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106 lines
3.8 KiB
Erlang
106 lines
3.8 KiB
Erlang
-module(term_codec).
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-export([encode/1, decode/1]).
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%% Erlang-side term <-> binary codec, built on the substrate fixes from
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%% commits 24e3bf53 (binary_to_list / list_to_binary), 3d80bd8c ($X char
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%% literals), 4852cca9 (atom_to_list / integer_to_list charlists).
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%%
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%% Wire format (netstring-ish; all length headers ASCII decimal):
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%%
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%% atom $a Len $: NameBytes
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%% integer $i Len $: DecimalBytes (negative ints carry leading $-)
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%% binary $b Len $: RawBytes
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%% tuple $t Count $: Enc1 Enc2 ... Encn
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%% list $l Count $: Enc1 Enc2 ... Encn (proper list)
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%% nil $l $0 $: (empty list)
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%%
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%% Each Enc is itself one of these forms — recursive. The format is
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%% byte-clean: binary bodies may contain any byte (newlines, NULs, etc.),
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%% so callers can frame entries with a 4-byte big-endian length prefix
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%% (Step 3b on-disk segment writer's job).
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%% encode/1: term -> binary
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encode(T) when is_atom(T) ->
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Cs = atom_to_list(T),
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list_to_binary([$a, integer_to_list(length(Cs)), $:, Cs]);
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encode(T) when is_integer(T) ->
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Cs = integer_to_list(T),
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list_to_binary([$i, integer_to_list(length(Cs)), $:, Cs]);
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encode(T) when is_binary(T) ->
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list_to_binary([$b, integer_to_list(byte_size(T)), $:, T]);
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encode(T) when is_tuple(T) ->
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L = tuple_to_list(T),
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list_to_binary([$t, integer_to_list(length(L)), $:,
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[encode(E) || E <- L]]);
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encode([]) ->
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list_to_binary([$l, $0, $:]);
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encode(T) when is_list(T) ->
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list_to_binary([$l, integer_to_list(length(T)), $:,
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[encode(E) || E <- T]]).
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%% decode/1: binary -> {ok, Term, RestBinary} | {error, badform}
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%% On success returns the remaining unconsumed bytes so callers can
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%% stream-decode multiple frames from one buffer.
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decode(B) when is_binary(B) ->
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decode_chars(binary_to_list(B)).
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decode_chars([$a | Rest]) ->
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{Len, Rest1} = read_len(Rest, 0),
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Rest2 = strip_colon(Rest1),
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{NameChars, Rest3} = split_at(Len, Rest2),
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{ok, list_to_atom(NameChars), list_to_binary(Rest3)};
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decode_chars([$i | Rest]) ->
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{Len, Rest1} = read_len(Rest, 0),
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Rest2 = strip_colon(Rest1),
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{NumChars, Rest3} = split_at(Len, Rest2),
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{ok, list_to_integer(NumChars), list_to_binary(Rest3)};
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decode_chars([$b | Rest]) ->
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{Len, Rest1} = read_len(Rest, 0),
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Rest2 = strip_colon(Rest1),
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{Bytes, Rest3} = split_at(Len, Rest2),
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{ok, list_to_binary(Bytes), list_to_binary(Rest3)};
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decode_chars([$t | Rest]) ->
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{N, Rest1} = read_len(Rest, 0),
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Rest2 = strip_colon(Rest1),
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{Elems, Rest3} = decode_n(N, Rest2, []),
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{ok, list_to_tuple(Elems), list_to_binary(Rest3)};
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decode_chars([$l | Rest]) ->
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{N, Rest1} = read_len(Rest, 0),
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Rest2 = strip_colon(Rest1),
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{Elems, Rest3} = decode_n(N, Rest2, []),
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{ok, Elems, list_to_binary(Rest3)};
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decode_chars(_) ->
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{error, badform}.
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read_len([C | Rest], Acc) when C >= $0, C =< $9 ->
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read_len(Rest, Acc * 10 + C - $0);
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read_len([$- | Rest], 0) ->
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%% Leading minus for negative integer-body lengths is invalid for
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%% lengths, but appears inside integer-body bytes (handled in
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%% the body, not here — read_len only consumes digits before $:).
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{0, [$- | Rest]};
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read_len(Rest, Acc) ->
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{Acc, Rest}.
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strip_colon([$: | Rest]) -> Rest;
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strip_colon(Other) -> erlang:error({badform, Other}).
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split_at(0, Rest) -> {[], Rest};
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split_at(N, [H | T]) ->
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{Hs, Tl} = split_at(N - 1, T),
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{[H | Hs], Tl};
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split_at(_, []) ->
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erlang:error({badform, short}).
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decode_n(0, Rest, Acc) ->
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{lists:reverse(Acc), Rest};
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decode_n(N, Bytes, Acc) ->
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{Term, Rest} = decode_one(Bytes),
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decode_n(N - 1, Rest, [Term | Acc]).
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decode_one(Bytes) ->
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case decode_chars(Bytes) of
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{ok, Term, RestBin} -> {Term, binary_to_list(RestBin)};
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{error, R} -> erlang:error({badform, R})
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end.
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