Hamming.hpp

#ifndef __HAMMING_HPP__
#define __HAMMING_HPP__

#include <bitset>
#include <string>
#include <assert.h>


#include <iostream>

/**
 * \brief Holds data for the Hamming_10_6_3 class. See next.
 */
class Hamming_10_6_3_data
{
public:
    std::bitset<10> g0;
    std::bitset<10> g1;
    std::bitset<10> g2;
    std::bitset<10> g3;
    std::bitset<10> g4;
    std::bitset<10> g5;

    std::bitset<10> h0;
    std::bitset<10> h1;
    std::bitset<10> h2;
    std::bitset<10> h3;

    int bad_bit_table[16];

    Hamming_10_6_3_data()
    {
        // G matrix come from the APCO 25 reference documentation:
        g0 = std::bitset<10>(std::string("1000001110"));
        g1 = std::bitset<10>(std::string("0100001101"));
        g2 = std::bitset<10>(std::string("0010001011"));
        g3 = std::bitset<10>(std::string("0001000111"));
        g4 = std::bitset<10>(std::string("0000100011"));
        g5 = std::bitset<10>(std::string("0000011100"));

        // H matrix, calculated transposing G. See wikipedia.
        h0 = std::bitset<10>(std::string("1110011000"));
        h1 = std::bitset<10>(std::string("1101010100"));
        h2 = std::bitset<10>(std::string("1011100010"));
        h3 = std::bitset<10>(std::string("0111100001"));

        // bad_bit_table indicates which is the bit to correct from the calculated syndromes.
        // It's calculated from the previous matrices: See the left-most column of the matrix whose rows are
        // h0,h1,h2 and h3. It's [1 1 1 0]. It means: when the syndrome value is the binary number 1110, the
        // bit that is wrong is the bit 9 (the left-most column is for the bit 9). Another example with
        // the right-most column: it's column is [0 0 0 1], so when the syndrome is 0001, the bit to fix is
        // bit 0 (one of the parity bits).
        bad_bit_table[ 1] = 0;
        bad_bit_table[ 2] = 1;
        bad_bit_table[ 4] = 2;
        bad_bit_table[ 8] = 3;
        bad_bit_table[12] = 4;
        bad_bit_table[ 3] = 5;
        bad_bit_table[ 7] = 6;
        bad_bit_table[11] = 7;
        bad_bit_table[13] = 8;
        bad_bit_table[14] = 9;

        // If the syndrome is zero, then the data is correct, no errors detected.
        bad_bit_table[ 0] = -2;

        // Incorrect values, if the syndrome gets one of this values, then we fail to correct the data.
        bad_bit_table[ 5] = -1;
        bad_bit_table[ 6] = -1;
        bad_bit_table[ 9] = -1;
        bad_bit_table[10] = -1;
        bad_bit_table[15] = -1;
    }
};

/**
 * An interface used by the two implementations defined here.
 */
class Hamming_Inteface
{
public:
    virtual ~Hamming_Inteface()
    {
        // Does nothing
    }

    /**
     * \brief Decodes using the Hamming (10,6,3) algorithm a sequence of 10 bits expressed as an integer.
     * \arg input The number to decode.
     * \arg output A pointer to int where the decoded result is stored.
     * \return Count of detected errors.
     */
    virtual int decode(int input, int* output) = 0;

    /**
     * \brief Decodes using the Hamming (10,6,3) algorithm a sequence of 10 bits expressed as a pointer
     * to six chars (data) and a pointer to four chars (parity). Altogether ten chars, each holding one bit.
     * \arg hex A pointer to six chars. If error are detected the values are modified acordingly.
     * \arg parity A pointer to four chars. Represent the parity of the data.
     * \return Count of detected errors.
     */
    virtual int decode(char* hex, char* parity) = 0;
};

/**
 * \brief Hamming (10,6,3) error correction implementation.
 */
class Hamming_10_6_3 : public Hamming_Inteface
{
private:
    static Hamming_10_6_3_data data;

public:
    /**
     * \brief Decodes using the Hamming (10,6,3) algorithm a sequence of 10 bits expressed as a std::bitset.
     * \arg input The sequence to decode.
     * \return Count of detected errors.
     */
    int decode(std::bitset<10>& input);

    int decode(int input, int* output)
    {
        assert (input < 1024 && input >= 0);

        std::bitset<10> bitset_input(input);
        int error_count = decode(bitset_input);
        if (error_count == 1) {
            // t has possibly been modified
            input = (int)bitset_input.to_ulong();
        } else {
            // both in case that there were no errors or there were irrecoverable errors we leave the data as
            // it is
        }

        // discard the four parity bits at the end
        *output = input >> 4;

        return error_count;
    }

    int decode(char* hex, char* parity)
    {
        // Make a bitset from hex and parity
        std::bitset<10> value;
        // in the bitset 9 is the left-most and 0 is the right-most
        for (unsigned int i=0; i<6; i++) {
            value[9-i] = hex[i];
        }
        for (unsigned int i=0; i<4; i++) {
            value[3-i] = parity[i];
        }

        int error_count = decode(value);

        // Modify hex if needed
        if (error_count == 1) {
            for (unsigned int i=0; i<6; i++) {
                hex[i] = value[9-i];
            }
        } else {
            // No errors or irrecoverable errors, in both cases don't touch the input
        }

        return error_count;
    }
};

/**
 * \brief Stores data for the Hamming_10_6_3_TableImpl that comes next.
 */
class Hamming_10_6_3_TableImpl_data
{
public:
    int fixed_values[1024];
    int error_counts[1024];

    /**
     * \brief On initialization it builds a table to calculate every possible outcome (there are 1024) before
     * hand. Uses Hamming_10_6_3 to build the table.
     */
    Hamming_10_6_3_TableImpl_data()
    {
        Hamming_10_6_3 hamming;

        // Build the tables
        for (int i=0; i<1024; i++) {
            int fixed;
            int error_count = hamming.decode(i, &fixed);
            fixed_values[i] = fixed;
            error_counts[i] = error_count;
        }
    }
};

/**
 * \brief Hamming (10,6,3) error correction implementation.
 */
class Hamming_10_6_3_TableImpl : public Hamming_Inteface
{
private:
    static Hamming_10_6_3_TableImpl_data data;

public:
    int decode(int input, int* output);

    static int convert_hex_to_int(char* hex)
    {
        // Make an int from hex
        int value = 0;
        // in the bitset 9 is the left-most and 0 is the right-most
        for (unsigned int i=0; i<6; i++) {
            value <<= 1;
            value |= hex[i];
        }

        return value;
    }

    static int convert_hex_parity_to_int(char* hex, char* parity)
    {
        // Make an int from hex and parity
        int value = 0;
        // in the bitset 9 is the left-most and 0 is the right-most
        for (unsigned int i=0; i<6; i++) {
            value <<= 1;
            value |= hex[i];
        }
        for (unsigned int i=0; i<4; i++) {
            value <<= 1;
            value |= parity[i];
        }

        return value;
    }

    static void convert_int_to_hex(int value, char* hex)
    {
        unsigned int v = value;
        for (unsigned int i=0; i<6; i++) {
            hex[5-i] = v & 1;
            v >>= 1;
        }
    }

    int decode(char* hex, char* parity)
    {
        int value = convert_hex_parity_to_int(hex, parity);
        int fixed;
        int error_count = decode(value, &fixed);

        // Modify hex if needed
        if (error_count == 1) {
            convert_int_to_hex(fixed, hex);
        } else {
            // No errors or irrecoverable errors, in both cases don't touch the input
        }

        return error_count;
    }
};


#endif // __HAMMING_HPP__