#include <stddef.h>
#include <stdint.h>

uint16_t hamming812(uint8_t byte) {
    return (__builtin_parity(byte & 0b11011010) << 15) |  // 16
           (__builtin_parity(byte & 0b10110110) << 14) |  // 15
           ((byte & 0b10000000) << 6) |                   // 14
           (__builtin_parity(byte & 0b01110001) << 12) |  // 13
           ((byte & 0b01110000) << 5) |                   // 12 - 10
           (__builtin_parity(byte & 0b00001111) << 8) |   // 9
           (byte & 0b00001111) << 4;                      // 8-5
}

void encode(void *data, void *encoded, size_t n) {
    for (size_t i = 0; i < n; ++i) {
        uint8_t byte = ((uint8_t *)data)[i];
        ((uint16_t *)encoded)[i] = hamming812(byte);
    }
}

uint8_t hamming_decode(uint16_t enc) {
    int err1 = __builtin_parity(enc & 0b1010101010100000);
    int err2 = __builtin_parity(enc & 0b0110011001100000);
    int err4 = __builtin_parity(enc & 0b0001111000010000);
    int err8 = __builtin_parity(enc & 0b0000000111110000);
    if (!(err1 | err2 | err4 | err8)) {
        return ((enc & 0b0010000000000000) >> 6) |
               ((enc & 0b0000111000000000) >> 5) |
               ((enc & 0b0000000011110000) >> 4);
    }
    int ind = err1 + (err2 << 1) + (err4 << 2) + (err8 << 3);
    uint16_t fixed_enc = enc ^ (1 << (16 - ind));
    return ((fixed_enc & 0b0010000000000000) >> 6) |
           ((fixed_enc & 0b0000111000000000) >> 5) |
           ((fixed_enc & 0b0000000011110000) >> 4);
}

void decode(void *encoded, void *data, size_t n) {
    for (size_t i = 0; i < n; ++i) {
        ((uint8_t *)data)[i] = hamming_decode(((uint16_t *)encoded)[i]);
    }
}