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A.18.5 The Generic Package Containers.Hashed_Maps

Static Semantics

1/2
{AI95-00302-03} The generic library package Containers.Hashed_Maps has the following declaration: 
2/3
{AI05-0084-1} {AI05-0212-1} with Ada.Iterator_Interfaces;
generic
   type Key_Type is private;
   type Element_Type is private;
   with function Hash (Key : Key_Type) return Hash_Type;
   with function Equivalent_Keys (Left, Right : Key_Type)
      return Boolean;
   with function "=" (Left, Right : Element_Type)
      return Boolean is <>;
package Ada.Containers.Hashed_Maps is
   pragma Preelaborate(Hashed_Maps);
   pragma Remote_Types(Hashed_Maps);
3/3
{AI05-0212-1}    type Map is tagged private
      with Constant_Indexing => Constant_Reference,
           Variable_Indexing => Reference,
           Default_Iterator  => Iterate,
           Iterator_Element  => Element_Type;
   pragma Preelaborable_Initialization(Map);
4/2
   type Cursor is private;
   pragma Preelaborable_Initialization(Cursor);
5/2
   Empty_Map : constant Map;
6/2
   No_Element : constant Cursor;
6.1/3
{AI05-0212-1}    function Has_Element (Position : Cursor) return Boolean;
6.2/3
{AI05-0212-1}    package Map_Iterator_Interfaces is new
       Ada.Iterator_Interfaces (Cursor, Has_Element);
7/2
   function "=" (Left, Right : Map) return Boolean;
8/2
   function Capacity (Container : Map) return Count_Type;
9/2
   procedure Reserve_Capacity (Container : in out Map;
                               Capacity  : in     Count_Type);
10/2
   function Length (Container : Map) return Count_Type;
11/2
   function Is_Empty (Container : Map) return Boolean;
12/2
   procedure Clear (Container : in out Map);
13/2
   function Key (Position : Cursor) return Key_Type;
14/2
   function Element (Position : Cursor) return Element_Type;
15/2
   procedure Replace_Element (Container : in out Map;
                              Position  : in     Cursor;
                              New_Item  : in     Element_Type);
16/2
   procedure Query_Element
     (Position : in Cursor;
      Process  : not null access procedure (Key     : in Key_Type;
                                            Element : in Element_Type));
17/2
   procedure Update_Element
     (Container : in out Map;
      Position  : in     Cursor;
      Process   : not null access procedure
                      (Key     : in     Key_Type;
                       Element : in out Element_Type));
17.1/3
{AI05-0212-1}    type Constant_Reference_Type
         (Element : not null access constant Element_Type) is private
      with Implicit_Dereference => Element;
17.2/3
{AI05-0212-1}    type Reference_Type (Element : not null access Element_Type) is private
      with Implicit_Dereference => Element;
17.3/3
{AI05-0212-1}    function Constant_Reference (Container : aliased in Map;
                                Position  : in Cursor)
      return Constant_Reference_Type;
17.4/3
{AI05-0212-1}    function Reference (Container : aliased in out Map;
                       Position  : in Cursor)
      return Reference_Type;
17.5/3
{AI05-0212-1}    function Constant_Reference (Container : aliased in Map;
                                Key       : in Key_Type)
      return Constant_Reference_Type;
17.6/3
{AI05-0212-1}    function Reference (Container : aliased in out Map;
                       Key       : in Key_Type)
      return Reference_Type;
17.7/3
{AI05-0001-1}    procedure Assign (Target : in out Map; Source : in Map);
17.8/3
{AI05-0001-1}    function Copy (Source : Map; Capacity : Count_Type := 0) return Map;
18/2
   procedure Move (Target : in out Map;
                   Source : in out Map);
19/2
   procedure Insert (Container : in out Map;
                     Key       : in     Key_Type;
                     New_Item  : in     Element_Type;
                     Position  :    out Cursor;
                     Inserted  :    out Boolean);
20/2
   procedure Insert (Container : in out Map;
                     Key       : in     Key_Type;
                     Position  :    out Cursor;
                     Inserted  :    out Boolean);
21/2
   procedure Insert (Container : in out Map;
                     Key       : in     Key_Type;
                     New_Item  : in     Element_Type);
22/2
   procedure Include (Container : in out Map;
                      Key       : in     Key_Type;
                      New_Item  : in     Element_Type);
23/2
   procedure Replace (Container : in out Map;
                      Key       : in     Key_Type;
                      New_Item  : in     Element_Type);
24/2
   procedure Exclude (Container : in out Map;
                      Key       : in     Key_Type);
25/2
   procedure Delete (Container : in out Map;
                     Key       : in     Key_Type);
26/2
   procedure Delete (Container : in out Map;
                     Position  : in out Cursor);
27/2
   function First (Container : Map)
      return Cursor;
28/2
   function Next (Position  : Cursor) return Cursor;
29/2
   procedure Next (Position  : in out Cursor);
30/2
   function Find (Container : Map;
                  Key       : Key_Type)
      return Cursor;
31/2
   function Element (Container : Map;
                     Key       : Key_Type)
      return Element_Type;
32/2
   function Contains (Container : Map;
                      Key       : Key_Type) return Boolean;
33/3
This paragraph was deleted.{AI05-0212-1}
34/2
   function Equivalent_Keys (Left, Right : Cursor)
      return Boolean;
35/2
