File: isisbn.c

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/*
**  ISISBN.C - Validate International Standard Book Numbers (ISBNs)
**
**  public domain by Maynard Hogg
*/

#include <ctype.h>

int isbn2(char *str)
{
      int i = 0;
      int test = 0;
      int c;

      while ('\0' != (c = *str++))
      {
            if (isdigit(c))
                  c -= '0';
            else if (i == 9 && 'X' == c)
                  c = 10;
            else continue;
            test += c * ++i;
      }
      return (i == 10 && test % 11 == 0);
}