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104 lines
2.3 KiB
Go
104 lines
2.3 KiB
Go
package accesslog
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import (
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"bytes"
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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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chunkSize int
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offset int64
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buffer []byte
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line []byte
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err error
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size int64
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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 &BackScanner{
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file: file,
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chunkSize: chunkSize,
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offset: size,
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size: size,
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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.buffer, '\n'); idx >= 0 {
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s.line = s.buffer[idx+1:]
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s.buffer = s.buffer[: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.buffer) > 0 {
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s.line = s.buffer
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s.buffer = 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 := make([]byte, chunkSize)
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n, err := s.file.ReadAt(chunk, newOffset)
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if err != nil && err != io.EOF {
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s.err = err
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return false
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}
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// Prepend the chunk to the buffer
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s.buffer = append(chunk[:n], s.buffer...)
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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.buffer, '\n'); idx >= 0 {
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s.line = s.buffer[idx+1:]
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s.buffer = s.buffer[: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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