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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(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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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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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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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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 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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