%% @doc property-based tests for `mimemail' module
%%
  Following limitations of mimemail are discovered and modelled in this suite :
%% * We may truncate leading and trailing whitespaces " " from header values
  * We may truncate trailing tabs and whitespaces from payload when Content - Transfer - Encoding is not base64
%% * For binary payload it's highly recommended to set `#{transfer_encoding => <<"base64">>}' explicitly
-module(prop_mimemail).

-export([
    prop_plaintext_encode_no_crash/1,
    prop_multipart_encode_no_crash/1,
    prop_plaintext_encode_decode_match/1,
    prop_multipart_encode_decode_match/1,
    prop_encode_decode_no_mime_version_match/1,
    prop_quoted_printable/1,
    prop_smtp_compatible/1
]).

-include_lib("proper/include/proper.hrl").
-include_lib("stdlib/include/assert.hrl").

prop_plaintext_encode_no_crash(doc) ->
    "Check that any plaintext mail can be encoded without crash".

prop_plaintext_encode_no_crash() ->
    ?FORALL(
        Mail,
        gen_plaintext_mail(),
        is_binary(mimemail:encode(Mail))
    ).

prop_multipart_encode_no_crash(doc) ->
    "Check that any multipart mail can be encoded without crash".

prop_multipart_encode_no_crash() ->
    ?FORALL(
        Mail,
        gen_multipart_mail(),
        is_binary(mimemail:encode(Mail))
    ).

prop_plaintext_encode_decode_match(doc) ->
    "Check that any plaintext mail can be encoded and decoded without"
    " information loss or corruption".

prop_plaintext_encode_decode_match() ->
    ?FORALL(
        Mail,
        gen_plaintext_mail(),
        begin
            Encoded = mimemail:encode(Mail),
            Recoded = mimemail:decode(Encoded),
            ?WHENFAIL(
                io:format(
                    "Orig:~n~p~nEncoded:~n~p~nRecoded:~n~p~n",
                    [Mail, Encoded, Recoded]
                ),
                match(Mail, Recoded)
            )
        end
    ).

prop_multipart_encode_decode_match(doc) ->
    "Check that any plaintext mail can be encoded and decoded without"
    " information loss or corruption".

prop_multipart_encode_decode_match() ->
    ?FORALL(
        Mail,
        gen_multipart_mail(),
        begin
            Encoded = mimemail:encode(Mail),
            Recoded = mimemail:decode(Encoded),
            ?WHENFAIL(
                io:format(
                    "Orig:~n~p~nEncoded:~n~p~nRecoded:~n~p~n",
                    [Mail, Encoded, Recoded]
                ),
                match(Mail, Recoded)
            )
        end
    ).

prop_encode_decode_no_mime_version_match(doc) ->
    "Make sure decoder is able to recover from situation when 'mime-version' header is missing".

prop_encode_decode_no_mime_version_match() ->
    ?FORALL(
        Mail,
        proper_types:oneof([gen_plaintext_mail(), gen_multipart_mail()]),
        begin
            Encoded = mimemail:encode(Mail),
            Recoded = mimemail:decode(
                strip_mime_version(Encoded),
                [
                    {allow_missing_version, true},
                    {encoding, <<"utf-8">>}
                ]
            ),
            ?WHENFAIL(
                io:format(
                    "Orig:~n~p~nEncoded:~n~p~nRecoded:~n~p~n",
                    [Mail, Encoded, Recoded]
                ),
                match(Mail, Recoded)
            )
        end
    ).

