I try to compress string in Java like php's gzcompress function does it. But I can't get the same byte arrays in result.
My php code:
<?php
echo 'This is a test';
$compressed = gzcompress('This is a test');
echo '<br>';
for ($i=0; $i<strlen($compressed); $i++) {
echo '['.$i.'] = '.ord($compressed[$i]).'<br>';
}
?>
The result:
This is a test
[0] = 120
[1] = 156
[2] = 11
[3] = 201
[4] = 200
[5] = 44
[6] = 86
[7] = 0
[8] = 162
[9] = 68
[10] = 133
[11] = 146
[12] = 212
[13] = 226
[14] = 18
[15] = 0
[16] = 36
[17] = 115
[18] = 4
[19] = 246
Java code:
public class Main {
public static byte[] compressString(String uncompressedString) throws IllegalArgumentException, IllegalStateException {
try {
byte[] utfEncodedBytes = uncompressedString.getBytes();
ByteArrayOutputStream baos = new ByteArrayOutputStream();
try (GZIPOutputStream gzipOutputStream = new GZIPOutputStream(baos)) {
gzipOutputStream.write(utfEncodedBytes);
gzipOutputStream.finish();
}
return baos.toByteArray();
} catch (Exception e) {
throw new IllegalStateException("GZIP compression failed: " + e, e);
}
}
public static void main(String[] args) {
String input = "This is a test";
System.out.println("Input: " + input);
byte[] compressed = compressString(input);
for (int i = 0; i < compressed.length; i++) {
System.out.println("[" + i + "] = " + (compressed[i] & 0xFF));
}
}
}
The result:
Input: This is a test
[0] = 31
[1] = 139
[2] = 8
[3] = 0
[4] = 0
[5] = 0
[6] = 0
[7] = 0
[8] = 0
[9] = 0
[10] = 11
[11] = 201
[12] = 200
[13] = 44
[14] = 86
[15] = 0
[16] = 162
[17] = 68
[18] = 133
[19] = 146
[20] = 212
[21] = 226
[22] = 18
[23] = 0
[24] = 50
[25] = 159
[26] = 122
[27] = 192
[28] = 14
[29] = 0
[30] = 0
[31] = 0
Middle parts of compressed arrays are the same. But they are different in the start and in the end.
The documentation for the gzcompress function specifically states that the compression algorithm it uses is zlib, which is
not the same as gzip compression, which includes some header data. See gzencode() for gzip compression.
Java's GZIPOutputStream does proper gzip compression including the right headers. DeflaterOutputStream does plain deflate without the gzip headers, which may be closer to what you are after, or there are third party solutions such as JZlib.
According to the the RFC http://www.gzip.org/zlib/rfc-gzip.html
Its states that the start of the file should contain.
ID1 (IDentification 1) ID2 (IDentification 2) These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139 (0x8b, \213), to identify the file as being in gzip format.
CM (Compression Method) This identifies the compression method used in the file. CM = 0-7 are reserved. CM = 8 denotes the "deflate" compression method, which is the one customarily used by gzip and which is documented elsewhere.
So the first three bytes should be 31, 139, 8. The fourth byte should be a value between 0-31 (using bits 0 to 4, and 5,6, and 7 are reserved). I suspect the output you see in PHP is not gz format.