1	:- use_module(library(error)).
2	:- use_module(library(dcg/basics)).
3
4	:- op(950, xfx, will_be).
5
6	%%% Basic definitions
7
8	% apply check to value when bound
9	will_be(Value, Goal) :-
10	freeze(Value, assertion(call(Goal, Value))).
11
12	byte(Value) :- must_be(between(0, 255), Value).
13
14	nonnegative_integer(Value) :- must_be(nonneg, Value).
15
16	sequence_of_bytes([]).
17	sequence_of_bytes([First|Rest]) :-
18	First will_be byte,
19	Rest will_be sequence_of_bytes.
20
21	netstring_encoding(PayloadBytes) -->	Netstring is a sequence of bytes.
22	netstring_prefix_for_payload(PayloadBytes),	It consists of prefix,
23	PayloadBytes,	payload,
24	`,`.	and terminator. Terminator is single ASCII comma `,`.
25	netstring_encoding(netstring(PayloadBytes), Bytes) :-
26	assertion(ground(PayloadBytes); ground(Bytes)),
27	Bytes will_be sequence_of_bytes,
28	phrase(netstring_encoding(PayloadBytes), Bytes).
29
30		Netstring prefix is the shortest ASCII decimal representation for length of payload in bytes, followed by ASCII colon `:`. That is number starting with non-zero digit unless payload is empty, in which case it's `0`.
31
32
33
34	netstring_prefix_for_payload(PayloadBytes, A, B) :-
35	( var(PayloadBytes)
36	-> netstring_prefix_codes(Prefix, A, B),
37	number_codes(PayloadLength, Prefix),
38	length(PayloadBytes, PayloadLength)
39	; length(PayloadBytes, PayloadLength),
40	number_codes(PayloadLength, Prefix),
41	netstring_prefix_codes(Prefix, A, B)
42	).
43	netstring_prefix_codes(`0`) --> `0:`, !.
44	netstring_prefix_codes([C|Cs]) -->
45	nonzero_digit(C),
46	!,
47	digits(Cs),
48	`:`.
49
50	nonzero_digit(Code) -->
51	[Code],
52	{ assertion(ground(Code)) },
53	{ member(Code, `123456789`) }.
54
55	netstring_of(Goal, Bytes) :-
56	ground(Goal),
57	!,
58	call(Goal, PayloadBytes),
59	netstring_encoding(netstring(PayloadBytes), Bytes).
60
61	netstring_of(Goal, Bytes) :-
62	ground(Bytes),
63	!,
64	netstring_encoding(netstring(PayloadBytes), Bytes),
65	call(Goal, PayloadBytes).
66
67
68	% :- use_module(library(pldoc/doc_files)).
69	% doc_save('spec.pl', [format(html), doc_root(.)]).