match(
    {TypeA, SubTypeA, HeadersA, ParamsA, BodyA},
    {TypeB, SubTypeB, HeadersB, ParamsB, BodyB}
) ->
    ?assertEqual(TypeA, TypeB),
    ?assertEqual(SubTypeA, SubTypeB),
    ?assert(is_map(ParamsA)),
    ?assert(is_map(ParamsB)),
    maps:fold(
        fun
            (transfer_encoding, _, _) ->
                %never added during decoding
                [];
            (disposition, _, _) when
                not is_binary(BodyA);
                BodyA =:= <<>>
            ->
                [];
            (disposition = K, V, _) when is_binary(BodyA) ->
                %% disposition only applied for non-empty bodies
                case re:replace(BodyA, "\s+", "", [global, {return, binary}]) of
                    <<>> -> [];
                    _ -> ?assertEqual(V, maps:get(K, ParamsB))
                end;
            (K, KVA, _) when
                K =:= content_type_params;
                K =:= disposition_params
            ->
  assert all Content - Type / Disposition from original mime do present in
                %% recoded mime; keys should be lowercased
                KVB = maps:get(K, ParamsB),
                lists:foreach(
                    fun({PKA, PVA}) ->
                        ?assert(lists:member({binstr:to_lower(PKA), PVA}, KVB))
                    end,
                    KVA
                );
            (K, V, _) ->
                ?assertEqual(V, maps:get(K, ParamsB))
        end,
        [],
        ParamsA
    ),
    %% XXX: we have to strip values of the body and headers, because it seems some types of
    %% encoding do remove some of whitespaces from payload. Not sure if it's ok...
    lists:foreach(
        fun({K, VA}) ->
            VB = proplists:get_value(K, HeadersB),
            ?assertEqual(
                string:trim(VA, both, " "),
                string:trim(VB, both, " "),
                #{
                    header => K,
                    b_headers => HeadersB
                }
            )
        end,
        HeadersA
    ),
    case is_binary(BodyA) of
        true ->
            ?assertEqual(BodyA, BodyB),
            true;
        false ->
            Bodies = lists:zip(BodyA, BodyB),
            lists:all(
                fun({SubBodyA, SubBodyB}) ->
                    match(SubBodyA, SubBodyB)
                end,
                Bodies
            )
    end.

prop_quoted_printable(doc) ->
    "Make sure quoted-printable encoder works as expected: "
    "* No lines longer than 76 chars "
    "* decode(encode(data)) returns the same result as original input".

prop_quoted_printable() ->
    ?FORALL(
        Body,
        proper_types:oneof([
            ?SIZED(Size, printable_ascii(Size * 50)),
            ?SIZED(Size, printable_ascii_and_cariage(Size * 50)),
            printable_ascii(),
            printable_ascii_and_cariage(),
            nonull_utf8(),
            proper_types:binary()
        ]),
        begin
            [QPEncoded] = mimemail:encode_quoted_printable(Body),
            ?assertEqual(Body, mimemail:decode_quoted_printable(QPEncoded)),
            ?assertNot(has_lines_over(QPEncoded, 76), #{encoded => QPEncoded, orig => Body}),
            true
        end
    ).

prop_smtp_compatible(doc) ->
    "Makes sure mimemail never produces output that is not compatible with SMTP, "
    "See  and :"
    "* Should not contain bare '\r' or '\n' (ie, $\r or $\n in any other form than '\r\n' pair). "
    "* Should not contain ASCII codes above 127"
    "* Should not contain 0 byte"
    "* Should not have too long (over 1000 chars) lines".

prop_smtp_compatible() ->
    ?FORALL(
        Mail,
        proper_types:oneof([gen_multipart_mail(), gen_plaintext_mail()]),
        begin
            SevenByte = mimemail:encode(Mail),
            ?assertNot(has_bare_cr_or_lf(SevenByte), SevenByte),
            ?assertNot(has_bytes_above_127(SevenByte), SevenByte),
            ?assertNot(has_zero_byte(SevenByte), SevenByte),
            ?assertNot(has_lines_over(SevenByte, 1000), SevenByte),
            true
        end
    ).

has_bare_cr_or_lf(Mime) ->
    WithoutCRLF = binary:replace(Mime, <<"\r\n">>, <<"">>, [global]),
    case binary:match(WithoutCRLF, [<<"\r">>, <<"\n">>]) of
        nomatch -> false;
        {_, _} -> true
    end.

has_bytes_above_127(<<C, _/binary>>) when C > 127 ->
    true;
has_bytes_above_127(<<_, Tail/binary>>) ->
    has_bytes_above_127(Tail);
has_bytes_above_127(<<>>) ->
    false.

has_zero_byte(Mime) ->
    case binary:match(Mime, <<0>>) of
        nomatch -> false;
        {match, _} -> true
    end.

has_lines_over(Mime, Limit) ->
    lists:any(
        fun(Line) ->
            byte_size(Line) > Limit
        end,
        binary:split(Mime, <<"\r\n">>, [global])
    ).

strip_mime_version(MimeBin) ->
    binary:replace(MimeBin, <<"MIME-Version: 1.0\r\n">>, <<>>).
%% re:replace(MimeBin, "mime-version: 1\\.0\\s*", "", [caseless, {return, binary}]).

