feat: implement experimental BackScanner for reading files backward and add tests for various scenarios

internal/net/gphttp/accesslog/back_scanner.go

@@ -0,0 +1,104 @@
+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
+}