Document that StreamingXXHash32.asChecksum() returns only 28 bits #115

StreamingXXHash32.java:106 returns getValue() & 0xFFFFFFFL. The mask has been there since 2013 (c2d417e). LZ4BlockOutputStream/LZ4BlockInputStream use this Checksum by default, so the LZ4Block on-wire checksum is truncated to 28 bits. Other implementations of the format (e.g. Netty) reproduce this, so changing the behaviour would break the wire format.

Don't change the behaviour. Instead:

• Javadoc on asChecksum(): for compatibility with the LZ4Block format, getValue() returns only the low 28 bits. Use StreamingXXHash32.getValue() for the full hash.
• Mention the 28-bit mask in the format description of LZ4BlockOutputStream.
• Optionally write the constant as 0x0FFFFFFFL and add a test that pins the value.