consider wide chars on calculating tab width

Cargo.toml

@@ -43,6 +43,7 @@ tui = { version = "0.19", default-features = false, optional = true }
 arbitrary = { version = "1", features = ["derive"], optional = true }
 crossterm-027 = { package = "crossterm", version = "0.27", optional = true }
 ratatui = { version = "0.23.0", default-features = false, optional = true }
+unicode-width = "0.1.11"
 
 [[example]]
 name = "minimal"

src/highlight.rs

@@ -5,7 +5,6 @@ use crate::tui::style::Style;
     feature = "ratatui-your-backend",
 ))]
 use crate::tui::text::Line as Spans;
-
 use crate::tui::text::Span;
 #[cfg(not(any(
     feature = "ratatui-crossterm",
@@ -13,11 +12,11 @@ use crate::tui::text::Span;
     feature = "ratatui-your-backend",
 )))]
 use crate::tui::text::Spans;
-
 use crate::util::{num_digits, spaces};
 use std::borrow::Cow;
 use std::cmp::Ordering;
 use std::iter;
+use unicode_width::UnicodeWidthChar as _;
 
 enum Boundary {
     Cursor(Style),
@@ -58,7 +57,7 @@ fn line_display_text(s: &str, tab_len: u8, mask: Option<char>) -> Cow<'_, str> {
 
     let tab = spaces(tab_len);
     let mut buf = String::new();
-    let mut col = 0;
+    let mut width = 0;
     for (i, c) in s.char_indices() {
         if c == '\t' {
             if buf.is_empty() {
@@ -66,15 +65,15 @@ fn line_display_text(s: &str, tab_len: u8, mask: Option<char>) -> Cow<'_, str> {
                 buf.push_str(&s[..i]);
             }
             if tab_len > 0 {
-                let len = tab_len as usize - (col % tab_len as usize);
+                let len = tab_len as usize - (width % tab_len as usize);
                 buf.push_str(&tab[..len]);
-                col += len;
+                width += len;
             }
         } else {
             if !buf.is_empty() {
                 buf.push(c);
             }
-            col += 1;
+            width += c.width().unwrap_or(0);
         }
     }
 
@@ -245,5 +244,9 @@ mod tests {
         assert_eq!(&line_display_text("ab\t\t", 4,      None),          "ab      ");
         assert_eq!(&line_display_text("ab\t\t", 8,      None),  "ab              ");
         assert_eq!(&line_display_text("abcd\t", 4,      None),          "abcd    ");
+        assert_eq!(&line_display_text(  "あ\t", 0,      None),                "あ");
+        assert_eq!(&line_display_text(  "あ\t", 4,      None),              "あ  ");
+        assert_eq!(&line_display_text(  "🐶\t", 4,      None),              "🐶  ");
+        assert_eq!(&line_display_text(  "あ\t", 4, Some('x')),                "xx");
     }
 }

src/textarea.rs

@@ -23,6 +23,7 @@ use crate::tui::widgets::{Block, Widget};
 use crate::util::spaces;
 use crate::widget::{Renderer, Viewport};
 use crate::word::{find_word_end_forward, find_word_start_backward};
+use unicode_width::UnicodeWidthChar as _;
 
 /// A type to manage state of textarea.
 ///
@@ -686,26 +687,35 @@ impl<'a> TextArea<'a> {
 
     /// Insert a tab at current cursor position. Note that this method does nothing when the tab length is 0. This
     /// method returns if a tab string was inserted or not in the textarea.
-    /// textarea.
     /// ```
-    /// use tui_textarea::TextArea;
+    /// use tui_textarea::{TextArea, CursorMove};
     ///
     /// let mut textarea = TextArea::from(["hi"]);
     ///
+    /// textarea.move_cursor(CursorMove::End); // Move to the end of line
+    ///
     /// textarea.insert_tab();
-    /// assert_eq!(textarea.lines(), ["    hi"]);
+    /// assert_eq!(textarea.lines(), ["hi  "]);
+    /// textarea.insert_tab();
+    /// assert_eq!(textarea.lines(), ["hi      "]);
     /// ```
     pub fn insert_tab(&mut self) -> bool {
         if self.tab_len == 0 {
             return false;
         }
-        let tab = if self.hard_tab_indent {
-            "\t"
-        } else {
-            let len = self.tab_len - (self.cursor.1 % self.tab_len as usize) as u8;
-            spaces(len)
-        };
-        self.insert_str(tab)
+
+        if self.hard_tab_indent {
+            return self.insert_str("\t");
+        }
+
+        let (row, col) = self.cursor;
+        let width: usize = self.lines[row]
+            .chars()
+            .take(col)
+            .map(|c| c.width().unwrap_or(0))
+            .sum();
+        let len = self.tab_len - (width % self.tab_len as usize) as u8;
+        self.insert_str(spaces(len))
     }
 
     /// Insert a newline at current cursor position.

tests/mod.rs

@@ -1,2 +1,3 @@
 mod cursor;
 mod history;
+mod textarea;

tests/textarea.rs

@@ -0,0 +1,26 @@
+use tui_textarea::{CursorMove, TextArea};
+
+#[test]
+fn test_insert_soft_tab() {
+    for test in [
+        ("", 0, "    "),
+        ("a", 1, "a   "),
+        ("abcd", 4, "abcd    "),
+        ("a", 0, "    a"),
+        ("ab", 1, "a   b"),
+        ("abcdefgh", 4, "abcd    efgh"),
+        ("あ", 1, "あ  "),
+        ("🐶", 1, "🐶  "),
+        ("あ", 0, "    あ"),
+        ("あい", 1, "あ  い"),
+    ] {
+        let (input, col, expected) = test;
+        let mut t = TextArea::from([input.to_string()]);
+        t.move_cursor(CursorMove::Jump(0, col));
+        assert!(t.insert_tab(), "{:?}", test);
+        let lines = t.into_lines();
+        assert_eq!(lines.len(), 1, "{:?}, {:?}", lines, test);
+        let line = &lines[0];
+        assert_eq!(line, expected, "{:?}", test);
+    }
+}