https://blog.openacid.com/storage/ec-2/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <getopt.h>
#include "erasure_code.h"   // use <isa-l.h> instead when linking against installed

#define MMAX 255
#define KMAX 255

typedef unsigned char u8;

// 計算指定的校驗塊
void ec_encode_data_spec(int len, int srcs, int dest_i,
                    unsigned char *v,unsigned char **src,
                    unsigned char *dest)
{
    int i, j;
    unsigned char s;

    for (i = 0; i < len; i++) {
        s = 0;
      for (j = 0; j < srcs; j++) {
        s ^= gf_mul(src[j][i], v[j * 32 + dest_i * srcs * 32 + 1]);
        dest[i] = s;
            }
    }
}

// 計算增量校驗塊
void ec_encode_data_delta_spec(int len, int k, int vec_i,
                                            int dest_i, unsigned char *v,
                            unsigned char *data,
                            unsigned char *dest)
{
    int i;
    for (i = 0; i < len; i++) {
       dest[i] = gf_mul(data[i], v[vec_i * 32 + dest_i * k * 32 + 1]);
    }
}

// 計算增量校驗塊并更新
void ec_encode_data_update_spec(int len, int k, int vec_i,
                                            int dest_i, unsigned char *v,
                            unsigned char *data,
                            unsigned char *dest)
{
    int i;
    unsigned char s;

    for (i = 0; i < len; i++) {
       s = dest[i];
       s ^= gf_mul(data[i], v[vec_i * 32 + dest_i * k * 32 + 1]);
       dest[i] = s;
    }
}

int main(int argc, char *argv[])
{
    u8 *frag_ptrs[MMAX];
    u8 *encode_matrix;
    u8 *g_tbls;
  int k = 4,p =2;
    int m = k + p;
  int len =5;
    encode_matrix = malloc(m * k);
    g_tbls = malloc(k * p * 32);

    for (int i = 0; i < m; i++) {
        if (NULL == (frag_ptrs[i] = malloc(len))) {
            printf("alloc error: Fail\n");
            return -1;
        }
    }

    for (int i = 0; i < k; i++)
        for (int j = 0; j < len; j++)
            frag_ptrs[i][j] = i+1;

    printf(" encode (m,k,p)=(%d,%d,%d) len=%d\n", m, k, p, len);

    gf_gen_rs_matrix(encode_matrix, m, k);
  for (int i=0;i<m;i++) {
        for (int j=0;j<k;j++) {
            printf("%d ",encode_matrix[i*k+j]);
    }
        printf("\n");
  }
  printf("-------------------\n");
    // Initialize g_tbls from encode matrix
    ec_init_tables(k, p, &encode_matrix[k * k], g_tbls);


  printf("ec encode\n");
  printf("------------------\n");
    // Generate EC parity blocks from sources
    ec_encode_data(len, k, p, g_tbls, frag_ptrs, &frag_ptrs[k]);

  for (int i = 0; i < m; i++) {
    for (int j = 0; j < len; j++)
        printf("%d ",frag_ptrs[i][j]);
    printf("\n");
    }

    u8* old_delta_ec = malloc(p*len);
  for (int i=0;i<p;i++) {
        ec_encode_data_delta_spec(len,k,0,i,g_tbls,frag_ptrs[0],&old_delta_ec[i*len]);
    }
    printf("change data--------------\n");
  for (int i=0;i<len;i++) {
        frag_ptrs[0][i] *= 10;
    }
    for (int i=0;i<k;i++) {
        for (int j=0;j<len;j++) {
            printf("%d ",frag_ptrs[i][j]);
        }
        printf("\n");
    }

    u8* new_delta_ec = malloc(p*len);
  for (int i=0;i<p;i++) {
        ec_encode_data_delta_spec(len,k,0,i,g_tbls,frag_ptrs[0],&new_delta_ec[i*len]);
    }
  printf("encode delta-------------------\n");
  for (int j=0;j<p;j++) {
        for (int i=0;i<len;i++) {
            printf("%d ",frag_ptrs[k+j][i]^old_delta_ec[j*len+i]^new_delta_ec[j*len+i]);
        }
        printf("\n");
    }


  printf("encode update--------------\n");
    for (int i=0;i<p;i++) {
    ec_encode_data_update_spec(len,k,0,i,g_tbls,frag_ptrs[0],frag_ptrs[k+i]);
    }
  for (int i = 0; i < p; i++) {
    for (int j = 0; j < len; j++)
        printf("%d ",frag_ptrs[k+i][j] ^ old_delta_ec[i*len+j]);
    printf("\n");
    }

  printf("re encode-----------\n");
    ec_encode_data(len, k, p, g_tbls, frag_ptrs, &frag_ptrs[k]);

  for (int i = 0; i < m; i++) {
    for (int j = 0; j < len; j++)
        printf("%d ",frag_ptrs[i][j]);
    printf("\n");
    }
}