%%
%% Generators
%%

  top - level   ( )
gen_multipart_mail() ->
    {<<"multipart">>, proper_types:oneof([<<"mixed">>, <<"alternative">>]), gen_top_headers(), gen_props(outer),
        %% Resizing to not create too many sub-bodies, because it's slow
        ?SIZED(
            Size,
            proper_types:resize(
                max(1, Size div 2),
                proper_types:list(
                    proper_types:oneof(
                        [
                            gen_embedded_plaintext_mail(),
                            gen_embedded_html_mail(),
                            gen_embedded_attachment_mail()
                        ]
                    )
                )
            )
        )}.

  top - level plaintext  ( )
gen_plaintext_mail() ->
    {<<"text">>, <<"plain">>, gen_top_headers(), gen_props(outer),
        proper_types:oneof([gen_body(), gen_nonempty_body()])}.

  Plaintext mimemail ( ) , that is safe to use inside multipart mails
gen_embedded_plaintext_mail() ->
    {<<"text">>, <<"plain">>, gen_headers(), gen_props(embedded), gen_nonempty_body()}.

%% Pseudo-HTML mimemail(), that is safe to use inside multipart mails
gen_embedded_html_mail() ->
    {<<"text">>, <<"html">>, gen_headers(),
        #{
            content_type_params => [{<<"charset">>, <<"utf-8">>}],
            disposition => <<"inline">>
        },
        ?LET(
            Body,
            gen_body(),
            <<"<!doctype html><html><body><p>", Body/binary, "</p></body></html>">>
        )}.

gen_embedded_attachment_mail() ->
    {<<"application">>, <<"pdf">>, gen_headers(), gen_attachment_props(),
        proper_types:non_empty(proper_types:binary())}.

%% like gen_headers/0, but `From' is always there
gen_top_headers() ->
    ?LET(KV, gen_headers(), lists:ukeysort(1, [{<<"From">>, <<"">>} | KV])).

%% [{binary(), binary()}]
gen_headers() ->
    AddrHeaders = [<<"To">>, <<"Cc">>, <<"Bcc">>, <<"Reply-To">>, <<"From">>],
    ContentHeaders = [
        <<"Content-Type">>, <<"Content-Disposition">>, <<"Content-Transfer-Encoding">>
    ],
    SpecialHeaders = AddrHeaders ++ ContentHeaders,
    ?LET(
        KV,
        proper_types:list(
            proper_types:frequency(
                [
                    {5,
                        ?SUCHTHAT(
                            {K, _},
                            gen_any_header(),
                            not lists:member(K, SpecialHeaders)
                        )},
                    {1, {proper_types:oneof(AddrHeaders), <<"">>}}
                ]
            )
        ),
        lists:ukeysort(1, KV)
    ).

%% This can generate invalid header when it requires some specific format
gen_any_header() ->
    {
        header_name(),
        proper_types:oneof(
            [
                nonull_utf8(),
                printable_ascii_and_cariage(),
                printable_ascii()
            ]
        )
    }.

%% #{atom() => any()}
gen_props(Location) ->
    Disposition =
        case Location of
            outer -> [];
            embedded -> [{disposition, proper_types:oneof([<<"inline">>, <<"attachment">>])}]
        end,
    ?LET(
        KV,
        proper_types:list(
            proper_types:oneof(
                Disposition ++
                    [
                        {content_type_params, [{<<"charset">>, <<"utf-8">>}]},
                        {transfer_encoding, proper_types:oneof([<<"base64">>, <<"quoted-printable">>])}
                    ]
            )
        ),
        maps:from_list(KV)
    ).

gen_attachment_props() ->
    ?LET(
        KV,
        proper_types:list(
            proper_types:oneof(
                [
                    {content_type_params, gen_params()},
                    {disposition_params, gen_params()}
                ]
            )
        ),
        maps:from_list([
            {disposition, <<"attachment">>},
            {transfer_encoding, <<"base64">>}
            | KV
        ])
    ).

gen_params() ->
    proper_types:list(
        {
            header_name(),
            header_name()
        }
    ).

