column.d

// The column struct, with the array
struct Column
{
    State[] states;
    private bool[State] _uniques;

    bool add(ref State state)
    {
        if (state in _uniques)
        {
            return false;
        }

        state.end_column = &this;
        _uniques[state] = true;
        states ~= state;

        return true;
    }
}

// The iteration
foreach (state; col.states)
{
    // ...
    col.add(State(...));
}