Ada 2012 Reference Manual

A.4.9 String Hashing

Static Semantics

The library function Strings.Hash has the following declaration:

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

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).

The library function Strings.Fixed.Hash has the following declaration:

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

The generic library function Strings.Bounded.Hash has the following declaration:

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);

Equivalent to Strings.Hash (Bounded.To_String (Key));

The library function Strings.Unbounded.Hash has the following declaration:

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

Equivalent to Strings.Hash (To_String (Key));

The library function Strings.Hash_Case_Insensitive has the following declaration:

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

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).

The library function Strings.Fixed.Hash_Case_Insensitive has the following declaration:

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;

The generic library function Strings.Bounded.Hash_Case_Insensitive has the following declaration:

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);

Equivalent to Strings.Hash_Case_Insensitive (Bounded.To_String (Key));

The library function Strings.Unbounded.Hash_Case_Insensitive has the following declaration:

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);

Equivalent to Strings.Hash_Case_Insensitive (To_String (Key));

Implementation Advice

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.