the string in bytes. Since a
single unicode scalar value may consist of multiple bytes, this is not
useful to determine the position inside the prompt.
The panic occured in the State::push() method and resulted from the
attempt to insert at a byte index that does not represent a character
boundary.
See:
https://doc.rust-lang.org/stable/std/primitive.str.html#method.len
https://doc.rust-lang.org/stable/std/string/struct.String.html#method.insert

src/prompt.rs

@@ -1,5 +1,8 @@
+use std::iter::once;
+
 use crate::Status;
 use crossterm::event::{KeyCode, KeyEvent, KeyEventKind, KeyModifiers};
+use itertools::chain;
 use ratatui::{prelude::*, widgets::StatefulWidget};
 
 /// A prompt that can be drawn to a terminal.
@@ -81,6 +84,14 @@ pub trait State {
     /// A mutable reference to the value of the prompt.
     fn value_mut(&mut self) -> &mut String;
 
+    fn len(&self) -> usize {
+        self.value().chars().count()
+    }
+
+    fn is_empty(&self) -> bool {
+        self.value().len() == 0
+    }
+
     fn handle_key_event(&mut self, key_event: KeyEvent) {
         if key_event.kind == KeyEventKind::Release {
             return;
@@ -114,23 +125,23 @@ pub trait State {
 
     fn delete(&mut self) {
         let position = self.position();
-        if position == self.value().len() {
+        if position == self.len() {
             return;
         }
         self.value_mut().remove(position);
     }
 
     fn backspace(&mut self) {
         let position = self.position().saturating_sub(1);
-        if position == self.value().len() {
+        if position == self.len() {
             return;
         }
         *self.position_mut() = position;
         self.value_mut().remove(position);
     }
 
     fn move_right(&mut self) {
-        if self.position() == self.value().len() {
+        if self.position() == self.len() {
             return;
         }
         *self.position_mut() = self.position().saturating_add(1);
@@ -141,7 +152,7 @@ pub trait State {
     }
 
     fn move_end(&mut self) {
-        *self.position_mut() = self.value().len();
+        *self.position_mut() = self.len();
     }
 
     fn move_start(&mut self) {
@@ -159,8 +170,19 @@ pub trait State {
     }
 
     fn push(&mut self, c: char) {
-        let position = self.position();
-        self.value_mut().insert(position, c);
+        if self.position() == self.len() {
+            self.value_mut().push(c);
+        } else {
+            // We cannot use String::insert() as it operates on bytes, which can lead to incorrect modifications with
+            // multibyte characters. Instead, we handle text manipulation at the character level using Rust's char type
+            // for Unicode correctness. Check docs of String::insert() and String::chars() for futher info.
+            *self.value_mut() = chain![
+                self.value().chars().take(self.position()),
+                once(c),
+                self.value().chars().skip(self.position())
+            ]
+            .collect();
+        }
         *self.position_mut() = self.position().saturating_add(1);
     }
 }

src/text_prompt.rs

@@ -38,7 +38,7 @@ impl TextRenderStyle {
     pub fn render(&self, state: &TextState) -> String {
         match self {
             Self::Default => state.value().to_string(),
-            Self::Password => "*".repeat(state.value().len()),
+            Self::Password => "*".repeat(state.len()),
             Self::Invisible => String::new(),
         }
     }
@@ -91,7 +91,7 @@ impl<'a> StatefulWidget for TextPrompt<'a> {
         let width = area.width as usize;
         let height = area.height as usize;
         let value = self.render_style.render(state);
-        let value_length = value.len();
+        let value_length = value.chars().count();
 
         let line = Line::from(vec![
             state.status().symbol(),
@@ -100,7 +100,7 @@ impl<'a> StatefulWidget for TextPrompt<'a> {
             " › ".cyan().dim(),
             Span::raw(value),
         ]);
-        let prompt_length = line.width() - value_length;
+        let prompt_length = line.to_string().chars().count() - value_length;
         let lines = wrap(line, width).take(height).collect_vec();
 
         // constrain the position to the area

src/text_state.rs

@@ -90,4 +90,28 @@ impl State for TextState<'_> {
 }
 
 #[cfg(test)]
-mod tests {}
+mod tests {
+    use crate::{State, TextState};
+
+    #[test]
+    fn insert_multibyte_start() {
+        let mut test = TextState::new().with_value("ää");
+        test.move_start();
+        test.push('Ö');
+        assert_eq!(test.value(), "Öää");
+    }
+    #[test]
+    fn insert_multibyte_middle() {
+        let mut test = TextState::new().with_value("ää");
+        test.move_right();
+        test.push('Ö');
+        assert_eq!(test.value(), "äÖä");
+    }
+    #[test]
+    fn insert_multibyte_end() {
+        let mut test = TextState::new().with_value("ää");
+        test.move_end();
+        test.push('Ö');
+        assert_eq!(test.value(), "ääÖ");
+    }
+}