Re: ANN: Simple components v 1.3

[Jeffrey Carter <spam@spam.com>]

Mats Weber wrote:

> I think not, because with maps, you have to be able to extract an item 
> given its key, which you cannot do with Generic_Sets as there is no key 
> type. Following is the function that does that in my package:
> 
>    function Search (Key : Key_Type; Within : Bag) return Item_Type;
>       -- Returns the first item in FROM that has a key equal to KEY.
>       -- NONEXISTENT_KEY will be raised if no such item is
>       -- found in the bag.

It should not be necessary for the component to know about the 
difference between keys and values. To my mind, a map is a way of using 
a searchable structure, such as a balanced tree or a skip list. For 
example, using PragmARC.Skip_List_Unbounded, one would define the 
Element type actual as

type Map_Data is record
    Key   : Unbounded_String;
    Value : Data := Null_Data;
end record;

Then the necessary comparison routines:

function "=" (Left, Right : Map_Data) return Boolean is
begin -- "="
    return Left.Key = Right.Key;
end "=";

function "<" (Left, Right : Map_Data) return Boolean is
begin -- "<"
    return Left.Key < Right.Key;
end "<";

After instantiating Skip_List_Unbounded, one can insert and search by key:

Key : constant Unbounded_String :=
    To_Unbound_String ("Your key here");

Thing : Map_Data := (Key, Some_Value);
Map   : Skip_List;
Found : Result;
...
Insert (List => Map, Item => Thing);
Thing := (Key, Null_Data); -- Value component not used in Search
Found := Search (Map, Thing);

if Found.Found then
    -- Do something with Found.Item, type Map_Data, which contains
    -- (Key => Key, Value => Some_Value)
end if;

-- 
Jeff Carter
"I wave my private parts at your aunties."
Monty Python & the Holy Grail
13