With the add modifier, you could completely remove the switch statement, so instead of this:

switch(_mod.op_type){
            case 0: array_push(modifiers[0], _mod); break;
            case 1: array_push(modifiers[1], _mod); break;
            case 2: array_push(modifiers[2], _mod); break;
        }

You would simply have this:

array_push(modifiers[_mod.op_type], _mod);

You could probably also write some functions to do the calculations and have them stored in a global array that corresponds to op_type, which would let you shorten the calculations code as well.