%% binary(), guaranteed to be not `<<>>'. Also, try to generate relatively large body
gen_nonempty_body() ->
    proper_types:oneof(
        [
            proper_types:non_empty(?SIZED(Size, printable_ascii(Size * 30))),
            proper_types:non_empty(?SIZED(Size, printable_ascii_and_cariage(Size * 30))),
            proper_types:non_empty(nonull_utf8())
        ]
    ).

%% binary()
gen_body() ->
    proper_types:oneof(
        [
            printable_ascii(),
            printable_ascii_and_cariage(),
            nonull_utf8()
        ]
    ).

%% `[0-9a-zA-Z_-]*'
header_name() ->
  let 's limit header names to 20 characters . Too long header names can easily create very long lines
    ?LET(
        OrigHdr,
        proper_types:non_empty(
            binary_of(
                "-_" ++
                    lists:seq($0, $9) ++
                    lists:seq($A, $Z) ++
                    lists:seq($a, $z)
            )
        ),
        case OrigHdr of
            <<Max20:20/binary, _/binary>> -> Max20;
            _ -> OrigHdr
        end
    ).

printable_ascii_and_cariage() ->
    ?SIZED(Size, printable_ascii_and_cariage(Size)).

printable_ascii_and_cariage(Size) ->
    binary_of("\t\r\n" ++ lists:seq(32, 126), Size).

printable_ascii() ->
    ?SIZED(Size, printable_ascii(Size)).

printable_ascii(Size) ->
    binary_of(lists:seq(32, 126), Size).

binary_of(Bytes) ->
    ?SIZED(Size, binary_of(Bytes, Size)).

binary_of(Bytes, Size) ->
    ?LET(
        List,
        proper_types:resize(Size, proper_types:list(proper_types:oneof(Bytes))),
        list_to_binary(List)
    ).

  any utf-8 , except 0
nonull_utf8() ->
    ?SUCHTHAT(
        Chars,
        proper_unicode:utf8(),
        case Chars of
            <<>> ->
                true;
            _ ->
                binary:match(Chars, <<0>>) =:= nomatch
        end
    ).

 @doc property-based tests for `mimemail' module

 * We may truncate leading and trailing whitespaces " " from header values
 * For binary payload it's highly recommended to set `#{transfer_encoding => <<"base64">>}' explicitly
never added during decoding
 disposition only applied for non-empty bodies
 recoded mime; keys should be lowercased
 XXX: we have to strip values of the body and headers, because it seems some types of
 encoding do remove some of whitespaces from payload. Not sure if it's ok...
 re:replace(MimeBin, "mime-version: 1\\.0\\s*", "", [caseless, {return, binary}]).

 Generators

 Resizing to not create too many sub-bodies, because it's slow
 Pseudo-HTML mimemail(), that is safe to use inside multipart mails
 like gen_headers/0, but `From' is always there
 [{binary(), binary()}]
 This can generate invalid header when it requires some specific format
 #{atom() => any()}
 binary(), guaranteed to be not `<<>>'. Also, try to generate relatively large body
 binary()
 `[0-9a-zA-Z_-]*'
  Following limitations of mimemail are discovered and modelled in this suite :
  * We may truncate trailing tabs and whitespaces from payload when Content - Transfer - Encoding is not base64
-module(prop_mimemail).

-export([
    prop_plaintext_encode_no_crash/1,
    prop_multipart_encode_no_crash/1,
    prop_plaintext_encode_decode_match/1,
    prop_multipart_encode_decode_match/1,
    prop_encode_decode_no_mime_version_match/1,
    prop_quoted_printable/1,
    prop_smtp_compatible/1
]).

