fold: Adding combining character support (#9328)

* Adding combining character support for fold

* add fullwidth to the spell ignore list

* addressing comments and cargo fmt fixes

* clippy fixes for test files

---------

Co-authored-by: Christopher Illarionova <drydench@amazon.com>
Co-authored-by: Sylvestre Ledru <sylvestre@debian.org>

src/uu/fold/src/fold.rs

@@ -434,6 +434,15 @@ fn process_utf8_line<W: Write>(line: &str, ctx: &mut FoldContext<'_, W>) -> URes
     let mut iter = line.char_indices().peekable();
 
     while let Some((byte_idx, ch)) = iter.next() {
+        // Include combining characters with the base character
+        while let Some(&(_, next_ch)) = iter.peek() {
+            if unicode_width::UnicodeWidthChar::width(next_ch).unwrap_or(1) == 0 {
+                iter.next();
+            } else {
+                break;
+            }
+        }
+
         let next_idx = iter.peek().map(|(idx, _)| *idx).unwrap_or(line_bytes.len());
 
         if ch == '\n' {

tests/by-util/test_fold.rs

@@ -2,6 +2,8 @@
 //
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
+// spell-checker:ignore fullwidth
+
 use uutests::new_ucmd;
 
 #[test]
@@ -597,3 +599,36 @@ fn test_all_tab_advances_at_non_utf8_character() {
         .succeeds()
         .stdout_is_fixture_bytes("non_utf8_tab_stops_w16.expected");
 }
+
+#[test]
+fn test_combining_characters_nfc() {
+    // e acute NFC form (single character)
+    let e_acute_nfc = "\u{00E9}"; // é as single character
+    new_ucmd!()
+        .arg("-w2")
+        .pipe_in(format!("{e_acute_nfc}{e_acute_nfc}{e_acute_nfc}"))
+        .succeeds()
+        .stdout_is(format!("{e_acute_nfc}{e_acute_nfc}\n{e_acute_nfc}"));
+}
+
+#[test]
+fn test_combining_characters_nfd() {
+    // e acute NFD form (base + combining acute)
+    let e_acute_nfd = "e\u{0301}"; // e + combining acute accent
+    new_ucmd!()
+        .arg("-w2")
+        .pipe_in(format!("{e_acute_nfd}{e_acute_nfd}{e_acute_nfd}"))
+        .succeeds()
+        .stdout_is(format!("{e_acute_nfd}{e_acute_nfd}\n{e_acute_nfd}"));
+}
+
+#[test]
+fn test_fullwidth_characters() {
+    // e fullwidth (takes 2 columns)
+    let e_fullwidth = "\u{FF45}"; // ｅ
+    new_ucmd!()
+        .arg("-w2")
+        .pipe_in(format!("{e_fullwidth}{e_fullwidth}"))
+        .succeeds()
+        .stdout_is(format!("{e_fullwidth}\n{e_fullwidth}"));
+}