First code

.gitignore

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+# File di build e compilazione
+/_build/
+*.beam
+*.o
+*.plt
+
+# Dipendenze scaricate
+/deps/
+
+# File di test e log
+.eunit/
+logs/
+erl_crash.dump
+
+# File generati da strumenti
+.rebar/
+.rebar3/
+.cover/
+*.d
+
+# File temporanei di Mnesia (database locale)
+Mnesia.*
+
+# File temporanei di editor
+*~
+*.swp
+*.swo
+
+# File temporanei di sistema
+.DS_Store
+Thumbs.db
+
+# Escludi solo i file beam da ebin, NON l'intera cartella (serve per i .app)
+ebin/*.beam

rebar.config

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+{deps, [
+  {cowboy, "2.10.0"},
+  {jsx, "3.1.0"}
+]}.

rebar.lock

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+{"1.2.0",
+[{<<"cowboy">>,{pkg,<<"cowboy">>,<<"2.10.0">>},0},
+ {<<"cowlib">>,{pkg,<<"cowlib">>,<<"2.12.1">>},1},
+ {<<"jsx">>,{pkg,<<"jsx">>,<<"3.1.0">>},0},
+ {<<"ranch">>,{pkg,<<"ranch">>,<<"1.8.0">>},1}]}.
+[
+{pkg_hash,[
+ {<<"cowboy">>, <<"FF9FFEFF91DAE4AE270DD975642997AFE2A1179D94B1887863E43F681A203E26">>},
+ {<<"cowlib">>, <<"A9FA9A625F1D2025FE6B462CB865881329B5CAFF8F1854D1CBC9F9533F00E1E1">>},
+ {<<"jsx">>, <<"D12516BAA0BB23A59BB35DCCAF02A1BD08243FCBB9EFE24F2D9D056CCFF71268">>},
+ {<<"ranch">>, <<"8C7A100A139FD57F17327B6413E4167AC559FBC04CA7448E9BE9057311597A1D">>}]},
+{pkg_hash_ext,[
+ {<<"cowboy">>, <<"3AFDCCB7183CC6F143CB14D3CF51FA00E53DB9EC80CDCD525482F5E99BC41D6B">>},
+ {<<"cowlib">>, <<"163B73F6367A7341B33C794C4E88E7DBFE6498AC42DCD69EF44C5BC5507C8DB0">>},
+ {<<"jsx">>, <<"0C5CC8FDC11B53CC25CF65AC6705AD39E54ECC56D1C22E4ADB8F5A53FB9427F3">>},
+ {<<"ranch">>, <<"49FBCFD3682FAB1F5D109351B61257676DA1A2FDBE295904176D5E521A2DDFE5">>}]}
+].

src/activitypub_server.app.src

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+{application, activitypub_server,
+ [
+  {description, "A simple ActivityPub server in Erlang"},
+  {vsn, "0.1.0"},
+  {modules, [
+    activitypub_server_app,
+    activitypub_server_sup,
+    hello_handler,
+    webfinger_handler
+  ]},
+  {registered, []},
+  {applications, [
+    kernel,
+    stdlib,
+    cowboy,
+    jsx
+  ]},
+  {mod, {activitypub_server_app, []}},
+  {env, []}
+ ]}.

src/activitypub_server_app.erl

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+%% activitypub_server_app.erl
+%% Modulo principale dell'applicazione OTP
+
+-module(activitypub_server_app).
+-behaviour(application).
+
+%% Esporta le funzioni richieste dal behaviour application
+-export([start/2, stop/1]).
+
+start(_Type, _Args) ->
+
+    %% creiamo un utente di test, solo per vedere se mnesia va
+    user_db:init(),
+    user_db:add_user(<<"admin">>, #{email => <<"admin@example.com">>}),
+
+    Dispatch = cowboy_router:compile([
+        {'_', [
+            {"/", hello_handler, []},
+            {"/.well-known/webfinger", webfinger_handler, []}
+        ]}
+    ]),
+    {ok, _} = cowboy:start_clear(
+        http_listener,
+        [{port, 8080}],
+        #{env => #{dispatch => Dispatch}}
+    ),
+    activitypub_server_sup:start_link().
+
+
+stop(_State) ->
+    ok.

