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A.4.9 String Hashing

Static Semantics

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{20302AI95−00302−03} 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(Hash);

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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).

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Implementation defined: The values returned by Strings.Hash.

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{20302AI95−00302−03} 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;
     pragma Pure(Hash);

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{20302AI95−00302−03} 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(Hash);

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

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{20302AI95−00302−03} 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(Hash);

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

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{20302AI95−00302−03} 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.

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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.

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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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{20302AI95−00302−03} {extensions to Ada 95} The Strings.Hash, Strings.Fixed.Hash, Strings.Bounded.Hash, and Strings.Unbounded.Hash functions are new.