matthijsln edited a comment on pull request #214:
URL: https://github.com/apache/commons-compress/pull/214#issuecomment-892504852
This would work without a copy buffer:
```
public static byte[] readRange(final ReadableByteChannel input, final
int len) throws IOException {
final ByteBuffer b = ByteBuffer.allocate(len);
while (b.hasRemaining()) {
if (input.read(b) <= 0) {
break;
}
}
return Arrays.copyOf(b.array(), b.position());
}
```
With a copy buffer:
```
public static byte[] readRange(final ReadableByteChannel input, final
int len) throws IOException {
final ByteArrayOutputStream output = new ByteArrayOutputStream();
final ByteBuffer b = ByteBuffer.allocate(Math.min(len,
COPY_BUF_SIZE));
int read = 0;
while (read < len) {
// Make sure we never read more than len bytes
b.limit(Math.min(len - read, b.capacity()));
final int readNow = input.read(b);
if (readNow <= 0) {
break;
}
output.write(b.array(), 0, readNow);
b.rewind();
read += readNow;
}
return output.toByteArray();
}
```
Additional test case for reading more than the copy buffer length (of course
COPY_BUF_SIZE can be made more accessible instead of using reflection):
```
@Test
public void readRangeMoreThanCopyBufferSize() throws Exception {
final Field COPY_BUF_SIZE =
IOUtils.class.getDeclaredField("COPY_BUF_SIZE");
COPY_BUF_SIZE.setAccessible(true);
final int copyBufSize = (int)COPY_BUF_SIZE.get(null);
// Make an input that requires two read loops
byte[] input = new byte[copyBufSize + 10];
try (SeekableByteChannel in = new
SeekableInMemoryByteChannel(input)) {
// Ask for less than the input length, but more than the buffer
size
int toRead = copyBufSize + 1;
byte[] read = IOUtils.readRange(in, toRead);
assertEquals(toRead, read.length);
assertEquals(toRead, in.position());
}
}
```
Hope this is correct :) What version is preferable? I don't think a copy
buffer in itself is needed, although the maximum memory requirement is `len *
2` instead of `len + COPY_BUF_SIZE`.
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