unit_parser

correct_unit(ListOfUnits) :-
    maplist(nafree,ListOfUnits).

expression_to_unit(identifier(_Pos,Name),Out) :-
    unit_alias(Name,AliasDef)
    -> Out = AliasDef
    ; valid_unit_symbol(Name),
    Out = [[0,Name,1]].

expression_to_unit(power_of(_Pos,identifier(Pos,Name),Exponent),Unit) :- !,
    expression_to_unit(identifier(Pos,Name),UnitOfIdentifier),
    unary_minus_exponent(Exponent,NumericExponent),
    power_of_multiply_units(NumericExponent,UnitOfIdentifier,Unit).

expression_to_unit(power_of(_Pos,integer(_Pos2,10),Exponent),Unit) :- !,
    unary_minus_exponent(Exponent,NumericExponent),
    Unit = [[NumericExponent,na,na]].

expression_to_unit(mult_or_cart(_Pos,Arg1,Arg2),Unit) :- !,
    expression_to_unit(Arg1,Unit1), expression_to_unit(Arg2,Unit2),
    multiply_units(Unit1,Unit2,Unit).

expression_to_unit(div(_Pos,Arg1,Arg2),Unit) :- !,
    expression_to_unit(Arg1,Unit1), expression_to_unit(Arg2,Unit2),
    units_domain_division(Unit1,Unit2,Unit).

expression_to_unit(integer(_Pos,10),[[1,na,na]]) :- !.

multiply_units([[X,na,na]],[[0,Unit,X2]],[[X,Unit,X2]]) :- !, Unit \= na, X2 \= na.

multiply_units([[0,Unit,X2]],[[X,na,na]],[[X,Unit,X2]]) :- !, Unit \= na, X2 \= na.

multiply_units(U1,U2,U) :- !, append(U1,U2,U).

nafree([A,B,C]) :- A \= na, B \= na, C \= na.

unary_minus_exponent(unary_minus(_Pos,integer(_Pos2,X)), MX) :- !, MX is -X.

unary_minus_exponent(integer(_Pos,X), X) :- !.

unit_args_to_list(Arg,O) :- atom(Arg), Arg \= [], !,
    atom_codes(Arg,ArgC), % also allow units to be passed as atoms
    unit_args_to_list(ArgC,O).

unit_args_to_list(Arg,O) :-
    catch(parse_formula(Arg,Expression),_Exception, fail),
    expression_to_unit(Expression,O), correct_unit(O), !.

unit_args_to_list(Arg,_) :-
    atom_codes(ArgAtom,Arg),
    add_error(incorrect_unit_definition, 'Incorrect unit definition: ', ArgAtom).

valid_unit_symbol(m).

valid_unit_symbol(kg).

valid_unit_symbol(s).

valid_unit_symbol('A').

valid_unit_symbol('K').

valid_unit_symbol(mol).

valid_unit_symbol(cd).