head: remove clippy::cognitive_complexity by moving some content in a function (#5366)

* head: remove a clippy::cognitive_complexity by moving some content into a function

* Remove duplicate comment

src/uu/head/src/parse.rs

@@ -13,7 +13,6 @@ pub enum ParseError {
 }
 /// Parses obsolete syntax
 /// head -NUM\[kmzv\] // spell-checker:disable-line
-#[allow(clippy::cognitive_complexity)]
 pub fn parse_obsolete(src: &str) -> Option<Result<impl Iterator<Item = OsString>, ParseError>> {
     let mut chars = src.char_indices();
     if let Some((_, '-')) = chars.next() {
@@ -30,7 +29,22 @@ pub fn parse_obsolete(src: &str) -> Option<Result<impl Iterator<Item = OsString>
             }
         }
         if has_num {
-            match src[1..=num_end].parse::<usize>() {
+            process_num_block(&src[1..=num_end], last_char, &mut chars)
+        } else {
+            None
+        }
+    } else {
+        None
+    }
+}
+
+/// Processes the numeric block of the input string to generate the appropriate options.
+fn process_num_block(
+    src: &str,
+    last_char: char,
+    chars: &mut std::str::CharIndices,
+) -> Option<Result<impl Iterator<Item = OsString>, ParseError>> {
+    match src.parse::<usize>() {
         Ok(num) => {
             let mut quiet = false;
             let mut verbose = false;
@@ -38,7 +52,7 @@ pub fn parse_obsolete(src: &str) -> Option<Result<impl Iterator<Item = OsString>
             let mut multiplier = None;
             let mut c = last_char;
             loop {
-                        // not that here, we only match lower case 'k', 'c', and 'm'
+                // note that here, we only match lower case 'k', 'c', and 'm'
                 match c {
                     // we want to preserve order
                     // this also saves us 1 heap allocation
@@ -89,13 +103,8 @@ pub fn parse_obsolete(src: &str) -> Option<Result<impl Iterator<Item = OsString>
         }
         Err(_) => Some(Err(ParseError::Overflow)),
     }
-        } else {
-            None
-        }
-    } else {
-        None
-    }
 }
+
 /// Parses an -c or -n argument,
 /// the bool specifies whether to read from the end
 pub fn parse_num(src: &str) -> Result<(u64, bool), ParseSizeError> {