presto-main/src/main/java/io/prestosql/operator/exchange/LocalPartitionGenerator.java/LocalPartitionGenerator/LocalPartitionGenerator
class LocalPartitionGenerator:    public LocalPartitionGenerator(HashGenerator hashGenerator, int partitionCount)
    {
        this.hashGenerator = hashGenerator;
        checkArgument(Integer.bitCount(partitionCount) == 1, "partitionCount must be a power of 2");
        this.partitionCount = partitionCount;
        hashMask = partitionCount - 1;
    }