fix(tail): add support for negative indexing (#1865)

closes: https://github.com/uutils/coreutils/issues/1860

src/uu/tail/src/tail.rs

@@ -80,6 +80,7 @@ pub fn uumain(args: impl uucore::Args) -> i32 {
                 .short("c")
                 .long(options::BYTES)
                 .takes_value(true)
+                .allow_hyphen_values(true)
                 .help("Number of bytes to print"),
         )
         .arg(
@@ -93,6 +94,7 @@ pub fn uumain(args: impl uucore::Args) -> i32 {
                 .short("n")
                 .long(options::LINES)
                 .takes_value(true)
+                .allow_hyphen_values(true)
                 .help("Number of lines to print"),
         )
         .arg(
@@ -343,9 +345,9 @@ pub fn parse_size(mut size_slice: &str) -> Result<u64, ParseSizeErr> {
         // sole B is not a valid suffix
         Err(ParseSizeErr::parse_failure(size_slice))
     } else {
-        let value: Option<u64> = size_slice.parse().ok();
+        let value: Option<i64> = size_slice.parse().ok();
         value
-            .map(|v| Ok(multiplier * v))
+            .map(|v| Ok((multiplier as i64 * v.abs()) as u64))
             .unwrap_or_else(|| Err(ParseSizeErr::parse_failure(size_slice)))
     }
 }

tests/by-util/test_tail.rs

@@ -329,3 +329,17 @@ fn test_multiple_input_quiet_flag_overrides_verbose_flag_for_suppressing_headers
         .run()
         .stdout_is_fixture("foobar_multiple_quiet.expected");
 }
+
+#[test]
+fn test_negative_indexing() {
+    let positive_lines_index = new_ucmd!().arg("-n").arg("5").arg(FOOBAR_TXT).run();
+
+    let negative_lines_index = new_ucmd!().arg("-n").arg("-5").arg(FOOBAR_TXT).run();
+
+    let positive_bytes_index = new_ucmd!().arg("-c").arg("20").arg(FOOBAR_TXT).run();
+
+    let negative_bytes_index = new_ucmd!().arg("-c").arg("-20").arg(FOOBAR_TXT).run();
+
+    assert_eq!(positive_lines_index.stdout, negative_lines_index.stdout);
+    assert_eq!(positive_bytes_index.stdout, negative_bytes_index.stdout);
+}