go4.org@v0.0.0-20230225012048-214862532bf5/readerutil/bufreaderat.go (about)

     1  /*
     2  Copyright 2018 The go4 Authors
     3  
     4  Licensed under the Apache License, Version 2.0 (the "License");
     5  you may not use this file except in compliance with the License.
     6  You may obtain a copy of the License at
     7  
     8       http://www.apache.org/licenses/LICENSE-2.0
     9  
    10  Unless required by applicable law or agreed to in writing, software
    11  distributed under the License is distributed on an "AS IS" BASIS,
    12  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13  See the License for the specific language governing permissions and
    14  limitations under the License.
    15  */
    16  
    17  package readerutil
    18  
    19  import "io"
    20  
    21  // NewBufferingReaderAt returns an io.ReaderAt that reads from r as
    22  // necessary and keeps a copy of all data read in memory.
    23  func NewBufferingReaderAt(r io.Reader) io.ReaderAt {
    24  	return &bufReaderAt{r: r}
    25  }
    26  
    27  type bufReaderAt struct {
    28  	r   io.Reader
    29  	buf []byte
    30  }
    31  
    32  func (br *bufReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
    33  	endOff := off + int64(len(p))
    34  	need := endOff - int64(len(br.buf))
    35  	if need > 0 {
    36  		buf := make([]byte, need)
    37  		var rn int
    38  		rn, err = io.ReadFull(br.r, buf)
    39  		br.buf = append(br.buf, buf[:rn]...)
    40  	}
    41  	if int64(len(br.buf)) >= off {
    42  		n = copy(p, br.buf[off:])
    43  	}
    44  	if n == len(p) {
    45  		err = nil
    46  	}
    47  	return
    48  }