-include_lib("proper/include/proper.hrl").
-include_lib("stdlib/include/assert.hrl").

prop_plaintext_encode_no_crash(doc) ->
    "Check that any plaintext mail can be encoded without crash".

prop_plaintext_encode_no_crash() ->
    ?FORALL(
        Mail,
        gen_plaintext_mail(),
        is_binary(mimemail:encode(Mail))
    ).

prop_multipart_encode_no_crash(doc) ->
    "Check that any multipart mail can be encoded without crash".

prop_multipart_encode_no_crash() ->
    ?FORALL(
        Mail,
        gen_multipart_mail(),
        is_binary(mimemail:encode(Mail))
    ).

prop_plaintext_encode_decode_match(doc) ->
    "Check that any plaintext mail can be encoded and decoded without"
    " information loss or corruption".

prop_plaintext_encode_decode_match() ->
    ?FORALL(
        Mail,
        gen_plaintext_mail(),
        begin
            Encoded = mimemail:encode(Mail),
            Recoded = mimemail:decode(Encoded),
            ?WHENFAIL(
                io:format(
                    "Orig:~n~p~nEncoded:~n~p~nRecoded:~n~p~n",
                    [Mail, Encoded, Recoded]
                ),
                match(Mail, Recoded)
            )
        end
    ).

prop_multipart_encode_decode_match(doc) ->
    "Check that any plaintext mail can be encoded and decoded without"
    " information loss or corruption".

prop_multipart_encode_decode_match() ->
    ?FORALL(
        Mail,
        gen_multipart_mail(),
        begin
            Encoded = mimemail:encode(Mail),
            Recoded = mimemail:decode(Encoded),
            ?WHENFAIL(
                io:format(
                    "Orig:~n~p~nEncoded:~n~p~nRecoded:~n~p~n",
                    [Mail, Encoded, Recoded]
                ),
                match(Mail, Recoded)
            )
        end
    ).

prop_encode_decode_no_mime_version_match(doc) ->
    "Make sure decoder is able to recover from situation when 'mime-version' header is missing".

prop_encode_decode_no_mime_version_match() ->
    ?FORALL(
        Mail,
        proper_types:oneof([gen_plaintext_mail(), gen_multipart_mail()]),
        begin
            Encoded = mimemail:encode(Mail),
            Recoded = mimemail:decode(
                strip_mime_version(Encoded),
                [
                    {allow_missing_version, true},
                    {encoding, <<"utf-8">>}
                ]
            ),
            ?WHENFAIL(
                io:format(
                    "Orig:~n~p~nEncoded:~n~p~nRecoded:~n~p~n",
                    [Mail, Encoded, Recoded]
                ),
                match(Mail, Recoded)
            )
        end
    ).

match(
    {TypeA, SubTypeA, HeadersA, ParamsA, BodyA},
    {TypeB, SubTypeB, HeadersB, ParamsB, BodyB}
) ->
    ?assertEqual(TypeA, TypeB),
    ?assertEqual(SubTypeA, SubTypeB),
    ?assert(is_map(ParamsA)),
    ?assert(is_map(ParamsB)),
    maps:fold(
        fun
            (transfer_encoding, _, _) ->
                [];
            (disposition, _, _) when
                not is_binary(BodyA);
                BodyA =:= <<>>
            ->
                [];
            (disposition = K, V, _) when is_binary(BodyA) ->
                case re:replace(BodyA, "\s+", "", [global, {return, binary}]) of
                    <<>> -> [];
                    _ -> ?assertEqual(V, maps:get(K, ParamsB))
                end;
            (K, KVA, _) when
                K =:= content_type_params;
                K =:= disposition_params
            ->
  assert all Content - Type / Disposition from original mime do present in
                KVB = maps:get(K, ParamsB),
                lists:foreach(
                    fun({PKA, PVA}) ->
                        ?assert(lists:member({binstr:to_lower(PKA), PVA}, KVB))
                    end,
                    KVA
                );
            (K, V, _) ->
                ?assertEqual(V, maps:get(K, ParamsB))
        end,
        [],
        ParamsA
    ),
    lists:foreach(
        fun({K, VA}) ->
            VB = proplists:get_value(K, HeadersB),
            ?assertEqual(
                string:trim(VA, both, " "),
                string:trim(VB, both, " "),
                #{
                    header => K,
                    b_headers => HeadersB
                }
            )
        end,
        HeadersA
    ),
    case is_binary(BodyA) of
        true ->
            ?assertEqual(BodyA, BodyB),
            true;
        false ->
            Bodies = lists:zip(BodyA, BodyB),
            lists:all(
                fun({SubBodyA, SubBodyB}) ->
                    match(SubBodyA, SubBodyB)
                end,
                Bodies
            )
    end.

