There are cases where a scalar value (usually an integer) is represented by a complex structure. One example would be variable-length integers (#8). While some of these constructs are representable with the current means, their handling is cumbersome. That is caused by the fact that a single logical entity (like the Tag type or the Length in the ASN.1 BER encoding) cannot be represented as a single message field. In each place where such a logical entity is used, a differentiation between the possible variants is required to determine which message field for the desired value must be actually accessed.
Another example would be the ASN.1 BER encoding, where each message and each message field is represented by a TLV structure. Using the generated code to serialize such a message would require to determine and set the values of the Tag and the Length for each field manually, which gets very cumbersome for nested messages.
(see also example specifications on issue_8 branch)
package QUIC is
type Length is (L6, L14, L30, L62) with Size => 2;
type U6 is range 0 .. 2 ** 6 - 1 with Size => 6;
type U14 is range 0 .. 2 ** 14 - 1 with Size => 14;
type U30 is range 0 .. 2 ** 30 - 1 with Size => 30;
type U62 is range 0 .. 2 ** 62 - 1 with Size => 62;
type Variable_Length_Integer is
message
Length : Length
then U6
if Length = L6
then U14
if Length = L14
then U30
if Length = L30
then U62
if Length = L62;
U6 : U6
then null;
U14 : U14
then null;
U30 : U30
then null;
U62 : U62;
end message with
Deserialize =>
(case Length is
L6 => U6,
L14 => U14,
L30 => U30,
L62 => U62),
Serialize =>
(case Variable_Length_Integer'Value is
0 .. 2 ** 6 - 1 =>
(Length => L6,
U6 => Variable_Length_Integer'Value),
2 ** 6 .. 2 ** 14 - 1 =>
(Length => L14,
U14 => Variable_Length_Integer'Value),
2 ** 14 .. 2 ** 30 - 1 =>
(Length => L30,
U30 => Variable_Length_Integer'Value),
2 ** 30 .. 2 ** 62 - 1 =>
(Length => L62,
U62 => Variable_Length_Integer'Value));
type Integer is range 0 .. 2 ** 62 - 1
with
Structure => Variable_Length_Integer;
end QUIC;
package MQTT is
type Integer is range 0 .. 2 ** 32 - 1 with
Structure => Variable_Byte_Integer;
type U7 is range 0 .. 2 ** 7 -1 with Size => 7;
type Variable_Byte_Integer_Element is
message
More : Boolean;
Value : U7;
end message;
type Variable_Byte_Integer is sequence of Variable_Byte_Integer_Element
with
Deserialize =>
(Initial => 0,
Next => Variable_Byte_Integer'Current + Variable_Byte_Integer'Element.Value * 128 ** (Variable_Byte_Integer'Position - 1),
Until => (Variable_Byte_Integer'Element.More = False)),
Serialize =>
(Size => Variable_Byte_Integer'Value'Size,
Set => (More => Variable_Byte_Integer'Current >= 128,
Value => Variable_Byte_Integer'Current mod 128),
Next => Variable_Byte_Integer'Current / 128,
Until => Variable_Byte_Integer'Current = 0);
end MQTT;
package BER is
--
-- Identifier
--
type Tag_Class is (TC_Universal, TC_Application, TC_Context_Specific, TC_Private) with
Size => 2;
type Tag_Type_Short is range 0 .. 31 with
Size => 5;
type Tag_Type_Long_Octet is
message
More : Boolean;
Tag_Type_Part : Tag_Type_Part;
end message;
type Tag_Type_Long_Structure is sequence of Tag_Type_Long_Structure_Octet with
Deserialize =>
(Initial => 0,
Next => Tag_Type_Long_Structure'Current * 128 + Tag_Type_Long_Structure'Element.Tag_Type_Part,
Until => (Tag_Type_Long_Structure'Element.More = False)),
Serialize =>
(Size => Tag_Type_Long_Structure'Value'Size,
Set => (More => Tag_Type_Long_Structure'Position < Tag_Type_Long_Structure'Last_Element,
Value => Tag_Type_Long_Structure'Current mod 128),
