fix: Fix prompt for multi-byte characters. (#41)

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

Co-authored-by: Josh McKinney <[email protected]>

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");
+    }
+}