fix comments. make code more concise.

Signed-off-by: guo-shaoge <shaoge1994@163.com>

util/chunk/disk.go

@@ -406,27 +406,17 @@ func (r *ReaderWithCache) ReadAt(p []byte, off int64) (readCnt int, err error) {
 
  // When got here, user input is not filled fully, so we need read data from cache.
  err = nil
- if readCnt == 0 {
- // 'readCnt == 0' means all user requested data resides in r.cache.
- beg := off - r.cacheOff
- if beg < 0 {
- return readCnt, errors2.Trace(errors2.New("off must be greater than r.cacheOff when readCnt is 0"))
- }
- end := int(beg) + len(p)
- if end > len(r.cache) {
- err = io.EOF
- end = len(r.cache)
- }
- readCnt = copy(p, r.cache[beg:end])
- } else {
- // readCnt != 0 means only partial data of user requested resides in r.cache.
- p = p[readCnt:]
- end := len(p)
- if end > len(r.cache) {
- err = io.EOF
- end = len(r.cache)
- }
- readCnt += copy(p, r.cache[:end])
+ p = p[readCnt:]
+ beg := off - r.cacheOff
+ if beg < 0 {
+ // This happens when only partial data of user requested resides in r.cache.
+ beg = 0
+ }
+ end := int(beg) + len(p)
+ if end > len(r.cache) {
+ err = io.EOF
+ end = len(r.cache)
  }
+ readCnt += copy(p, r.cache[beg:end])
  return readCnt, err
 }