package accesslog

import (
	"bytes"
	"io"
)

// BackScanner provides an interface to read a file backward line by line.
type BackScanner struct {
	file      AccessLogIO
	chunkSize int
	offset    int64
	buffer    []byte
	line      []byte
	err       error
	size      int64
}

// NewBackScanner creates a new Scanner to read the file backward.
// chunkSize determines the size of each read chunk from the end of the file.
func NewBackScanner(file AccessLogIO, chunkSize int) *BackScanner {
	size, err := file.Seek(0, io.SeekEnd)
	if err != nil {
		return &BackScanner{err: err}
	}
	return &BackScanner{
		file:      file,
		chunkSize: chunkSize,
		offset:    size,
		size:      size,
	}
}

// Scan advances the scanner to the previous line, which will then be available
// via the Bytes method. It returns false when there are no more lines.
func (s *BackScanner) Scan() bool {
	if s.err != nil {
		return false
	}

	// Read chunks until a newline is found or the file is fully read
	for {
		// Check if there's a line in the buffer
		if idx := bytes.LastIndexByte(s.buffer, '\n'); idx >= 0 {
			s.line = s.buffer[idx+1:]
			s.buffer = s.buffer[:idx]
			if len(s.line) > 0 {
				return true
			}
			continue
		}

		for {
			if s.offset <= 0 {
				// No more data to read; check remaining buffer
				if len(s.buffer) > 0 {
					s.line = s.buffer
					s.buffer = nil
					return true
				}
				return false
			}

			newOffset := max(0, s.offset-int64(s.chunkSize))
			chunkSize := s.offset - newOffset
			chunk := make([]byte, chunkSize)

			n, err := s.file.ReadAt(chunk, newOffset)
			if err != nil && err != io.EOF {
				s.err = err
				return false
			}

			// Prepend the chunk to the buffer
			s.buffer = append(chunk[:n], s.buffer...)
			s.offset = newOffset

			// Check for newline in the updated buffer
			if idx := bytes.LastIndexByte(s.buffer, '\n'); idx >= 0 {
				s.line = s.buffer[idx+1:]
				s.buffer = s.buffer[:idx]
				if len(s.line) > 0 {
					return true
				}
				break
			}
		}
	}
}

// Bytes returns the most recent line generated by a call to Scan.
func (s *BackScanner) Bytes() []byte {
	return s.line
}

// FileSize returns the size of the file.
func (s *BackScanner) FileSize() int64 {
	return s.size
}

// Err returns the first non-EOF error encountered by the scanner.
func (s *BackScanner) Err() error {
	return s.err
}