add comments on the function logic

src/uu/cut/src/cut.rs

@@ -290,7 +290,10 @@ fn cut_fields_whitespace<R: Read>(
 
             return Ok(true);
         }
-
+        // The logic is identical to `cut_fields_delimiter` function above, which uses
+        // `Searcher` that iterates over and returns the first position of the delimiter character.
+        // The main difference is that `WhitespaceSearcher` returns a pair of the first and last
+        // delimiter character positions, since each delimiter sequence length can vary.
         for &Range { low, high } in ranges {
             if low - fields_pos > 0 {
                 low_idx = match delim_search.nth(low - fields_pos - 1) {

src/uu/cut/src/whitespace_searcher.rs

@@ -24,6 +24,9 @@ impl<'a> WhitespaceSearcher<'a> {
 impl<'a> Iterator for WhitespaceSearcher<'a> {
     type Item = (usize, usize);
 
+    // Iterate over sequences of consecutive whitespace (space and/or tab) characters.
+    // Returns (first, last) positions of each sequence, where `first` is inclusive and `last` is exclusive.
+    // The delimiter sequence byte-length is equal to `last - first`
     fn next(&mut self) -> Option<Self::Item> {
         if let Some(match_idx) = memchr2(b' ', b'\t', self.haystack) {
             let mut skip = match_idx + 1;