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126 lines
2.7 KiB
Go
126 lines
2.7 KiB
Go
package accesslog
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import (
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"bytes"
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"errors"
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"io"
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)
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// BackScanner provides an interface to read a file backward line by line.
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type BackScanner struct {
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file supportRotate
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size int64
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chunkSize int
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chunkBuf []byte
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offset int64
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chunk []byte
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line []byte
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err error
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}
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// NewBackScanner creates a new Scanner to read the file backward.
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// chunkSize determines the size of each read chunk from the end of the file.
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func NewBackScanner(file supportRotate, chunkSize int) *BackScanner {
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size, err := file.Seek(0, io.SeekEnd)
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if err != nil {
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return &BackScanner{err: err}
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}
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return newBackScanner(file, size, make([]byte, chunkSize))
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}
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func newBackScanner(file supportRotate, fileSize int64, buf []byte) *BackScanner {
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return &BackScanner{
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file: file,
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size: fileSize,
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offset: fileSize,
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chunkSize: len(buf),
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chunkBuf: buf,
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}
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}
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// Scan advances the scanner to the previous line, which will then be available
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// via the Bytes method. It returns false when there are no more lines.
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func (s *BackScanner) Scan() bool {
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if s.err != nil {
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return false
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}
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// Read chunks until a newline is found or the file is fully read
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for {
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// Check if there's a line in the buffer
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if idx := bytes.LastIndexByte(s.chunk, '\n'); idx >= 0 {
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s.line = s.chunk[idx+1:]
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s.chunk = s.chunk[:idx]
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if len(s.line) > 0 {
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return true
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}
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continue
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}
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for {
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if s.offset <= 0 {
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// No more data to read; check remaining buffer
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if len(s.chunk) > 0 {
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s.line = s.chunk
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s.chunk = nil
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return true
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}
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return false
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}
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newOffset := max(0, s.offset-int64(s.chunkSize))
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chunkSize := s.offset - newOffset
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chunk := s.chunkBuf[:chunkSize]
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n, err := s.file.ReadAt(chunk, newOffset)
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if err != nil {
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if !errors.Is(err, io.EOF) {
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s.err = err
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}
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return false
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} else if n == 0 {
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return false
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}
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// Prepend the chunk to the buffer
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clone := append([]byte{}, chunk[:n]...)
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s.chunk = append(clone, s.chunk...)
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s.offset = newOffset
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// Check for newline in the updated buffer
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if idx := bytes.LastIndexByte(s.chunk, '\n'); idx >= 0 {
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s.line = s.chunk[idx+1:]
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s.chunk = s.chunk[:idx]
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if len(s.line) > 0 {
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return true
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}
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break
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}
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}
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}
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}
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// Bytes returns the most recent line generated by a call to Scan.
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func (s *BackScanner) Bytes() []byte {
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return s.line
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}
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// FileSize returns the size of the file.
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func (s *BackScanner) FileSize() int64 {
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return s.size
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}
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// Err returns the first non-EOF error encountered by the scanner.
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func (s *BackScanner) Err() error {
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return s.err
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}
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func (s *BackScanner) Reset() error {
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_, err := s.file.Seek(0, io.SeekStart)
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if err != nil {
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return err
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}
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*s = *newBackScanner(s.file, s.size, s.chunkBuf)
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return nil
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}
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