The following example shows how to perform random read accesses on a compressed datafile.

#include <LEDA/coding/compress.h> // includes all compression classes
#include <LEDA/core/random.h>     // for random access ...

using namespace leda;

using std::cout;
using std::endl;
using std::ostream;
using std::istream;

string db_file_name = "database";
const int db_sz = 1000;
const int record_sz = 20;

typedef BlockCoder<PPMIICoder> Coder;
// Note: random access is also possible without BlockCoder but much slower!

/*
    The code for generating and accessing the database uses standard C++ streams.
    It does not even know that the data is actually compressed during writing and
    decompressed during reading!
*/

void generate_database(std::ostream& db)
{
    string fill; while (fill.length() <= record_sz) fill += " ";
        // used to enlarge the records to the desired length

    for (int i = 0; i < db_sz; ++i) {
        // generate record
        string record = string("record #%d", i);
        record += fill(1, record_sz - record.length());

        // write record into the database
        db << record;
    }
}

string retrieve_record(std::istream& db, int record_num)
{
    // jump to the correct position in the stream
    int pos = record_num * record_sz;
        // Note: pos is the position in the uncompressed data!
    db.seekg(pos);

    // read the record
    char buffer[record_sz+1];
    db.read(buffer, record_sz);

    // add a terminating zero and return it
    buffer[record_sz] = 0;
    return string(buffer);
}

int main()
{
    // build the database
    {
        encoding_ofstream<Coder> db(db_file_name);
        generate_database(db);
        db.close();
        if (db.fail()) cout << "Error writing " << db_file_name << endl;
    }

    // access the database and retrieve some records
    {
        autodecoding_ifstream db(db_file_name);
        random_source rand(0, db_sz);
        for (int i = 0; i < 30; ++i) {
            int record_num = rand();
            cout << record_num << ": " << retrieve_record(db, record_num) << endl;
        }
    }

    return 0;
}