Implementation of Verhoeff scheme adapted to aplhanumeric data

ver36.c

/* Group operation in D18 (precomputed) */
static int d18_op[36][36] = {
  { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35},
  { 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,17, 0,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,18},
  { 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,17, 0, 1,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,18,19},
  { 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,17, 0, 1, 2,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,18,19,20},
  { 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,17, 0, 1, 2, 3,22,23,24,25,26,27,28,29,30,31,32,33,34,35,18,19,20,21},
  { 5, 6, 7, 8, 9,10,11,12,13,14,15,16,17, 0, 1, 2, 3, 4,23,24,25,26,27,28,29,30,31,32,33,34,35,18,19,20,21,22},
  { 6, 7, 8, 9,10,11,12,13,14,15,16,17, 0, 1, 2, 3, 4, 5,24,25,26,27,28,29,30,31,32,33,34,35,18,19,20,21,22,23},
  { 7, 8, 9,10,11,12,13,14,15,16,17, 0, 1, 2, 3, 4, 5, 6,25,26,27,28,29,30,31,32,33,34,35,18,19,20,21,22,23,24},
  { 8, 9,10,11,12,13,14,15,16,17, 0, 1, 2, 3, 4, 5, 6, 7,26,27,28,29,30,31,32,33,34,35,18,19,20,21,22,23,24,25},
  { 9,10,11,12,13,14,15,16,17, 0, 1, 2, 3, 4, 5, 6, 7, 8,27,28,29,30,31,32,33,34,35,18,19,20,21,22,23,24,25,26},
  {10,11,12,13,14,15,16,17, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,28,29,30,31,32,33,34,35,18,19,20,21,22,23,24,25,26,27},
  {11,12,13,14,15,16,17, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,29,30,31,32,33,34,35,18,19,20,21,22,23,24,25,26,27,28},
  {12,13,14,15,16,17, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,30,31,32,33,34,35,18,19,20,21,22,23,24,25,26,27,28,29},
  {13,14,15,16,17, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,31,32,33,34,35,18,19,20,21,22,23,24,25,26,27,28,29,30},
  {14,15,16,17, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,32,33,34,35,18,19,20,21,22,23,24,25,26,27,28,29,30,31},
  {15,16,17, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,33,34,35,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32},
  {16,17, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,34,35,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33},
  {17, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,35,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34},
  {18,35,34,33,32,31,30,29,28,27,26,25,24,23,22,21,20,19, 0,17,16,15,14,13,12,11,10, 9, 8, 7, 6, 5, 4, 3, 2, 1},
  {19,18,35,34,33,32,31,30,29,28,27,26,25,24,23,22,21,20, 1, 0,17,16,15,14,13,12,11,10, 9, 8, 7, 6, 5, 4, 3, 2},
  {20,19,18,35,34,33,32,31,30,29,28,27,26,25,24,23,22,21, 2, 1, 0,17,16,15,14,13,12,11,10, 9, 8, 7, 6, 5, 4, 3},
  {21,20,19,18,35,34,33,32,31,30,29,28,27,26,25,24,23,22, 3, 2, 1, 0,17,16,15,14,13,12,11,10, 9, 8, 7, 6, 5, 4},
  {22,21,20,19,18,35,34,33,32,31,30,29,28,27,26,25,24,23, 4, 3, 2, 1, 0,17,16,15,14,13,12,11,10, 9, 8, 7, 6, 5},
  {23,22,21,20,19,18,35,34,33,32,31,30,29,28,27,26,25,24, 5, 4, 3, 2, 1, 0,17,16,15,14,13,12,11,10, 9, 8, 7, 6},
  {24,23,22,21,20,19,18,35,34,33,32,31,30,29,28,27,26,25, 6, 5, 4, 3, 2, 1, 0,17,16,15,14,13,12,11,10, 9, 8, 7},
  {25,24,23,22,21,20,19,18,35,34,33,32,31,30,29,28,27,26, 7, 6, 5, 4, 3, 2, 1, 0,17,16,15,14,13,12,11,10, 9, 8},
  {26,25,24,23,22,21,20,19,18,35,34,33,32,31,30,29,28,27, 8, 7, 6, 5, 4, 3, 2, 1, 0,17,16,15,14,13,12,11,10, 9},
  {27,26,25,24,23,22,21,20,19,18,35,34,33,32,31,30,29,28, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0,17,16,15,14,13,12,11,10},
  {28,27,26,25,24,23,22,21,20,19,18,35,34,33,32,31,30,29,10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0,17,16,15,14,13,12,11},
  {29,28,27,26,25,24,23,22,21,20,19,18,35,34,33,32,31,30,11,10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0,17,16,15,14,13,12},
  {30,29,28,27,26,25,24,23,22,21,20,19,18,35,34,33,32,31,12,11,10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0,17,16,15,14,13},
  {31,30,29,28,27,26,25,24,23,22,21,20,19,18,35,34,33,32,13,12,11,10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0,17,16,15,14},
  {32,31,30,29,28,27,26,25,24,23,22,21,20,19,18,35,34,33,14,13,12,11,10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0,17,16,15},
  {33,32,31,30,29,28,27,26,25,24,23,22,21,20,19,18,35,34,15,14,13,12,11,10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0,17,16},
  {34,33,32,31,30,29,28,27,26,25,24,23,22,21,20,19,18,35,16,15,14,13,12,11,10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0,17},
  {35,34,33,32,31,30,29,28,27,26,25,24,23,22,21,20,19,18,17,16,15,14,13,12,11,10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0}
};