prop_quoted_printable(doc) ->
    "Make sure quoted-printable encoder works as expected: "
    "* No lines longer than 76 chars "
    "* decode(encode(data)) returns the same result as original input".

prop_quoted_printable() ->
    ?FORALL(
        Body,
        proper_types:oneof([
            ?SIZED(Size, printable_ascii(Size * 50)),
            ?SIZED(Size, printable_ascii_and_cariage(Size * 50)),
            printable_ascii(),
            printable_ascii_and_cariage(),
            nonull_utf8(),
            proper_types:binary()
        ]),
        begin
            [QPEncoded] = mimemail:encode_quoted_printable(Body),
            ?assertEqual(Body, mimemail:decode_quoted_printable(QPEncoded)),
            ?assertNot(has_lines_over(QPEncoded, 76), #{encoded => QPEncoded, orig => Body}),
            true
        end
    ).

prop_smtp_compatible(doc) ->
    "Makes sure mimemail never produces output that is not compatible with SMTP, "
    "See  and :"
    "* Should not contain bare '\r' or '\n' (ie, $\r or $\n in any other form than '\r\n' pair). "
    "* Should not contain ASCII codes above 127"
    "* Should not contain 0 byte"
    "* Should not have too long (over 1000 chars) lines".

prop_smtp_compatible() ->
    ?FORALL(
        Mail,
        proper_types:oneof([gen_multipart_mail(), gen_plaintext_mail()]),
        begin
            SevenByte = mimemail:encode(Mail),
            ?assertNot(has_bare_cr_or_lf(SevenByte), SevenByte),
            ?assertNot(has_bytes_above_127(SevenByte), SevenByte),
            ?assertNot(has_zero_byte(SevenByte), SevenByte),
            ?assertNot(has_lines_over(SevenByte, 1000), SevenByte),
            true
        end
    ).

has_bare_cr_or_lf(Mime) ->
    WithoutCRLF = binary:replace(Mime, <<"\r\n">>, <<"">>, [global]),
    case binary:match(WithoutCRLF, [<<"\r">>, <<"\n">>]) of
        nomatch -> false;
        {_, _} -> true
    end.

has_bytes_above_127(<<C, _/binary>>) when C > 127 ->
    true;
has_bytes_above_127(<<_, Tail/binary>>) ->
    has_bytes_above_127(Tail);
has_bytes_above_127(<<>>) ->
    false.

has_zero_byte(Mime) ->
    case binary:match(Mime, <<0>>) of
        nomatch -> false;
        {match, _} -> true
    end.

has_lines_over(Mime, Limit) ->
    lists:any(
        fun(Line) ->
            byte_size(Line) > Limit
        end,
        binary:split(Mime, <<"\r\n">>, [global])
    ).

strip_mime_version(MimeBin) ->
    binary:replace(MimeBin, <<"MIME-Version: 1.0\r\n">>, <<>>).


  top - level   ( )
gen_multipart_mail() ->
    {<<"multipart">>, proper_types:oneof([<<"mixed">>, <<"alternative">>]), gen_top_headers(), gen_props(outer),
        ?SIZED(
            Size,
            proper_types:resize(
                max(1, Size div 2),
                proper_types:list(
                    proper_types:oneof(
                        [
                            gen_embedded_plaintext_mail(),
                            gen_embedded_html_mail(),
                            gen_embedded_attachment_mail()
                        ]
                    )
                )
            )
        )}.

  top - level plaintext  ( )
gen_plaintext_mail() ->
    {<<"text">>, <<"plain">>, gen_top_headers(), gen_props(outer),
        proper_types:oneof([gen_body(), gen_nonempty_body()])}.

  Plaintext mimemail ( ) , that is safe to use inside multipart mails
gen_embedded_plaintext_mail() ->
    {<<"text">>, <<"plain">>, gen_headers(), gen_props(embedded), gen_nonempty_body()}.

gen_embedded_html_mail() ->
    {<<"text">>, <<"html">>, gen_headers(),
        #{
            content_type_params => [{<<"charset">>, <<"utf-8">>}],
            disposition => <<"inline">>
        },
        ?LET(
            Body,
            gen_body(),
            <<"<!doctype html><html><body><p>", Body/binary, "</p></body></html>">>
        )}.

