% Copyright (c) 2025 Onomondo ApS & sysmocom - s.f.m.c. GmbH. All rights reserved.
%
% SPDX-License-Identifier: AGPL-3.0-only
%
% Author: Philipp Maier <pmaier@sysmocom.de> / sysmocom - s.f.m.c. GmbH

-module(utils).
-export([binary_to_hex/1, hex_to_binary/1, join_binary_list/1, integer_to_bytes/1, lpad_binary/3]).

% Converts a single hex digit (e.g. <<"A">>) into its integer representation.
hexstr_digit_to_int(HexDigit) ->
    [HexDigitInt] = HexDigit,
    HexDigitIntCap = HexDigitInt bor 32,
    case HexDigitIntCap of
        48 -> 0;
        49 -> 1;
        50 -> 2;
        51 -> 3;
        52 -> 4;
        53 -> 5;
        54 -> 6;
        55 -> 7;
        56 -> 8;
        57 -> 9;
        97 -> 10;
        98 -> 11;
        99 -> 12;
        100 -> 13;
        101 -> 14;
        102 -> 15
    end.

% Converts a single hex octet (e.g. <<"1A">>) into its integer representation.
hexstr_octet_to_int(HexOctet) ->
    HexDigits = [[X] || <<X:8>> <= HexOctet],
    HexDigitsInt = [hexstr_digit_to_int(X) || X <- HexDigits],
    [HexOctetIntH | T] = HexDigitsInt,
    [HexOctetIntL | _] = T,
    HexOctetIntH * 16 + HexOctetIntL.

% Converts a binary into a printable hex-string (also a binary).
binary_to_hex(Binary) ->
    EncList = [io_lib:format("~2.16.0B", [X]) || <<X:8>> <= Binary],
    list_to_binary(string:join(io_lib:format("~s", [EncList]), "")).

% Converts a printable hex-string (int the form of a binary) to a binary.
hex_to_binary(HexStr) ->
    OctetsStr = [<<X:16>> || <<X:16>> <= HexStr],
    list_to_binary([hexstr_octet_to_int(X) || X <- OctetsStr]).

% Joins (concatenates) a list of binaries into a single large binary
join_binary_list([]) ->
    % An empty list will result into an empty binary
    <<>>;
join_binary_list([Single]) ->
    % A list that contains only of a single binary will result into that single binry.
    Single;
join_binary_list([H | T]) ->
    % Lists with multiple binaries get joined into a single large binary
    lists:foldl(fun(Binary, Joined) -> <<Joined/binary, Binary/binary>> end, H, T);
join_binary_list(Binary) ->
    % In case someone uses this function on a binary directly, we transparently return that binary.
    Binary.

% Convert an integer into a binary that contains the integer as a series of bytes (network byte order)
integer_to_bytes(Integer) ->
    % Convert the integer to a list with hex digits (nibbles)
    IntegerDigits = integer_to_list(Integer, 16),
    % Make sure that the list contains an even number of digits (nibbles)
    IntegerDigitsPadded =
        case length(IntegerDigits) rem 2 of
            1 ->
                ["0" | IntegerDigits];
            _ ->
                IntegerDigits
        end,
    % Merge the list of hex digits into a single hex string (binary) and convert that binary into a binary
    IntegerAsHexstring = list_to_binary(IntegerDigitsPadded),
    hex_to_binary(IntegerAsHexstring).

% Padd a binary from the left with a specified padding byte
lpad_binary(Binary, Padding, Length) ->
    if
        byte_size(Binary) >= Length ->
            Binary;
        true ->
            BinaryPadded = <<Padding/binary, Binary/binary>>,
            lpad_binary(BinaryPadded, Padding, Length)
    end.