   function Equivalent_Keys (Left  : Cursor;
                             Right : Key_Type)
      return Boolean;
36/2
   function Equivalent_Keys (Left  : Key_Type;
                             Right : Cursor)
      return Boolean;
37/2
   procedure Iterate
     (Container : in Map;
      Process   : not null access procedure (Position : in Cursor));
37.1/3
{AI05-0212-1}    function Iterate (Container : in Map)
      return Map_Iterator_Interfaces.Forward_Iterator'Class;
38/2
private
39/2
   ... -- not specified by the language
40/2
end Ada.Containers.Hashed_Maps;
41/2
 {AI95-00302-03} An object of type Map contains an expandable hash table, which is used to provide direct access to nodes. The capacity of an object of type Map is the maximum number of nodes that can be inserted into the hash table prior to it being automatically expanded.
41.a/2
Implementation Note: The expected implementation for a Map uses a hash table which is grown when it is too small, with linked lists hanging off of each bucket. Note that in that implementation a cursor needs a back pointer to the Map object to implement iteration; that could either be in the nodes, or in the cursor object. To provide an average O(1) access time, capacity would typically equal the number of buckets in such an implementation, so that the average bucket linked list length would be no more than 1.0.
41.b/2
There is no defined relationship between elements in a hashed map. Typically, iteration will return elements in the order that they are hashed in. 
42/2
 {AI95-00302-03} Two keys K1 and K2 are defined to be equivalent if Equivalent_Keys (K1, K2) returns True.
43/2
 {AI95-00302-03} The actual function for the generic formal function Hash is expected to return the same value each time it is called with a particular key value. For any two equivalent key values, the actual for Hash is expected to return the same value. If the actual for Hash behaves in some other manner, the behavior of this package is unspecified. Which subprograms of this package call Hash, and how many times they call it, is unspecified.
43.a/2
Implementation Note: The implementation is not required to protect against Hash raising an exception, or returning random numbers, or any other “bad” behavior. It's not practical to do so, and a broken Hash function makes the container unusable.
43.b/2
The implementation can call Hash whenever it is needed; we don't want to specify how often that happens. The result must remain the same (this is logically a pure function), or the behavior is unspecified. 
44/2
 {AI95-00302-03} The actual function for the generic formal function Equivalent_Keys on Key_Type values is expected to return the same value each time it is called with a particular pair of key values. It should define an equivalence relationship, that is, be reflexive, symmetric, and transitive. If the actual for Equivalent_Keys behaves in some other manner, the behavior of this package is unspecified. Which subprograms of this package call Equivalent_Keys, and how many times they call it, is unspecified.
44.a/2
Implementation Note: As with Hash, the implementation is not required to protect against Equivalent_Keys raising an exception or returning random results. Similarly, the implementation can call this operation whenever it is needed. The result must remain the same (this is a logically pure function), or the behavior is unspecified.
45/2
 {AI95-00302-03} If the value of a key stored in a node of a map is changed other than by an operation in this package such that at least one of Hash or Equivalent_Keys give different results, the behavior of this package is unspecified.
45.a/2
Implementation Note: The implementation is not required to protect against changes to key values other than via the operations declared in the Hashed_Maps package.
45.b/2
To see how this could happen, imagine an instance of Hashed_Maps where the key type is an access-to-variable type and Hash returns a value derived from the components of the designated object. Then, any operation that has a key value could modify those components and change the hash value:
45.c/2
Key (Map).Some_Component := New_Value;
45.d/2
This is really a design error on the part of the user of the map; it shouldn't be possible to modify keys stored in a map. But we can't prevent this error anymore than we can prevent someone passing as Hash a random number generator. 
46/2
 {AI95-00302-03} Which nodes are the first node and the last node of a map, and which node is the successor of a given node, are unspecified, other than the general semantics described in A.18.4.
46.a/2
Implementation Note: Typically the first node will be the first node in the first bucket, the last node will be the last node in the last bucket, and the successor will be obtained by following the collision list, and going to the next bucket at the end of each bucket. 
47/2
function Capacity (Container : Map) return Count_Type;
48/2
{AI95-00302-03} Returns the capacity of Container.
49/2
procedure Reserve_Capacity (Container : in out Map;
                            Capacity  : in     Count_Type);
50/2
{AI95-00302-03} Reserve_Capacity allocates a new hash table such that the length of the resulting map can become at least the value Capacity without requiring an additional call to Reserve_Capacity, and is large enough to hold the current length of Container. Reserve_Capacity then rehashes the nodes in Container onto the new hash table. It replaces the old hash table with the new hash table, and then deallocates the old hash table. Any exception raised during allocation is propagated and Container is not modified.
