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A.4.9 String Hashing

Static Semantics

1/2
The library function Strings.Hash has the following declaration: 
2/3
with Ada.Containers;
function Ada.Strings.Hash (Key : String) return Containers.Hash_Type;
pragma Pure(Ada.Strings.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). 
4/2
The library function Strings.Fixed.Hash has the following declaration: 
5/3
with Ada.Containers, Ada.Strings.Hash;
function Ada.Strings.Fixed.Hash (Key : String) return Containers.Hash_Type
   renames Ada.Strings.Hash;
6/2
The generic library function Strings.Bounded.Hash has the following declaration: 
7/3
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(Ada.Strings.Bounded.Hash);
8/3
Equivalent to Strings.Hash (Bounded.To_String (Key));
9/2
The library function Strings.Unbounded.Hash has the following declaration: 
10/3
with Ada.Containers;
function Ada.Strings.Unbounded.Hash (Key : Unbounded_String)
   return Containers.Hash_Type;
pragma Preelaborate(Ada.Strings.Unbounded.Hash);
11/3
Equivalent to Strings.Hash (To_String (Key));
11.1/3
  The library function Strings.Hash_Case_Insensitive has the following declaration:
11.2/3
with Ada.Containers;
function Ada.Strings.Hash_Case_Insensitive (Key : String)
   return Containers.Hash_Type;
pragma Pure(Ada.Strings.Hash_Case_Insensitive);
11.3/3
Returns an implementation-defined value which is a function of the value of Key, converted to lower case. If A and B are strings such that Strings.Equal_Case_Insensitive (A, B) (see A.4.10) is True, then Hash_Case_Insensitive(A) equals Hash_Case_Insensitive(B). 
11.4/3
  The library function Strings.Fixed.Hash_Case_Insensitive has the following declaration:
11.5/3
with Ada.Containers, Ada.Strings.Hash_Case_Insensitive;
function Ada.Strings.Fixed.Hash_Case_Insensitive (Key : String)
   return Containers.Hash_Type renames Ada.Strings.Hash_Case_Insensitive;
11.6/3
  The generic library function Strings.Bounded.Hash_Case_Insensitive has the following declaration:
11.7/3
with Ada.Containers;
generic
   with package Bounded is
      new Ada.Strings.Bounded.Generic_Bounded_Length (<>);
function Ada.Strings.Bounded.Hash_Case_Insensitive
   (Key : Bounded.Bounded_String) return Containers.Hash_Type;
pragma Preelaborate(Ada.Strings.Bounded.Hash_Case_Insensitive);
11.8/3
Equivalent to Strings.Hash_Case_Insensitive (Bounded.To_String (Key)); 
11.9/3
  The library function Strings.Unbounded.Hash_Case_Insensitive has the following declaration:
11.10/3
with Ada.Containers;
function Ada.Strings.Unbounded.Hash_Case_Insensitive
   (Key : Unbounded_String) return Containers.Hash_Type;
pragma Preelaborate(Ada.Strings.Unbounded.Hash_Case_Insensitive);
11.11/3
Equivalent to Strings.Hash_Case_Insensitive (To_String (Key)); 

Implementation Advice

12/2
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. 
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