presto-spi/src/main/java/io/prestosql/spi/Page.java/Page/determinePositionCount
class Page:    private static int determinePositionCount(Block... blocks)
    {
        requireNonNull(blocks, "blocks is null");
        if (blocks.length == 0) {
            throw new IllegalArgumentException("blocks is empty");
        }

        return blocks[0].getPositionCount();
    }