51/2
Reserve_Capacity tampers with the cursors of Container.
51.a/2
Implementation Note: This routine is used to preallocate the internal hash table to the specified capacity such that future Inserts do not require expansion of the hash table. Therefore, the implementation should allocate the needed memory to make that true at this point, even though the visible semantics could be preserved by waiting until enough elements are inserted.
51.b/3
{AI05-0005-1} While Reserve_Capacity can be used to reduce the capacity of a map, we do not specify whether an implementation actually supports reduction of the capacity. Since the actual capacity can be anything greater than or equal to Capacity, an implementation never has to reduce the capacity.
51.c/2
Reserve_Capacity tampers with the cursors, as rehashing probably will change the order that elements are stored in the map. 
52/2
procedure Clear (Container : in out Map);
53/2
{AI95-00302-03} In addition to the semantics described in A.18.4, Clear does not affect the capacity of Container.
53.1/3
procedure Assign (Target : in out Map; Source : in Map);
53.2/3
{AI05-0001-1} {AI05-0248-1} In addition to the semantics described in A.18.4, if the length of Source is greater than the capacity of Target, Reserve_Capacity (Target, Length (Source)) is called before assigning any elements.
53.3/3
function Copy (Source : Map; Capacity : Count_Type := 0) return Map;
53.4/3
{AI05-0001-1} Returns a map whose keys and elements are initialized from the keys and elements of Source. If Capacity is 0, then the map capacity is the length of Source; if Capacity is equal to or greater than the length of Source, the map capacity is at least the specified value. Otherwise, the operation propagates Capacity_Error.
53.a/2
Implementation Note: In:
53.b/2
procedure Move (Target : in out Map;
                Source : in out Map);
53.c/2
The intended implementation is that the internal hash table of Target is first deallocated; then the internal hash table is removed from Source and moved to Target. 
54/2
procedure Insert (Container : in out Map;
                  Key       : in     Key_Type;
                  New_Item  : in     Element_Type;
                  Position  :    out Cursor;
                  Inserted  :    out Boolean);
55/2
{AI95-00302-03} In addition to the semantics described in A.18.4, if Length (Container) equals Capacity (Container), then Insert first calls Reserve_Capacity to increase the capacity of Container to some larger value.
55.a/2
Implementation Note: Insert should only compare keys that hash to the same bucket in the hash table.
55.b/2
We specify when Reserve_Capacity is called to bound the overhead of capacity expansion operations (which are potentially expensive). Moreover, expansion can be predicted by comparing Capacity(Map) to Length(Map). Since we don't specify by how much the hash table is expanded, this only can be used to predict the next expansion, not later ones.
55.c/2
Implementation Note: In:
55.d/2
procedure Exclude (Container : in out Map;
                   Key       : in     Key_Type);
55.e/2
Exclude should only compare keys that hash to the same bucket in the hash table.
55.f/2
Implementation Note: In:
55.g/2
procedure Delete (Container : in out Map;
                  Key       : in     Key_Type);
55.h/2
Delete should only compare keys that hash to the same bucket in the hash table. The node containing the element may be deallocated now, or it may be saved and reused later.
55.i/2
Implementation Note: In:
55.j/2
function First (Container : Map) return Cursor;
55.k/2
In a typical implementation, this will be the first node in the lowest numbered hash bucket that contains a node.
55.l/2
Implementation Note: In:
55.m/2
function Next (Position  : Cursor) return Cursor;
55.n/2
In a typical implementation, this will return the next node in a bucket; if Position is the last node in a bucket, this will return the first node in the next nonempty bucket.
55.o/2
A typical implementation will need to a keep a pointer at the map container in the cursor in order to implement this function. 
55.p/2
Implementation Note: In:
55.q/2
function Find (Container : Map;
               Key       : Key_Type) return Cursor;
55.r/2
Find should only compare keys that hash to the same bucket in the hash table.
56/2
function Equivalent_Keys (Left, Right : Cursor)
      return Boolean;
57/2
{AI95-00302-03} Equivalent to Equivalent_Keys (Key (Left), Key (Right)).
58/2
function Equivalent_Keys (Left  : Cursor;
                          Right : Key_Type) return Boolean;
59/2
{AI95-00302-03} Equivalent to Equivalent_Keys (Key (Left), Right).
60/2
function Equivalent_Keys (Left  : Key_Type;
                          Right : Cursor) return Boolean;
61/2
{AI95-00302-03} Equivalent to Equivalent_Keys (Left, Key (Right)).
61.1/3
function Iterate (Container : in Map)
   return Map_Iterator_Interfaces.Forward_Iterator'Class;
61.2/3
{AI05-0212-1} {AI05-0265-1} {AI05-0269-1} Iterate returns an iterator object (see 5.5.1) that will generate a value for a loop parameter (see 5.5.2) designating each node in Container, starting with the first node and moving the cursor according to the successor relation. Tampering with the cursors of Container is prohibited while the iterator object exists (in particular, in the sequence_of_statements of the loop_statement whose iterator_specification denotes this object). The iterator object needs finalization.

