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Description

I am trying to insert compressed data into an sql table, then extract and decompress it. At first I compress a string then insert it into a table (section 1). Compression works fine. Even When I try to decompress it before insertion I get the string back. The problem comes after inserting the string and trying to extract it back later on.

As you can see in section 2 of the code. The decompression doesn't work anymore. I don't know why it doesn't work anymore after I extract it from the table.

Source code

#include <zlc/zlibcomplete.hpp>
#include <string>
#include "3rdparty/sqlite_modern_cpp.h"

using namespace std;
using namespace zlibcomplete;

int main()
{
   //SECTION 1
   database db("data.db");
   db << "CREATE TABLE IF NOT EXISTS paragraph(ID INTEGER PRIMARY KEY, LINE TEXT);";
   string line = "hello world!";
   //Compress string by using zlib
   GZipCompressor compressor(9, auto_flush);
   string compressed_line = compressor.compress(line);
   compressor.finish();
   
   //IF I DECOMPRESS STRING HERE IT WORKS FINE!!!
   GZipDecompressor decompressor;
   string output = decompressor.decompress(line);
  
   //insert compressed string into table paragraph
   db <<"insert into paragraph(ID,LINE) VALUES (?,?);"
      << 1
      << compressed_line;


  //SECTION 2: Problem starts here where I try to return the compressed value
   db <<"select line from paragraph where id = ?;"
      << "1"
      >>[&](string line)
      {
        //Decompress string by using zlib
        GZipDecompressor decompressor;
        string output = decompressor.decompress(line);
        //out is empty I don't know why???
        cout << output  << endl;
      }; 
   return 0;
}

Update after correction

First download base64 classes as pointed out in this solution.

#include <zlc/zlibcomplete.hpp>
#include <string>
#include "3rdparty/sqlite_modern_cpp.h"
#include "base64.h"

using namespace std;
using namespace zlibcomplete;

int main()
{
   //SECTION 1
   database db("data.db");
   db << "CREATE TABLE IF NOT EXISTS paragraph(ID INTEGER PRIMARY KEY, LINE TEXT);";
   string line = "hello world!";
   //Compress string by using zlib
   GZipCompressor compressor(9, auto_flush);
   string compressed_line = compressor.compress(line);
   compressor.finish();
  std::string compressed = base64_encode(reinterpret_cast<const unsigned char*>(compressed_line.c_str()), compressed_line.length());
  
   //insert compressed string into table paragraph
   db <<"insert into paragraph(ID,LINE) VALUES (?,?);"
      << 1
      << compressed;


   db <<"select line from paragraph where id = ?;"
      << "1"
      >>[&](string line)
      {
        //decode HERE
        std::string compressed_decoded = base64_decode(line);
        //Decompress string by using zlib
        GZipDecompressor decompressor;
        string output = decompressor.decompress(line);
        cout << output  << endl;
      }; 
   return 0;
}
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Hani Goc
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1 Answers1

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Your code is unable to handle zeros in the compressed string. If the compressed string has a character 0 then text gets truncated and is no longer possible to make the reverse operation.

marom
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  • I don't really understand. should I remove the 0s from the string? is it on the compression level? – Hani Goc Sep 04 '15 at 13:39
  • is it similar to this problem http://stackoverflow.com/questions/27529570/simple-zlib-c-string-compression-and-decompression – Hani Goc Sep 04 '15 at 13:41
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    The point is that is the converted string has zeros you cannot remove them. You can either encode the string with Base64 (so you are sure that text has valid chars) or use some 'escape sequence' , so encode a single '\0' char with two non-null chars – marom Sep 04 '15 at 13:43
  • yeah trying to solve it using the source code from http://stackoverflow.com/questions/27529570/simple-zlib-c-string-compression-and-decompression Still have the last part lolololol – Hani Goc Sep 04 '15 at 14:05