Erlangコード最適化メモ: JSONデコード処理(1): まず基本形 ref: http://qiita.com/sile/items/6df1400ce45f870b4b03

json_decode_1.erl

-module(json_decode_1).

-export([decode/1]).

-type json_value()  :: null | boolean() | json_number() |
                       json_string() | json_array() | json_object().
-type json_number() :: non_neg_integer().
-type json_string() :: binary().
-type json_array()  :: [json_value()].
-type json_object() :: {[json_object_member()]}.
-type json_object_member() :: {json_string(), json_value()}.

%% @doc JSON文字列をデコードする
%%
%% 不正なJSON文字列が渡された場合は、badargエラーが送出される
-spec decode(binary()) -> json_value().
decode(Json) ->
    {Value, _RestBin} = value(skip_whitespace(Json)),
    Value.

-spec skip_whitespace(binary()) -> binary().
skip_whitespace(<<$  , Bin/binary>>) -> skip_whitespace(Bin);
skip_whitespace(<<$\t, Bin/binary>>) -> skip_whitespace(Bin);
skip_whitespace(<<$\r, Bin/binary>>) -> skip_whitespace(Bin);
skip_whitespace(<<$\n, Bin/binary>>) -> skip_whitespace(Bin);
skip_whitespace(Bin)                 -> Bin.

-spec value(binary()) -> {json_value(), binary()}.
value(<<"null", Bin/binary>>)                  -> {null, Bin};
value(<<"false", Bin/binary>>)                 -> {false, Bin};
value(<<"true", Bin/binary>>)                  -> {true, Bin};
value(<<$[, Bin/binary>>)                      -> array(skip_whitespace(Bin));
value(<<${, Bin/binary>>)                      -> object(skip_whitespace(Bin));
value(<<$", Bin/binary>>)                      -> string(Bin, "");
value(<<C, Bin/binary>>) when $0 =< C, C =< $9 -> number(C - $0, Bin);
value(Bin)                                     -> error(badarg, [Bin]).

-spec array(binary()) -> {json_array(), binary()}.
array(<<$], Bin/binary>>) -> {[], Bin};
array(Bin)                -> array(Bin, []).

-spec array(binary(), [json_value()]) -> {json_array(), binary()}.
array(Bin, Values) ->
    {Value, Bin2} = value(Bin),
    Values2 = [Value | Values],
    case skip_whitespace(Bin2) of
        <<$], Bin3/binary>> -> {lists:reverse(Values2), Bin3};
        <<$,, Bin3/binary>> -> array(skip_whitespace(Bin3), Values2);
        _                   -> error(badarg, [Bin, Values])
    end.

-spec object(binary()) -> {json_object(), binary()}.
object(<<$}, Bin/binary>>) -> {{[]}, Bin};
object(Bin)                -> object(Bin, []).

-spec object(binary(), [json_object_member()]) -> {json_object(), binary()}.
object(<<$", Bin/binary>>, Members) ->
    {Key, Bin2} = string(Bin, ""),
    case skip_whitespace(Bin2) of
        <<$:, Bin3/binary>> ->
            {Value, Bin4} = value(skip_whitespace(Bin3)),
            Members2 = [{Key, Value} | Members],
            case skip_whitespace(Bin4) of
                <<$}, Bin5/binary>> -> {{lists:reverse(Members2)}, Bin5};
                <<$,, Bin5/binary>> -> object(skip_whitespace(Bin5), Members2);
                _                   -> error(badarg, [<<$", Bin/binary>>, Members])
            end;
        _ -> error(badarg, [<<$", Bin/binary>>, Members])
    end;
object(Bin, Members) -> error(badarg, [Bin, Members]).

-spec string(binary(), string()) -> {json_string(), binary()}.
string(<<$",       Bin/binary>>, Acc) -> {list_to_binary(lists:reverse(Acc)), Bin};
string(<<$\\, $",  Bin/binary>>, Acc) -> string(Bin, [$" | Acc]);
string(<<$\\, $/,  Bin/binary>>, Acc) -> string(Bin, [$/ | Acc]);
string(<<$\\, $\\, Bin/binary>>, Acc) -> string(Bin, [$\\ | Acc]);
string(<<$\\, $b,  Bin/binary>>, Acc) -> string(Bin, [$\b | Acc]);
string(<<$\\, $f,  Bin/binary>>, Acc) -> string(Bin, [$\f | Acc]);
string(<<$\\, $n,  Bin/binary>>, Acc) -> string(Bin, [$\n | Acc]);
string(<<$\\, $r,  Bin/binary>>, Acc) -> string(Bin, [$\r | Acc]);
string(<<$\\, $t,  Bin/binary>>, Acc) -> string(Bin, [$\t | Acc]);
string(<<$\\,      Bin/binary>>, Acc) -> error(badarg, [<<$\\, Bin/binary>>, Acc]);
string(<<0:1, C:7, Bin/binary>>, Acc) -> string(Bin, [C | Acc]);
string(Bin,                      Acc) -> error(badarg, [Bin, Acc]).

-spec number(json_number(), binary()) -> {json_number(), binary()}.
number(N, <<C, Bin/binary>>) when $0 =< C, C =< $9 -> number(N * 10 + C - $0, Bin);
number(N, Bin)                                     -> {N, Bin}.