pony-struct

This package performs conversions between Pony values and C structs represented as Pony ByteSeq objects. This can be used in handling binary data stored in files or from network connections, among other sources. It uses Format Strings as compact descriptions of the layout of the C structs and the intended conversion to/from Pony values.

It is inspired by the Python struct module.

Usage

Struct.pack

Pack an Array[(Bool | Number | String | Array[U8] val)] according to a format string:

let byte_seqs = Struct.pack(">Iqd5ss5p",
  [ 1024
    1544924117813084
    3.141592653589793
    "hello"
    "!"
    recover [as U8: 1;2;3;4;5] end
    ] )?

Struct.unpack

Unpack a bytestream (a buffered Reader) into an Array[(Bool | Number | String | Array[U8] val)] according to a format string:

let unpacked = Struct.unpack(">Iq5s", consume reader)?
let length: U32 = unpacked(0)?
let nanosecond_offset = unpacked(1)? as I64
let message: String = unpacked(2)?

Byte Order

Byte order can be optionally specified with the first character in the format string. If left out, Big Endian is used as the default.

Byte order	character
Big Endian	>
Little Endian	<

The format conversion specification is described in the table below.

Format	C Type	Pony type	Standard size
x	pad byte	no value	1
c	char	String	1
b	signed char	I8	1
B	unsigned char	U8	1
?	_Bool	Bool	1
h	short	I16	2
H	unsigned short	U16	2
i	int	I32	4
I	unsigned int	U32	4
l	long	I32	4
L	unsigned long	U32	4
q	long long	I64	8
Q	unsigned long long	U64	8
u	unsigned bigint	U128	16
f	float	f32	4
d	double	f64	8
s	char[]	String
p	char[]	Array[U8]

Each type may be preceded with an integer representing its length (in the case of String and Array[U8]) or number repetitions (all other types).

All types passed to pack must have a reference capability of val.