/* Inverse element in D18 */
static int d18_inv[36] = {
   0,17,16,15,14,13,12,11,10, 9, 8, 7, 6, 5, 4, 3, 2, 1,
  18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35
};

/* Permutation */
static int P[] = {
  29,0,32,11,35,20,7,27,2,4,19,28,30,1,5,12,3,9,16,
  22,6,33,8,24,26,21,14,10,34,31,15,25,17,13,23,18
};
/* Apply n-th composition of the permutation */
static int perm(int x, int n) {
  n %= 600;
  while (n--)
    x = P[x];
  return x;
}

/* Translate from alphanumeric ASCII to 0-36 integers */
static int ascii2int(char x) {
  return (x & 0xF0) == 0x30 ? x - '0' : x - 'A' + 10;
}
/* Translate from integers to alphanumeric ASCII characters */
static char ascii[] = {
  '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
  'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
};
static char int2ascii(int x) {
  return ascii[x];
}


/* Compute checksum, correct if zero */
int ver36_check(char *data, int len) {
  int i = 0, c = 0;
  while (len--)
    c = d18_op[c][perm(ascii2int(data[len]),i++)];
  return c;
}
/* Replace last character with correct check digit */
int ver36_insert(char *data, int len) {
  int i, c = 0;
  for (i = 1; i < len; i++)
    c = d18_op[c][perm(ascii2int(data[len-i-1]), i)];
  c = d18_inv[c];
  data[len-1] = int2ascii(c);
  return c;
}



#include <stdio.h>
#include <ctype.h>
#include <string.h>

static int filteralnum(char *str) {
  int i = 0;
  char *p = str;
  while (*p) {
    if (isalnum(*p))
      str[i++] = toupper(*p);
    p++;
  }
  str[i] = '\0';
  return i;
}

int main(int argc, char **argv) {
  for (--argc, ++argv; argc--; argv++) {
    int len = filteralnum(*argv);
    if (ver36_check(*argv, len)) {
      printf("%s  WRONG  ", *argv);
      ver36_insert(*argv, len);
      printf("CORRECT IS %s\n", *argv);
    }
    else
      printf("%s  OK\n", *argv);
  }
  return 0;
}