package frp

import (
	"io"
	"os"
	"strings"
)

// ReadTailLog 读取文件末尾 n 行
func ReadTailLog(filePath string, n int) ([]string, error) {
	file, err := os.Open(filePath)
	if err != nil {
		return nil, err
	}
	defer file.Close()

	info, err := file.Stat()
	if err != nil {
		return nil, err
	}
	fileSize := info.Size()
	if fileSize == 0 {
		return []string{}, nil
	}

	const chunkSize = 4096
	var lines []string
	var leftover []byte
	offset := fileSize

	for len(lines) < n && offset > 0 {
		readSize := chunkSize
		if offset < int64(chunkSize) {
			readSize = int(offset)
		}
		offset -= int64(readSize)

		buf := make([]byte, readSize)
		_, err := file.ReadAt(buf, offset)
		if err != nil && err != io.EOF {
			return nil, err
		}

		data := append(buf, leftover...)
		leftover = nil

		start := 0
		for i := len(data) - 1; i >= 0; i-- {
			if data[i] == '\n' {
				if i+1 < len(data) {
					line := string(data[i+1:])
					if line != "" {
						lines = append([]string{line}, lines...)
						if len(lines) >= n {
							break
						}
					}
				}
				start = i
			}
		}

		if len(lines) < n && start > 0 {
			leftover = data[:start]
		}
	}

	if len(lines) < n && len(leftover) > 0 {
		parts := strings.Split(string(leftover), "\n")
		for i := len(parts) - 1; i >= 0; i-- {
			if parts[i] != "" {
				lines = append([]string{parts[i]}, lines...)
				if len(lines) >= n {
					break
				}
			}
		}
	}

	return lines, nil
}