tail.Location is now the most customizable

as it is now a tuple; (offset, whence) from os.Seek

cmd/gotail/gotail.go

@@ -9,9 +9,10 @@ import (
 	"os"
 )
 
-func args2config() tail.Config {
+func args2config() (tail.Config, int64) {
 	config := tail.Config{Follow: true}
-	flag.Int64Var(&config.Location, "n", 0, "tail from the last Nth location")
+	n := int64(0)
+	flag.Int64Var(&n, "n", 0, "tail from the last Nth location")
 	flag.BoolVar(&config.Follow, "f", false, "wait for additional data to be appended to the file")
 	flag.BoolVar(&config.ReOpen, "F", false, "follow, and track file rename/rotation")
 	flag.BoolVar(&config.Poll, "p", false, "use polling, instead of inotify")
@@ -19,16 +20,20 @@ func args2config() tail.Config {
 	if config.ReOpen {
 		config.Follow = true
 	}
-	return config
+	return config, n
 }
 
 func main() {
-	config := args2config()
+	config, n := args2config()
 	if flag.NFlag() < 1 {
 		fmt.Println("need one or more files as arguments")
 		os.Exit(1)
 	}
 
+	if n != 0 {
+		config.Location = &tail.SeekInfo{-n, os.SEEK_END}
+	}
+
 	done := make(chan bool)
 	for _, filename := range flag.Args() {
 		go tailFile(filename, config, done)

tail.go

@@ -24,11 +24,14 @@ type Line struct {
 	// log line itself.
 }
 
+type SeekInfo struct {
+	Offset int64
+	Whence int // os.SEEK_*
+}
+
 // Config is used to specify how a file must be tailed.
 type Config struct {
-	Location int64 // For positive location, tail after first N
-	// lines; for negative, tail from last N lines;
-	// for zero, tail from end. If -1, do not seek.
+	Location    *SeekInfo // Seek before tailing
 	Follow      bool      // Continue looking for new lines (tail -f)
 	ReOpen      bool      // Reopen recreated files (tail -F)
 	MustExist   bool      // Fail early if the file does not exist
@@ -141,16 +144,9 @@ func (tail *Tail) tailFileSync() {
 	}
 
 	// Seek to requested location on first open of the file.
-	if tail.Location != -1 {
-		var err error
-		switch {
-		case tail.Location <= 0:
-			// Seek relative to file end
-			_, err = tail.file.Seek(tail.Location, os.SEEK_END)
-		default:
-			// Seek from file beginning
-			_, err = tail.file.Seek(tail.Location, os.SEEK_SET)
-		}
+	if tail.Location != nil {
+		_, err := tail.file.Seek(tail.Location.Offset, tail.Location.Whence)
+		// log.Printf("Seeked %s - %+v\n", tail.Filename, tail.Location)
 		if err != nil {
 			tail.Killf("Seek error on %s: %s", tail.Filename, err)
 			return