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Newsgroups: comp.lang.ada
Date: Wed, 1 Aug 2018 02:08:21 -0700 (PDT)
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Subject: Re: Simple hash or pseudo-random function
From: gautier_niouzes@hotmail.com
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Xref: reader02.eternal-september.org comp.lang.ada:54005
Date: 2018-08-01T02:08:21-07:00
List-Id: <comp.lang.ada>

Just a followup on the last remark: as expected, GNAT's Hash function gives consecutive values for consecutive characters at the end of the string. People may want to append some constant string to get randomness in the hash code.

with Ada.Text_IO; use Ada.Text_IO;
with Ada.Strings.Hash;

procedure Hash_test is
begin
  Put_Line (Ada.Strings.Hash ("Abcd")'Image);
  Put_Line (Ada.Strings.Hash ("Bbcd")'Image);
  Put_Line (Ada.Strings.Hash ("Cbcd")'Image);
  Put_Line ("--");
  Put_Line (Ada.Strings.Hash ("abCd")'Image);
  Put_Line (Ada.Strings.Hash ("abDd")'Image);
  Put_Line (Ada.Strings.Hash ("abEd")'Image);
  Put_Line ("--");
  Put_Line (Ada.Strings.Hash ("abcD")'Image);
  Put_Line (Ada.Strings.Hash ("abcE")'Image);
  Put_Line (Ada.Strings.Hash ("abcF")'Image);
end Hash_test;

 14682274
 795269473
 1575856672
--
 3516536994
 3516602593
 3516668192
--
 3518636130
 3518636131
 3518636132

NB: same behavior for ObjectAda (reverting here to pure Ada 2005 syntax):

with Ada.Text_IO; use Ada.Text_IO;
with Ada.Strings.Hash;
with Ada.Containers; use Ada.Containers;

procedure Hash_test is
begin
  Put_Line (Hash_Type'Image (Ada.Strings.Hash ("Abcd")));
  Put_Line (Hash_Type'Image (Ada.Strings.Hash ("Bbcd")));
  Put_Line (Hash_Type'Image (Ada.Strings.Hash ("Cbcd")));
  Put_Line ("--");                                    
  Put_Line (Hash_Type'Image (Ada.Strings.Hash ("abCd")));
  Put_Line (Hash_Type'Image (Ada.Strings.Hash ("abDd")));
  Put_Line (Hash_Type'Image (Ada.Strings.Hash ("abEd")));
  Put_Line ("--");                                    
  Put_Line (Hash_Type'Image (Ada.Strings.Hash ("abcD")));
  Put_Line (Hash_Type'Image (Ada.Strings.Hash ("abcE")));
  Put_Line (Hash_Type'Image (Ada.Strings.Hash ("abcF")));
end Hash_test;

 803596578
 1584183777
 2364770976
--
 10484002
 10549601
 10615200
--
 12583138
 12583139
 12583140