Tail a file in Golang

tail.go

package main

import (
	"log"
	"os"
	"strings"
)

// JoinStrings use a strings.Builder for concatenate the input string array.
// It concatenate the strings among the delimiter in input
func JoinStrings(strs []string, delimiter string) string {
	if len(strs) == 0 {
		return ""
	}
	var sb strings.Builder
	for i := range strs {
		sb.WriteString(strs[i])
		sb.WriteString(delimiter)
	}
	return strings.TrimSuffix(sb.String(), delimiter)
}

const FILE string = `test.txt`

// BUFF_BYTE is how many bytes to move
// BUFF_BYTE can be positive or negative
const BUFF_BYTE int64 = -1

// Whence is the point of reference for offset
// 0 = Beginning of file
// 1 = Current position
// 2 = End of file
const START_POS int = 2
const N_STRING = 100

func Tail(FILE string, BUFF_BYTE int64, START_POS, N_STRING int) string {
	// list of strings readed
	var stringsArray []string = make([]string, N_STRING)
	// Contains the data
	var buff []byte = make([]byte, -BUFF_BYTE)

	if !(START_POS >= 0 && START_POS <= 2) {
		log.Fatal("Wrong argument for Seek ...")
	}

	file, err := os.Open(FILE)
	if err != nil {
		log.Println("Unable to open file: " + FILE + " ERR: " + err.Error())
		log.Fatal(err)
	}
	defer file.Close()

	// Go to end of file
	_, err = file.Seek(BUFF_BYTE, START_POS)
	if err != nil {
		log.Println("Unable to seek to the end of the file: " + FILE + " ERR: " + err.Error())
		log.Fatal(err)
	}
	var linesReaded int = 0
	var nByte int                // Number of byte readed
	var stringBuffer string = "" // Contains the string until we don't found the new line
	var iteration int64 = 1
	var n int64 = -BUFF_BYTE // Just for pass the first check
	var lastPosition int64
	// Until we haven't read all the string
	for linesReaded < N_STRING {
		if n >= -BUFF_BYTE {
			n, err = file.Seek(iteration*BUFF_BYTE, START_POS)
			if err != nil {
				log.Println("2) Error during read of file | Lines readed: ", linesReaded, " Byte readed: ", nByte, " Iteration: ", iteration)
				log.Fatal(err)
			}
			lastPosition = n
		} else {
			// We have not enought data for fill the buffer, seeking to the start of the file
			file.Seek(0, 0)
			buff = make([]byte, lastPosition)
			_, err = file.Read(buff)
			if err != nil {
				log.Println("3) Error during read of file | Lines readed: ", linesReaded, " Byte readed: ", nByte, " Iteration: ", iteration)
				log.Fatal(err)
			}
			stringBuffer = string(buff) + stringBuffer
			stringsArray[N_STRING-linesReaded-1] = stringBuffer
			break
		}

		// Read the string related to the buffer
		nByte, err = file.Read(buff)
		if err != nil {
			log.Println("1) Error during read of file | Lines readed: ", linesReaded, " Byte readed: ", nByte, " Iteration: ", iteration)
			log.Fatal(err)
		}
		// Append the string in initial position
		stringBuffer = string(buff) + stringBuffer
		if strings.Contains(stringBuffer, "\n") {
			stringsArray[N_STRING-linesReaded-1] = stringBuffer
			stringBuffer = ""
			linesReaded++
			// Continue to read, we have not found a new line and we have enough file to read
		}
		iteration++
	}
	stringsArray = stringsArray[linesReaded-1:]
	return JoinStrings(stringsArray, "")
}

func main() {
	log.Println(Tail(FILE, BUFF_BYTE, START_POS, N_STRING))
}