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-module(bencode).
-export([bdecode/1]).
-import(string, [substr/2, substr/3, to_integer/1, chr/2]).
%% String
bdecode([Char | _Rest] = Str) when Char >= $0, Char =< $9 ->
ColIdx = chr(Str, $:),
IntStr = substr(Str, 1, ColIdx-1),
{Len, _} = to_integer(IntStr),
Result = substr(Str, ColIdx+1, Len),
Rest = substr(Str, ColIdx+Len+1),
{{string, Result}, Rest};
%% Int
bdecode([Char | Rest]) when Char == $i ->
End = chr(Rest, $e),
{Result, _} = to_integer(substr(Rest, 1, End-1)),
{{int, Result}, substr(Rest, End+1)};
%% List
bdecode([Char | Rest]) when Char == $l ->
{List, Rem} = dolist(Rest),
{{list, List}, Rem};
%% Dict
bdecode([Char | Rest]) when Char == $d ->
{Dict, Rem} = dodict(Rest),
{{dict, Dict}, Rem}.
dolist(Str) ->
dolist(Str, []).
dolist([Char | Rest], List) when Char == $e ->
{List, Rest};
dolist(Str, List) ->
{Result, Rest} = bdecode(Str),
dolist(Rest, lists:append(List, [Result])).
dodict(Str) ->
dodict(Str, dict:new()).
dodict([Char | Rest], Dict) when Char == $e ->
{Dict, Rest};
dodict(Str, Dict) ->
{{string, Key}, Rest} = bdecode(Str),
{Val, Rest2} = bdecode(Rest),
dodict(Rest2, dict:store(Key, Val, Dict)).
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