src/activitypub_server_sup.erl

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+-module(activitypub_server_sup).
+-behaviour(supervisor).
+
+-export([start_link/0]).
+-export([init/1]).
+
+start_link() ->
+    supervisor:start_link({local, ?MODULE}, ?MODULE, []).
+    
+
+init([]) ->
+    {ok, {{one_for_one, 1, 5}, []}}.
+

src/hello_handler.erl

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+-module(hello_handler).
+-behaviour(cowboy_handler).
+
+-export([init/2]).
+
+init(Req, State) ->
+    {ok, Resp} = cowboy_req:reply(
+        200,
+        #{<<"content-type">> => <<"text/plain">>},
+        <<"Hello, world!">>,
+        Req
+    ),
+    {ok, Resp, State}.

src/start_server.erl

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+-module(start_server).
+-export([main/0]).
+
+main() ->
+    application:ensure_all_started(activitypub_server).

src/user_db.erl

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+-module(user_db).
+-export([init/0, add_user/2, get_user/1, all_users/0]).
+
+-record(user, {username, data}).
+
+init() ->
+    mnesia:create_schema([node()]),
+    mnesia:start(),
+    mnesia:create_table(user, [
+        {attributes, record_info(fields, user)},
+        {disc_copies, [node()]}
+    ]).
+
+add_user(Username, Data) ->
+    F = fun() ->
+                mnesia:write(#user{username=Username, data=Data})
+        end,
+    mnesia:transaction(F).
+
+get_user(Username) ->
+    F = fun() ->
+                case mnesia:read({user, Username}) of
+                    [User] -> {ok, User};
+                    [] -> not_found
+                end
+        end,
+    {atomic, Result} = mnesia:transaction(F),
+    Result.
+
+all_users() ->
+    F = fun() ->
+                mnesia:match_object(#user{username='_', data='_'})
+        end,
+    {atomic, Users} = mnesia:transaction(F),
+    Users.

src/webfinger_handler.erl

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+%% webfinger_handler.erl
+%% Handler per l'endpoint /.well-known/webfinger
+-module(webfinger_handler).
+-behaviour(cowboy_handler).
+
+%% Cowboy vuole che esportiamo la funzione init/2
+-export([init/2]).
+
+init(Req, State) ->
+    Params = cowboy_req:parse_qs(Req),
+    Resource = proplists:get_value(<<"resource">>, Params),
+
+    case Resource of
+        undefined ->
+            {ok, Resp} = cowboy_req:reply(
+                400,
+                #{<<"content-type">> => <<"application/json">>},
+                <<"{\"error\": \"Missing resource parameter\"}">>,
+                Req
+            ),
+            {ok, Resp, State};
+        _ ->
+            case parse_acct(Resource) of
+                {ok, Username, Domain} ->
+                    %% Qui interroghiamo Mnesia!
+                    case user_db:get_user(Username) of
+                        {ok, _User} ->
+                            ActorUrl = <<"https://", Domain/binary, "/users/", Username/binary>>,
+                            WebfingerMap = #{
+                                <<"subject">> => <<"acct:", Username/binary, "@", Domain/binary>>,
+                                <<"links">> => [
+                                    #{
+                                        <<"rel">> => <<"self">>,
+                                        <<"type">> => <<"application/activity+json">>,
+                                        <<"href">> => ActorUrl
+                                    }
+                                ]
+                            },
+                            Json = jsx:encode(WebfingerMap),
+                            {ok, Resp} = cowboy_req:reply(
+                                200,
+                                #{<<"content-type">> => <<"application/jrd+json">>},
+                                Json,
+                                Req
+                            ),
+                            {ok, Resp, State};
+                        not_found ->
+                            {ok, Resp} = cowboy_req:reply(
+                                404,
+                                #{<<"content-type">> => <<"application/json">>},
+                                <<"{\"error\": \"User not found\"}">>,
+                                Req
+                            ),
+                            {ok, Resp, State}
+                    end;
+                error ->
+                    {ok, Resp} = cowboy_req:reply(
+                        400,
+                        #{<<"content-type">> => <<"application/json">>},
+                        <<"{\"error\": \"Invalid resource format\"}">>,
+                        Req
+                    ),
+                    {ok, Resp, State}
+            end
+    end.
+
+
+%% Funzione di utilità per estrarre username e dominio da "acct:username@domain"
+parse_acct(<<"acct:", Rest/binary>>) ->
+    case binary:split(Rest, <<"@">>) of
+        [Username, Domain] when Username =/= <<>>, Domain =/= <<>> ->
+            {ok, Username, Domain};
+        _ ->
+            error
+    end;
+parse_acct(_) ->
+    error.