readRawVarint32() — netty Function Reference

Source Code

codec-protobuf/src/main/java/io/netty/handler/codec/protobuf/ProtobufVarint32FrameDecoder.java lines 73–98

    static int readRawVarint32(ByteBuf buffer) {
        if (buffer.readableBytes() < 4) {
            return readRawVarint24(buffer);
        }
        int wholeOrMore = buffer.getIntLE(buffer.readerIndex());
        int firstOneOnStop = ~wholeOrMore & 0x80808080;
        if (firstOneOnStop == 0) {
            return readRawVarint40(buffer, wholeOrMore);
        }
        int bitsToKeep = Integer.numberOfTrailingZeros(firstOneOnStop) + 1;
        buffer.skipBytes(bitsToKeep >> 3);
        int thisVarintMask = firstOneOnStop ^ (firstOneOnStop - 1);
        int wholeWithContinuations = wholeOrMore & thisVarintMask;
        // mix them up as per varint spec while dropping the continuation bits:
        // 0x7F007F isolate the first byte and the third byte dropping the continuation bits
        // 0x7F007F00 isolate the second byte and the fourth byte dropping the continuation bits
        // the second and fourth byte are shifted to the right by 1, filling the gaps left by the first and third byte
        // it means that the first and second bytes now occupy the first 14 bits (7 bits each)
        // and the third and fourth bytes occupy the next 14 bits (7 bits each), with a gap between the 2s of 2 bytes
        // and another gap of 2 bytes after the forth and third.
        wholeWithContinuations = (wholeWithContinuations & 0x7F007F) | ((wholeWithContinuations & 0x7F007F00) >> 1);
        // 0x3FFF isolate the first 14 bits i.e. the first and second bytes
        // 0x3FFF0000 isolate the next 14 bits i.e. the third and forth bytes
        // the third and forth bytes are shifted to the right by 2, filling the gaps left by the first and second bytes
        return (wholeWithContinuations & 0x3FFF) | ((wholeWithContinuations & 0x3FFF0000) >> 2);
    }