%
% issn.erl
% Routines for ISSN numbers.
%
% Written by Yegor Samusev <yegor@samusev.pp.ru>.
%
% This source code is in the public domain.
%
-module(issn).
-export([is/1, calculate_check_digit/1, get_check_digit/1, is_valid/1]).

%
% > issn:is("0131-1417").
% true
%
is(ISSN) ->
    case length(ISSN) of
        9 -> check(ISSN, 1);
        _ -> false
    end.

check([], _) ->
    true;
check([Separator|S], 5) ->
    case Separator of
        $- -> check(S, 6);
        _ -> false
    end;
check([Char|_], 9) ->
    if
        Char == $X -> true;
        Char >= $0, Char =< $9 -> true;
        true -> false
    end;
check([Digit|S], N) ->
    if
        Digit >= $0, Digit =< $9 -> check(S, N + 1);
        true -> false
    end.

%
% > issn:calculate_check_digit("0131-1417").
% 55
%
calculate_check_digit(ISSN) ->
    Chars = lists:sublist(ISSN, 4) ++ lists:sublist(lists:nthtail(5, ISSN), 3),
    Digits = lists:map(fun(X) -> X - 48 end, Chars),
    calculate_check_digit(Digits, 8, []).

calculate_check_digit([], 1, Calculated) ->
    Remainder = lists:sum(Calculated) rem 11,
    case Remainder of
        0 -> $0;
        _ ->
            Digit = 11 - Remainder,
            case Digit of
                10 -> $X;
                _ -> Digit + 48
            end
    end;
calculate_check_digit([Digit|Digits], I, Calculated) ->
    calculate_check_digit(Digits, I - 1, Calculated ++ [Digit * I]).

%
% > issn:get_check_digit("0131-1417").
% 55
%
get_check_digit(ISSN) ->
    lists:nth(9, ISSN).

%
% > issn:is_valid("0131-1417").
% true
%
is_valid(ISSN) ->
    IsISSN = is(ISSN),
    [CheckDigit, Valid] = [get_check_digit(ISSN), calculate_check_digit(ISSN)],
    if
        IsISSN, CheckDigit == Valid -> true;
        true -> false
    end.