gen_embedded_attachment_mail() ->
    {<<"application">>, <<"pdf">>, gen_headers(), gen_attachment_props(),
        proper_types:non_empty(proper_types:binary())}.

gen_top_headers() ->
    ?LET(KV, gen_headers(), lists:ukeysort(1, [{<<"From">>, <<"">>} | KV])).

gen_headers() ->
    AddrHeaders = [<<"To">>, <<"Cc">>, <<"Bcc">>, <<"Reply-To">>, <<"From">>],
    ContentHeaders = [
        <<"Content-Type">>, <<"Content-Disposition">>, <<"Content-Transfer-Encoding">>
    ],
    SpecialHeaders = AddrHeaders ++ ContentHeaders,
    ?LET(
        KV,
        proper_types:list(
            proper_types:frequency(
                [
                    {5,
                        ?SUCHTHAT(
                            {K, _},
                            gen_any_header(),
                            not lists:member(K, SpecialHeaders)
                        )},
                    {1, {proper_types:oneof(AddrHeaders), <<"">>}}
                ]
            )
        ),
        lists:ukeysort(1, KV)
    ).

gen_any_header() ->
    {
        header_name(),
        proper_types:oneof(
            [
                nonull_utf8(),
                printable_ascii_and_cariage(),
                printable_ascii()
            ]
        )
    }.

gen_props(Location) ->
    Disposition =
        case Location of
            outer -> [];
            embedded -> [{disposition, proper_types:oneof([<<"inline">>, <<"attachment">>])}]
        end,
    ?LET(
        KV,
        proper_types:list(
            proper_types:oneof(
                Disposition ++
                    [
                        {content_type_params, [{<<"charset">>, <<"utf-8">>}]},
                        {transfer_encoding, proper_types:oneof([<<"base64">>, <<"quoted-printable">>])}
                    ]
            )
        ),
        maps:from_list(KV)
    ).

gen_attachment_props() ->
    ?LET(
        KV,
        proper_types:list(
            proper_types:oneof(
                [
                    {content_type_params, gen_params()},
                    {disposition_params, gen_params()}
                ]
            )
        ),
        maps:from_list([
            {disposition, <<"attachment">>},
            {transfer_encoding, <<"base64">>}
            | KV
        ])
    ).

gen_params() ->
    proper_types:list(
        {
            header_name(),
            header_name()
        }
    ).

gen_nonempty_body() ->
    proper_types:oneof(
        [
            proper_types:non_empty(?SIZED(Size, printable_ascii(Size * 30))),
            proper_types:non_empty(?SIZED(Size, printable_ascii_and_cariage(Size * 30))),
            proper_types:non_empty(nonull_utf8())
        ]
    ).

gen_body() ->
    proper_types:oneof(
        [
            printable_ascii(),
            printable_ascii_and_cariage(),
            nonull_utf8()
        ]
    ).

header_name() ->
  let 's limit header names to 20 characters . Too long header names can easily create very long lines
    ?LET(
        OrigHdr,
        proper_types:non_empty(
            binary_of(
                "-_" ++
                    lists:seq($0, $9) ++
                    lists:seq($A, $Z) ++
                    lists:seq($a, $z)
            )
        ),
        case OrigHdr of
            <<Max20:20/binary, _/binary>> -> Max20;
            _ -> OrigHdr
        end
    ).

printable_ascii_and_cariage() ->
    ?SIZED(Size, printable_ascii_and_cariage(Size)).

printable_ascii_and_cariage(Size) ->
    binary_of("\t\r\n" ++ lists:seq(32, 126), Size).

printable_ascii() ->
    ?SIZED(Size, printable_ascii(Size)).

printable_ascii(Size) ->
    binary_of(lists:seq(32, 126), Size).

binary_of(Bytes) ->
    ?SIZED(Size, binary_of(Bytes, Size)).

binary_of(Bytes, Size) ->
    ?LET(
        List,
        proper_types:resize(Size, proper_types:list(proper_types:oneof(Bytes))),
        list_to_binary(List)
    ).

  any utf-8 , except 0
nonull_utf8() ->
    ?SUCHTHAT(
        Chars,
        proper_unicode:utf8(),
        case Chars of
            <<>> ->
                true;
            _ ->
                binary:match(Chars, <<0>>) =:= nomatch
        end
    ).
