Merge pull request #121 from pamburus/feature/tail

new: Added `--tail` parameter for `--follow` mode

Cargo.toml

@@ -4,7 +4,7 @@ categories = ["command-line-utilities"]
 description = "Utility for viewing json-formatted log files."
 keywords = ["cli", "human", "log"]
 name = "hl"
-version = "0.24.2"
+version = "0.25.0"
 edition = "2021"
 build = "build.rs"
 

README.md

@@ -424,6 +424,7 @@ Options:
       --list-themes                                      List available themes and exit
   -s, --sort                                             Sort messages chronologically
   -F, --follow                                           Follow input streams and sort messages chronologically during time frame set by --sync-interval-ms option
+      --tail <TAIL>                                      Number of last messages to preload from each file in --follow mode [default: 10]
       --sync-interval-ms <SYNC_INTERVAL_MS>              Synchronization interval for live streaming mode enabled by --follow option [default: 100]
   -o, --output <OUTPUT>                                  Output file
       --dump-index                                       Dump index metadata and exit

src/app.rs

@@ -66,6 +66,7 @@ pub struct Options {
     pub input_info: Option<InputInfo>,
     pub dump_index: bool,
     pub app_dirs: Option<AppDirs>,
+    pub tail: u64,
 }
 
 impl Options {
@@ -396,7 +397,7 @@ impl App {
                     if let InputReference::File(filename) = &input_ref { 
                         meta = Some(fs::metadata(filename)?);
                     }
-                    let mut input = Some(input_ref.open()?);
+                    let mut input = Some(input_ref.open_tail(self.options.tail)?);
                     let is_file = |meta: &Option<fs::Metadata>| meta.as_ref().map(|m|m.is_file()).unwrap_or(false);
                     let process = |input: &mut Option<Input>, is_file: bool| {
                         if let Some(input) = input {

src/input.rs

@@ -1,4 +1,5 @@
 // std imports
+use std::cmp::min;
 use std::convert::TryInto;
 use std::fs::File;
 use std::io::{self, stdin, BufReader, Read, Seek, SeekFrom};
@@ -61,12 +62,56 @@ impl InputReference {
         self.hold()?.open()
     }
 
+    pub fn open_tail(&self, n: u64) -> io::Result<Input> {
+        match self {
+            Self::Stdin => self.open(),
+            Self::File(path) => {
+                let mut file = File::open(path)
+                    .map_err(|e| io::Error::new(e.kind(), format!("failed to open {}: {}", self.description(), e)))?;
+                Self::seek_tail(&mut file, n).ok();
+                Ok(Input::new(self.clone(), Box::new(file)))
+            }
+        }
+    }
+
     pub fn description(&self) -> String {
         match self {
             Self::Stdin => "<stdin>".into(),
             Self::File(filename) => format!("file '{}'", Color::Yellow.paint(filename.to_string_lossy())),
         }
     }
+
+    fn seek_tail(file: &mut File, lines: u64) -> io::Result<()> {
+        const BUF_SIZE: usize = 64 * 1024;
+        let mut scratch = [0; BUF_SIZE];
+        let mut count: u64 = 0;
+        let mut prev_pos = file.seek(SeekFrom::End(0))?;
+        let mut pos = prev_pos;
+        while pos > 0 {
+            pos -= min(BUF_SIZE as u64, pos);
+            pos = file.seek(SeekFrom::Start(pos))?;
+            if pos == prev_pos {
+                break;
+            }
+            let bn = min(BUF_SIZE, (prev_pos - pos) as usize);
+            let buf = scratch[..bn].as_mut();
+
+            file.read_exact(buf)?;
+
+            for i in (0..bn).rev() {
+                if buf[i] == b'\n' {
+                    if count == lines {
+                        file.seek(SeekFrom::Start(pos + i as u64 + 1))?;
+                        return Ok(());
+                    }
+                    count += 1;
+                }
+            }
+
+            prev_pos = pos;
+        }
+        Ok(())
+    }
 }
 
 // ---

src/main.rs

@@ -182,6 +182,10 @@ struct Opt {
     #[arg(long, short = 'F')]
     follow: bool,
 
+    /// Number of last messages to preload from each file in --follow mode.
+    #[arg(long, default_value = "10")]
+    tail: u64,
+
     /// Synchronization interval for live streaming mode enabled by --follow option.
     #[arg(long, default_value = "100")]
     sync_interval_ms: u64,
@@ -402,6 +406,7 @@ fn run() -> Result<()> {
         },
         dump_index: opt.dump_index,
         app_dirs: Some(app_dirs),
+        tail: opt.tail,
     });
 
     // Configure input.