design: Rename pdl.md to pdl-language.md

design/pdl-language.md

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+# PDL Language Definition
+
+PDL allows defining a protocol using HOPP and TAPE.
+
+## Data Types
+
+| Syntax     | TN      | CN | Description
+| ---------- | ------- | -: | -----------
+| I5         | SI      |    |   
+| I8         | LI      |  0 | 
+| I16        | LI      |  1 | 
+| I32        | LI      |  3 | 
+| I64        | LI      |  7 | 
+| I128[^2]   | LI      | 15 | 
+| I256[^2]   | LI      | 31 | 
+| U5         | SI      |    | 
+| U8         | LI      |  0 | 
+| U16        | LI      |  1 | 
+| U32        | LI      |  3 | 
+| U64        | LI      |  7 | 
+| U128[^2]   | LI      | 15 | 
+| U256[^2]   | LI      | 31 | 
+| F16        | FP      |  1 | 
+| F32        | FP      |  3 | 
+| F64        | FP      |  7 | 
+| F128[^2]   | FP      | 15 | 
+| F256[^2]   | FP      | 31 | 
+| String     | SBA/LBA |  * | UTF-8 string
+| Buffer     | SBA/LBA |  * | Byte array
+| []\<TYPE\> | OTA     |  * | Array of any type[^1]
+| Table      | KTV     |  * | Table with undefined schema
+| {...}      | KTV     |  * | Table with defined schema
+
+[^1]: Excluding SI and SBA. I5 and U5 cannot be used in an array, but String and
+Buffer are simply forced to use their "long" variant.
+
+[^2]: Some systems may lack support for this.
+
+## Tokens
+
+PDL files are divided into tokens, which assemble together into larger language
+structures. They are separated by whitespace.
+
+| Name     | Syntax             | Description
+| -------- | ------------------ | -----------
+| Magic    | `PDL/0`            | Must appear at the very start of the file.
+| Method   | `M[0-9A-Fa-f]{4}`  | A 16-bit hexadecimal method code.
+| Key      | `[0-9A-Fa-f]{4}`   | A 16-bit hexadecimal table key.
+| Ident    | `[A-Z][A-Za-z0-9]` | An identifier.
+| Comma    | `,`                | A comma separator.
+| LBrace   | `{`                | A left curly brace.
+| RBrace   | `}`                | A right curly brace.
+| LBracket | `[`                | A left square bracket.
+| RBracket | `]`                | A right square bracket.
+
+## Syntax
+
+All files must begin with a Magic token.
+
+Types are expressed with an Ident. A table can be used by either writing the
+name of the type (Table), or by defining a schema with curly braces. Arrays must
+be expressed using two matching square brackets before their element type.
+
+A table schema contains comma-separated fields in-between its braces. Each field
+has three parts: the key number (Key), the field name (Ident), and the field
+type. Tables, Arrays, etc. can be nested.
+
+Files directly contain messages and types, which start with a Method token and
+an Ident token respectively. A message consists of the method code (Method), the
+message name (Ident), and the message's root type. This is usually a table, but
+can be anything.
+
+Here is an example of all that:
+
+```
+PDL/0
+
+M0000 Connect {
+	0000 Name String,
+	0001 Password String,
+}
+
+M0001 UserList {
+	0000 Users []User,
+}
+
+User {
+	0000 Name      String,
+	0001 Bio       String,
+	0002 Followers U32,
+}
+```
+
+## EBNF Description
+
+Below is an EBNF description of the language.
+
+```
+<file>    -> <magic> (<message> | <typedef)*
+<magic>   -> "PDL/0"
+<method>  -> /M[0-9A-Fa-f]{4}/
+<key>     -> /[0-9A-Fa-f]{4}/
+<ident>   -> /[A-Z][A-Za-z0-9]/
+<field>   -> <key> <ident> <type>
+<type> -> <ident>
+        | "[" "]" <type>
+        | "{" (<field> ",")* [<field>] "}"
+<message> -> <method> <ident> <type>
+<typedef> -> <ident> <type>
+```