-module(discovery).
-export([parse_acct/1, parse_resource/1,
         actor_url_for/2, webfinger_body/3]).

%% Discovery primitives per design §13.7. Step 10a covers the
%% local-side webfinger endpoint (responding when a peer asks
%% "where does acct:alice@here live?"); the peer-fetch direction
%% (loading a peer's actor doc lazily on first inbound) is Step 10b
%% and gates on Blockers #2 (native http-request primitive).
%%
%% parse_acct/1 — accept a binary in either form:
%%   <<"acct:alice@host:port">>   (full prefixed URI)
%%   <<"alice@host:port">>         (bare account, prefix optional)
%% Returns {ok, User, Host} | {error, Reason}.
%%
%% parse_resource/1 — the resource= query parameter from
%% /.well-known/webfinger. Same shape as parse_acct.
%%
%% actor_url_for/2(User, Host) — synthesises the canonical
%% per-actor URL `<scheme>://<host>/actors/<user>`. v2 hardcodes
%% http://; TLS / https is v3 (Blockers gate).
%%
%% webfinger_body/3 — builds the JSON response body.

%% ── parse_acct / parse_resource ─────────────────────────────────

%% "acct:" -> 5 bytes: 97 99 99 116 58
parse_acct(Bin) when is_binary(Bin) ->
    AcctPrefix = <<97,99,99,116,58>>,
    case strip_prefix(AcctPrefix, Bin) of
        {ok, Rest} -> split_user_host(Rest);
        nomatch    -> split_user_host(Bin)
    end;
parse_acct(_) -> {error, bad_input}.

parse_resource(Bin) -> parse_acct(Bin).

%% strip_prefix/2 — return {ok, Rest} when Bin starts with Prefix,
%% else nomatch. Substrate has no proper prefix-match BIF; this
%% byte-walks.

strip_prefix(<<>>, Rest) -> {ok, Rest};
strip_prefix(<<B, PRest/binary>>, <<B, RRest/binary>>) ->
    strip_prefix(PRest, RRest);
strip_prefix(_, _) -> nomatch.

%% split_user_host/1 — split a `user@host[:port]` binary at the
%% first `@`. Returns {ok, User, Host} where Host may include the
%% optional port suffix.

split_user_host(Bin) ->
    case split_at(64, Bin) of  % 64 = '@'
        {Before, After} when byte_size(Before) > 0, byte_size(After) > 0 ->
            {ok, Before, After};
        _ ->
            {error, bad_acct}
    end.

split_at(Byte, Bin) ->
    split_at(Byte, Bin, <<>>).

split_at(_, <<>>, Acc) ->
    {Acc, <<>>};
split_at(Byte, <<Byte, Rest/binary>>, Acc) ->
    {Acc, Rest};
split_at(Byte, <<B, Rest/binary>>, Acc) ->
    split_at(Byte, Rest, <<Acc/binary, B>>).

%% ── URL synthesis ──────────────────────────────────────────────

%% "http://"  -> 7 bytes  | "/actors/" -> 8 bytes
actor_url_for(User, Host) ->
    Pre = <<104,116,116,112,58,47,47>>,            % "http://"
    Mid = <<47,97,99,116,111,114,115,47>>,         % "/actors/"
    <<Pre/binary, Host/binary, Mid/binary, User/binary>>.

%% ── webfinger JSON body ────────────────────────────────────────
%%
%% Mastodon-shape per RFC 7033:
%%   {"subject":"acct:<user>@<host>",
%%    "links":[{"rel":"self",
%%              "type":"application/activity+json",
%%              "href":"<actor_url>"}]}
%%
%% Hand-rolled byte concatenation — no JSON BIF on this port. The
%% caller has already validated User + Host; we don't need to
%% re-escape (Mastodon's webfinger inputs are alphanumeric +
%% .-_ in practice).

webfinger_body(User, Host, ActorUrl) ->
    AcctPre = <<123,34,115,117,98,106,101,99,116,34,58,34,97,99,99,116,58>>,  % '{"subject":"acct:'
    AcctAt  = <<64>>,                                                          % '@'
    LinksHd = <<34,44,34,108,105,110,107,115,34,58,91,123,34,114,101,108,34,58,34,115,101,108,102,34,44,
                34,116,121,112,101,34,58,34,97,112,112,108,105,99,97,116,105,111,110,47,97,99,116,
                105,118,105,116,121,43,106,115,111,110,34,44,34,104,114,101,102,34,58,34>>,         % '","links":[{"rel":"self","type":"application/activity+json","href":"'
    LinksTl = <<34,125,93,125,10>>,                                            % '"}]}\n'
    <<AcctPre/binary, User/binary, AcctAt/binary, Host/binary,
      LinksHd/binary, ActorUrl/binary, LinksTl/binary>>.