common/recordio: Add zero-copy buffer split.

common/recordio/reader.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package recordio
 
 import (
+        "bytes"
         "encoding/binary"
         "fmt"
         "io"
 )
 
 // ErrFrameTooLarge is an error that is returned if a frame that is larger than
 // the maximum allowed size (not including the frame header) is read.
 var ErrFrameTooLarge = fmt.Errorf("frame: frame size exceeds maximum")
 
 // Reader reads individual frames from a frame-formatted input Reader.
@@ skipping 77 matching lines @@
 type simpleByteReader struct {
         io.Reader
 
         buf [1]byte
 }
 
 func (r *simpleByteReader) ReadByte() (byte, error) {
         _, err := r.Read(r.buf[:])
         return r.buf[0], err
 }
+
+// Split splits the supplied buffer into its component records.
+//
+// This method implements zero-copy segmentation, so the individual records are
+// slices of the original data set.
+func Split(data []byte) (records [][]byte, err error) {
+        br := bytes.NewReader(data)
+
+        for br.Len() > 0 {
+                var size uint64
+                size, err = binary.ReadUvarint(br)
+                if err != nil {
+                        return
+                }
+                if size > uint64(br.Len()) {
+                        err = ErrFrameTooLarge
+                        return
+                }
+
+                // Pull out the record from the original byte stream without copying.
+                // Casting size to an integer is safe at this point, since we have asserted
+                // that it is less than the remaining length in the buffer, which is an int.
+                offset := len(data) - br.Len()
+                records = append(records, data[offset:offset+int(size)])
+
+                if _, err := br.Seek(int64(size), 1); err != nil {
+                        // Our measurements should protect us from this being an invalid seek.
+                        panic(err)
+                }
+        }
+        return records, nil
+}