Implementation Advice

62/2
 {AI95-00302-03} If N is the length of a map, the average time complexity of the subprograms Element, Insert, Include, Replace, Delete, Exclude and Find that take a key parameter should be O(log N). The average time complexity of the subprograms that take a cursor parameter should be O(1). The average time complexity of Reserve_Capacity should be O(N). 
62.a/2
Implementation Advice: The average time complexity of Element, Insert, Include, Replace, Delete, Exclude and Find operations that take a key parameter for Containers.Hashed_Maps should be O(log N). The average time complexity of the subprograms of Containers.Hashed_Maps that take a cursor parameter should be O(1). The average time complexity of Containers.Hashed_Maps.Reserve_Capacity should be O(N).
62.b/2
Reason: We do not mean to overly constrain implementation strategies here. However, it is important for portability that the performance of large containers has roughly the same factors on different implementations. If a program is moved to an implementation for which Find is O(N), that program could be unusable when the maps are large. We allow O(log N) access because the proportionality constant and caching effects are likely to be larger than the log factor, and we don't want to discourage innovative implementations.

Extensions to Ada 95

62.c/2
{AI95-00302-03} The generic package Containers.Hashed_Maps is new.

Incompatibilities With Ada 2005

62.d/3
{AI05-0001-1} Subprograms Assign and Copy are added to Containers.Hashed_Maps. If an instance of Containers.Hashed_Maps is referenced in a use_clause, and an entity E with the same defining_identifier as a new entity in Containers.Hashed_Maps is defined in a package that is also referenced in a use_clause, the entity E may no longer be use-visible, resulting in errors. This should be rare and is easily fixed if it does occur. 

Extensions to Ada 2005

62.e/3
{AI05-0212-1} Added iterator and indexing support to make hashed map containers more convenient to use. 

Wording Changes from Ada 2005

62.f/3
{AI05-0084-1} Correction: Added a pragma Remote_Types so that containers can be used in distributed programs. 

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