golang with scanner

這是一個建立於 的文章，其中的資訊可能已經有所發展或是發生改變。

        the articles in this blog ,we will introduce the performance for that who consider performance in your design of software.wo introduce implementtion of some common method.the method "make" that usually malloc memory from the memory from go platform. a class called type Scanner struct.

    type Scanner struct {        r            io.Reader // The reader provided by the client.        split        SplitFunc // The function to split the tokens.        maxTokenSize int       // Maximum size of a token; modified by tests.        token        []byte    // Last token returned by split.        buf          []byte    // Buffer used as argument to split.        start        int       // First non-processed byte in buf.        end          int       // End of data in buf.        err          error     // Sticky error.        empties      int       // Count of successive empty tokens.        scanCalled   bool      // Scan has been called; buffer is in use.        done         bool      // Scan has finished.    }

we can show scanner.Bytes(),just used buiild in scanner class that is origin data,not new malloc memory to restore data.

    func (s *Scanner) Bytes() []byte {        return s.token //refernce from class inner member    }    // Text returns the most recent token generated by a call to Scan    // as a newly allocated string holding its bytes.    func (s *Scanner) Text() string {        return string(s.token) //useing memory copy from s.token to new string that is new memeory allocation.    }

    using Text() method,if the data of size is too big,this will get bad performance.    next I will introduce Scan method,implemention of Scan code.all method in golang, that bring us a convenient,but background of implements so complex.the management of memory of operation system taked over by golang platform,the user don’t care of how memory malloc or free fom system.at this point is not the same to C language.

//reference from bufio/scan.gofunc (s *Scanner) Scan() bool {  //this method already malloc memory for buf                                                              if s.done {                                                                             return false                                                                      }                                                                                   s.scanCalled = true                                                                 // Loop until we have a token.                                                      for {                                                                                   // See if we can get a token with what we already have.                               // If we've run out of data but have an error, give the split function                // a chance to recover any remaining, possibly empty token.                           if s.end > s.start || s.err != nil {                                                      advance, token, err := s.split(s.buf[s.start:s.end], s.err != nil)                      if err != nil {                                                                             if err == ErrFinalToken {                                                                     s.token = token                                                                             s.done = true                                                                               return true                                                                             }                                                                                         s.setErr(err)                                                                             return false                                                                          }                                                                                       if !s.advance(advance) {                                                                    return false                                                                          }                                                                                       s.token = token                                                                         if token != nil {                                                                           if s.err == nil || advance > 0 {                                                              s.empties = 0                                                                           } else {                                                                                      // Returning tokens not advancing input at EOF.                                             s.empties++                                                                                 if s.empties > 100 {                                                                            panic("bufio.Scan: 100 empty tokens without progressing")                                 }                                                                                       }                                                                                         return true                                                                           }                                                                                   }                                                                                     // We cannot generate a token with what we are holding.                               // If we've already hit EOF or an I/O error, we are done.                             if s.err != nil {                                                                         // Shut it down.                                                                        s.start = 0                                                                             s.end = 0                                                                               return false                                                                        }                                                                                     // Must read more data.                                                               // First, shift data to beginning of buffer if there's lots of empty space            // or space is needed.                                                                if s.start > 0 && (s.end == len(s.buf) || s.start > len(s.buf)/2) {                       copy(s.buf, s.buf[s.start:s.end])  //copy []byte                                                     s.end -= s.start                                                                        s.start = 0                                                                         }                                                                                     // Is the buffer full? If so, resize.                                                 if s.end == len(s.buf) {                                                                  // Guarantee no overflow in the multiplication below.                                   const maxInt = int(^uint(0) >> 1)                                                       if len(s.buf) >= s.maxTokenSize || len(s.buf) > maxInt/2 {                                  s.setErr(ErrTooLong)                                                                      return false                                                                          }                                                                                       newSize := len(s.buf) * 2                                                               if newSize == 0 {                                                                           newSize = startBufSize                                                                }                                                                                       if newSize > s.maxTokenSize {                                                               newSize = s.maxTokenSize                                                              }                                                                                       newBuf := make([]byte, newSize)    //resize buf                                                     copy(newBuf, s.buf[s.start:s.end])                                                      s.buf = newBuf                                                                          s.end -= s.start                                                                        s.start = 0                                                                             continue                                                                            }                                                                                     // Finally we can read some input. Make sure we don't get stuck with                  // a misbehaving Reader. Officially we don't need to do this, but let's               // be extra careful: Scanner is for safe, simple jobs.                                for loop := 0; ; {                                                                        n, err := s.r.Read(s.buf[s.end:len(s.buf)])                                             s.end += n                                                                              if err != nil {                                                                             s.setErr(err)                                                                             break                                                                                 }                                                                                       if n > 0 {                                                                                  s.empties = 0                                                                             break                                                                                 }                                                                                       loop++                                                                                  if loop > maxConsecutiveEmptyReads {                                                        s.setErr(io.ErrNoProgress)                                                                break                                                                                 }                                                                                   }                                                                                 }                                                                               }