how i can generate reverse of this below method(hash2string)-this method generate string2hash value...i have required an output- text as given in input side.

Question

SAURABH 2013-6-10

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/78577-how-i-can-generate-reverse-of-this-below-method-hash2string-this-method-generate-string2hash-value

function hash=string2hash(str,type)
% This function generates a hash value from a text string
%
% hash=string2hash(str,type);
%
% inputs,
%   str : The text string, or array with text strings.
% outputs,
%   hash : The hash value, integer value between 0 and 2^32-1
%   type : Type of has 'djb2' (default) or 'sdbm'
%
% From c-code on : http://www.cse.yorku.ca/~oz/hash.html 
%
% djb2
%  this algorithm was first reported by dan bernstein many years ago 
%  in comp.lang.c
%
% sdbm
%  this algorithm was created for sdbm (a public-domain reimplementation of
%  ndbm) database library. it was found to do well in scrambling bits, 
%  causing better distribution of the keys and fewer splits. it also happens
%  to be a good general hashing function with good distribution.
%
% example,
%
%  hash=string2hash('hello world');
%  disp(hash);
%
% Function is written by D.Kroon University of Twente (June 2010)
% From string to double array
str=double(str);
if(nargin<2), type='djb2'; end
switch(type)
    case 'djb2'
        hash = 5381*ones(size(str,1),1); 
        for i=1:size(str,2), 
            hash = mod(hash * 33 + str(:,i), 2^32-1); 
        end
    case 'sdbm'
        hash = zeros(size(str,1),1);
        for i=1:size(str,2), 
            hash = mod(hash * 65599 + str(:,i), 2^32-1);
        end
    otherwise
        error('string_hash:inputs','unknown type');
end

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SAURABH 2013-6-10

编辑：Walter Roberson 2013-6-10

在 MATLAB Online 中打开

str=double(str);
if(nargin<2), type='djb2'; end
switch(type)
    case 'djb2'
        hash = 5381*ones(size(str,1),1); 
        for i=1:size(str,2), 
            hash = mod(hash * 33 + str(:,i), 2^32-1); 
        end
    case 'sdbm'
        hash = zeros(size(str,1),1);
        for i=1:size(str,2), 
            hash = mod(hash * 65599 + str(:,i), 2^32-1);
        end
    otherwise
        error('string_hash:inputs','unknown type');
end

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Answer 1

Walter Roberson 2013-6-10

0
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/78577-how-i-can-generate-reverse-of-this-below-method-hash2string-this-method-generate-string2hash-value#answer_88316

在 MATLAB Online 中打开

You cannot reverse that hash function. See http://dmytry.blogspot.ca/2009/11/horrible-hashes.html and notice

What is so stupid about it? For starters, even though the output of this function is 32 bits, not even for the 2 char ASCII strings do you have a guarantee for lack of collisions. In fact "SV" collides with "Pt", and "g5" collides with "as" [in the second version that uses xor], just to name two examples out of hundreds.

Therefor if your input string was even only as long as 2 characters, you might well not be able to reverse it because there would be other 2 character input strings that hashed to the same output string.