Annotated Ada Reference Manual — Legal Information

A.4.9 String Hashing

Static Semantics

1/2

{AI95-00302-03} The library function Strings.Hash has the following declaration:

2/2

with Ada.Containers;
function Ada.Strings.Hash (Key : String) return Containers.Hash_Type;
pragma Pure(Hash);

3/2

Returns an implementation-defined value which is a function of the value of Key. If A and B are strings such that A equals B, Hash(A) equals Hash(B).

3.a/2

Implementation defined: The values returned by Strings.Hash.

4/2

{AI95-00302-03} The library function Strings.Fixed.Hash has the following declaration:

5/2

with Ada.Containers, Ada.Strings.Hash;
function Ada.Strings.Fixed.Hash (Key : String) return Containers.Hash_Type
renames Ada.Strings.Hash;
pragma Pure(Hash);

6/2

{AI95-00302-03} The generic library function Strings.Bounded.Hash has the following declaration:

7/2

with Ada.Containers;
generic
   with package Bounded is
                     new Ada.Strings.Bounded.Generic_Bounded_Length (<>);
function Ada.Strings.Bounded.Hash (Key : Bounded.Bounded_String)
   return Containers.Hash_Type;
pragma Preelaborate(Hash);

8/2

Strings.Bounded.Hash is equivalent to the function call Strings.Hash (Bounded.To_String (Key));

9/2

{AI95-00302-03} The library function Strings.Unbounded.Hash has the following declaration:

10/2

with Ada.Containers;
function Ada.Strings.Unbounded.Hash (Key : Unbounded_String)
return Containers.Hash_Type;
pragma Preelaborate(Hash);

11/2

Strings.Unbounded.Hash is equivalent to the function call Strings.Hash (To_String (Key));

Implementation Advice

12/2

{AI95-00302-03} The Hash functions should be good hash functions, returning a wide spread of values for different string values. It should be unlikely for similar strings to return the same value.

12.a/2

Implementation Advice: Strings.Hash should be good a hash function, returning a wide spread of values for different string values, and similar strings should rarely return the same value.

12.b/2

Ramification: The other functions are defined in terms of Strings.Hash, so they don't need separate advice in the Annex.

Extensions to Ada 95

12.c/2

{AI95-00302-03} {extensions to Ada 95} The Strings.Hash, Strings.Fixed.Hash, Strings.Bounded.Hash, and Strings.Unbounded.Hash functions are new.