15.4 Field Encoding
>>>字段編碼
There are 3 types of field encoding operators used or will be used in FAST message. They are default
operator, copy operator and incremental operator.
>>>在FAST消息中,有三種類型的編碼操作符,分別是:默認操作符default,拷貝操作符copy和增量操作符incremental.
15.4.1 Default operator
>>>默認操作符default
The default encoding operator specifies that the value of a field is either present in the stream or it will
be the default value. If the field is optional and without default value, the presence map bit is off
meaning that the input value is absent.
>>>default 說明當前字段的值要麼是數據流中自帶的,要麼是一個默認值.如果這個字段是可選的並且不帶默認值,
>>>那麼presence map位是off,意味着沒有輸入值,它是不存在的,缺席的.
15.4.2 Constant encoding operator
>>>常量編碼操作符Constant
The constant encoding operator specifies that the value of a field is either absent or it is a constant
value. If the field is mandatory, the input value must be the constant value. For optional field, the
input value is the constant value if the presence map bit is on while the input value is absent if the
presence map bit is off.
>>>Constant 說明當前字段的值要麼是不存在的,缺席的,要麼是一個常量值.
>>>如果這個字段是強制的,那麼輸入值必須是常量值.對於可選字段,如果presence map位是on,則輸入值是常量值.
>>>然而如果presence map位是off,則輸入值是不存在的,缺席的.
15.4.3 Copy coding operator
>>>拷貝編碼操作符copy
The copy encoding operator specifies that the value of a field is either copy from previous value or
present in the stream. Input value is present in the stream if the presence bit is on while the input value
is copied from previous value if the presence bit is off.
>>>copy 說明當前字段的值要麼是先前值的拷貝,要麼是數據流中自帶的.
>>>如果presence map位是on,則輸入值在流中自帶,然而如果presence map位是off,則輸入值是先前值的拷貝.
15.4.4 Increment coding
>>>增量編碼操作符incremental
The increment encoding operator specifies that the value of a field is either present in the stream or it
will be the previous value plus increment value. The input value becomes previous value.
>>>incremental 說明當前字段的值要麼是流中自帶的,要麼是先前值加上增量值.這個輸入值成爲先前值.
SSE Level 2 Vendor Interface Specification(FAST)-15章-FAST Decoder（FAST解析-第二部分） ---完