fix: tail_file read from the ending not beginning (#70)

* Update tail_file.rs: read from the ending not beginning

* fix: `tail_file` return the first n lines, not last n lines

`newline_positions` is already in reverse order

* Update read.rs

* Update test_fs_service.rs

5 is a special number, so it can pass the `test_tail_file_normal` test, I add a new line

Author: jingangdidi

src/fs_service/io/read.rs

@@ -116,7 +116,7 @@ impl FileSystemService {
         let start_pos = if line_count <= n {
             0 // Read from start if fewer than n lines
         } else {
-            *newline_positions.get(line_count - n).unwrap_or(&0) + 1
+            *newline_positions.get(n-1).unwrap_or(&0) + 1
         };
 
         // Read forward from start_pos

src/tools/tail_file.rs

@@ -24,7 +24,7 @@ use crate::fs_service::FileSystemService;
 pub struct TailFile {
     /// The path of the file to get information for.
     pub path: String,
-    /// The number of lines to read from the beginning of the file.
+    /// The number of lines to read from the ending of the file.
     pub lines: u64,
 }
 

tests/test_fs_service.rs

@@ -1213,13 +1213,13 @@ async fn test_tail_file_normal() {
     let file_path = create_test_file_with_line_ending(
         &temp_dir.to_path_buf(),
         "dir1/test.txt",
-        vec!["line1", "line2", "line3", "line4", "line5"],
+        vec!["line1", "line2", "line3", "line4", "line5", "line6"],
         "\n",
     )
     .await;
 
     let result = service.tail_file(&file_path, 3).await.unwrap();
-    assert_eq!(result, "line3\nline4\nline5"); // No trailing newline
+    assert_eq!(result, "line4\nline5\nline6"); // No trailing newline
 }
 
 #[tokio::test]