A.4.9 String Hashing
Static Semantics
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The library function Strings.Hash has the following declaration:
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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).
Implementation defined: The values returned
by Strings.Hash.
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The library function Strings.Fixed.Hash has the following declaration:
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with Ada.Containers, Ada.Strings.Hash;
function Ada.Strings.Fixed.Hash (Key : String)
return Containers.Hash_Type
renames Ada.Strings.Hash;
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The generic library function Strings.Bounded.Hash has the following declaration:
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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);
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Equivalent to Strings.Hash (Bounded.To_String (Key));
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The library function Strings.Unbounded.Hash has the following declaration:
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with Ada.Containers;
function Ada.Strings.Unbounded.Hash (Key : Unbounded_String)
return Containers.Hash_Type;
pragma Preelaborate(Ada.Strings.Unbounded.Hash);
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Equivalent to Strings.Hash (To_String (Key));
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).
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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;
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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));
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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
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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.
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.
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
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The Strings.Hash, Strings.Fixed.Hash, Strings.Bounded.Hash,
and Strings.Unbounded.Hash functions are new.
Extensions to Ada 2005
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The Strings.Hash_Case_Insensitive, Strings.Fixed.Hash_Case_Insensitive,
Strings.Bounded.Hash_Case_Insensitive, and Strings.Unbounded.Hash_Case_Insensitive
functions are new.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