Next => Tag_Type_Long_Structure'Current / 128,
Reverse => True);
type Tag_Type_Long is range 0 .. 2 ** 63 - 1 with
Structure => Tag_Type_Long_Structure;
type Identifier is
message
Tag_Class : Tag_Class;
Primitive : Boolean;
null : Ignore
with Size => 5;
Tag_Type : Tag_Type
with First => Tag_Class'First;
end message;
type Tag_Type is range 0 .. 2 ** 63 - 1 with
Structure => Tag_Type_Structure;
type Tag_Type_Structure is
message
null : Ignore
with Size => 3;
Tag_Type_Short : Tag_Type_Short
then null
if Tag_Type_Short < 31,
then Tag_Type_Long = 31;
Tag_Type_Long : Tag_Type_Long;
end message with
Deserialize =>
(if Tag_Type_Short < 31 then Tag_Type_Short else Tag_Type_Long),
Serialize =>
(if Tag_Type_Structure'Value < 31
then (Tag_Type_Short => Tag_Type_Structure'Value)
else (Tag_Type_Long => Tag_Type_Structure'Value));
--
-- Length
--
type Octet is range 0 .. 2 ** 8 - 1 with
Size => 8;
type Long_Length_Octets is sequence of Octet with
Deserialize =>
(Initial => 0,
Next => Long_Length_Octets'Current * 256 + Long_Length_Octets'Element);
Serialize =>
(Value => Long_Length_Octets'Value mod 256,
Next => Long_Length_Octets'Value / 256,
Reverse => True);
type Long_Length is range 0 .. 2 ** 63 - 1 with
Structure => Long_Length_Octets;
type Short_Length is range 0 .. 2 ** 7 - 1 with
Size => 7;
type Length_Octets is
message
Form : Boolean;
Short_Length : Short_Length
then null
if Form = False or (Form = True and Short_Length = 127)
then Long_Length
with Size => Short_Length * 8
if Form = True;
Long_Length : Long_Length;
end message with
Deserialize =>
(if Form = False then Short_Length else Long_Length),
Serialize =>
(if Length_Octets'Value >= 128
then (Form => True,
Short_Length => Length_Octets'Value'Size / 8 + 1,
Long_Length => Length_Octets'Value)
else (Form => False,
Short_Length => Length_Octets'Value);
-- Serialize =>
-- (Form => Length_Octets'Value >= 128,
-- Short_Length => (if Length_Octets'Value < 128
-- then Length_Octets'Value
-- else (Length_Octets'Value'Size / 8 + 1)),
-- Long_Length => (if Length_Octets'Value >= 128
-- then Length_Octets'Value
-- else null));
-- Serialize =>
-- (Form => Length_Octets'Value >= 128,
-- if Form then
-- Long_Length => Length_Octets'Value,
-- Short_Length => Length_Octets'Value'Size / 8 + 1
-- else
-- Short_Length => Length_Octets'Value);
type Length is range 0 .. 2 ** 63 - 1 with
Structure => Length_Octets;
--
-- TLV
--
type TLV is
message
Identifier : Identifier:
Length : Length:
Content : Opaque
with Size => Length * 8;
end message;
Context and Problem Statement
There are cases where a scalar value (usually an integer) is represented by a complex structure. One example would be variable-length integers (#8). While some of these constructs are representable with the current means, their handling is cumbersome. That is caused by the fact that a single logical entity (like the Tag type or the Length in the ASN.1 BER encoding) cannot be represented as a single message field. In each place where such a logical entity is used, a differentiation between the possible variants is required to determine which message field for the desired value must be actually accessed.
Another example would be the ASN.1 BER encoding, where each message and each message field is represented by a TLV structure. Using the generated code to serialize such a message would require to determine and set the values of the Tag and the Length for each field manually, which gets very cumbersome for nested messages.
Use Cases
8
Design
Examples
(see also example specifications on
issue_8branch)