String Hashing
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

Ada Reference Manual — Legal Information

# 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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