fold: refactor compute_col_count and add character mode tests

src/uu/fold/src/fold.rs

@@ -260,57 +260,33 @@ fn next_tab_stop(col_count: usize) -> usize {
 }
 
 fn compute_col_count(buffer: &[u8], mode: WidthMode) -> usize {
-    match mode {
-        WidthMode::Characters => {
-            if let Ok(s) = std::str::from_utf8(buffer) {
-                let mut width = 0;
-                for ch in s.chars() {
-                    match ch {
-                        '\r' => width = 0,
-                        '\t' => width = next_tab_stop(width),
-                        '\x08' => width = width.saturating_sub(1),
-                        _ => width += 1,
+    if let Ok(s) = std::str::from_utf8(buffer) {
+        let mut width = 0;
+        for ch in s.chars() {
+            match ch {
+                '\r' => width = 0,
+                '\t' => width = next_tab_stop(width),
+                '\x08' => width = width.saturating_sub(1),
+                _ => {
+                    width += match mode {
+                        WidthMode::Characters => 1,
+                        WidthMode::Columns => UnicodeWidthChar::width(ch).unwrap_or(0),
                     }
                 }
-                width
-            } else {
-                let mut width = 0;
-                for &byte in buffer {
-                    match byte {
-                        CR => width = 0,
-                        TAB => width = next_tab_stop(width),
-                        0x08 => width = width.saturating_sub(1),
-                        _ => width += 1,
-                    }
-                }
-                width
             }
         }
-        WidthMode::Columns => {
-            if let Ok(s) = std::str::from_utf8(buffer) {
-                let mut width = 0;
-                for ch in s.chars() {
-                    match ch {
-                        '\r' => width = 0,
-                        '\t' => width = next_tab_stop(width),
-                        '\x08' => width = width.saturating_sub(1),
-                        _ => width += UnicodeWidthChar::width(ch).unwrap_or(0),
-                    }
-                }
-                width
-            } else {
-                let mut width = 0;
-                for &byte in buffer {
-                    match byte {
-                        CR => width = 0,
-                        TAB => width = next_tab_stop(width),
-                        0x08 => width = width.saturating_sub(1),
-                        _ => width += 1,
-                    }
-                }
-                width
+        width
+    } else {
+        let mut width = 0;
+        for &byte in buffer {
+            match byte {
+                CR => width = 0,
+                TAB => width = next_tab_stop(width),
+                0x08 => width = width.saturating_sub(1),
+                _ => width += 1,
             }
         }
+        width
     }
 }
 

tests/by-util/test_fold.rs

@@ -2,7 +2,7 @@
 //
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
-// spell-checker:ignore fullwidth
+// spell-checker:ignore fullwidth refgh tefgh nefgh
 
 use bytecount::count;
 use unicode_width::UnicodeWidthChar;
@@ -958,3 +958,37 @@ fn test_fullwidth_characters() {
         .succeeds()
         .stdout_is(format!("{e_fullwidth}\n{e_fullwidth}"));
 }
+
+#[test]
+fn test_character_mode_special_chars() {
+    for (args, input, expected) in [
+        // backspace decreases column
+        (&["-c", "-w", "5"][..], "abcde\x08fg\n", "abcde\x08f\ng\n"),
+        // carriage return resets column
+        (&["-c", "-w", "5"], "abcd\refgh\n", "abcd\refgh\n"),
+        // tab at start exceeds width
+        (&["-c", "-w", "4"], "\tabc\n", "\t\nabc\n"),
+        // multiple tabs
+        (&["-c", "-w", "10"], "a\tb\tc\n", "a\tb\n\tc\n"),
+        // basic folding
+        (&["-c", "-w", "3"], "abcdef\n", "abc\ndef\n"),
+        // preserves empty lines
+        (&["-c", "-w", "5"], "abc\n\ndef\n", "abc\n\ndef\n"),
+        // word boundary with -s
+        (&["-c", "-s", "-w", "5"], "ab cd ef\n", "ab \ncd ef\n"),
+        // tab as word boundary
+        (&["-c", "-s", "-w", "10"], "abcd\tefgh\n", "abcd\t\nefgh\n"),
+        // wide chars count as 1 in -c mode
+        (
+            &["-c", "-w", "3"],
+            "\u{FF1A}\u{FF1A}\u{FF1A}\u{FF1A}\n",
+            "\u{FF1A}\u{FF1A}\u{FF1A}\n\u{FF1A}\n",
+        ),
+    ] {
+        new_ucmd!()
+            .args(args)
+            .pipe_in(input)
+            .succeeds()
+            .stdout_is(expected);
+    }
+}