# syntax: GAWK -f DESCENDING_PRIMES.AWK
BEGIN {
    start = 1
    stop = 99999999
    for (i=start; i<=stop; i++) {
      leng = length(i)
      flag = 1
      for (j=1; j<leng; j++) {
        if (substr(i,j,1) <= substr(i,j+1,1)) {
          flag = 0
          break
        }
      }
      if (flag) {
        if (is_prime(i)) {
          printf("%9d%1s",i,++count%10?"":"\n")
        }
      }
    }
    printf("\n%d-%d: %d descending primes\n",start,stop,count)
    exit(0)
}
function is_prime(n,  d) {
    d = 5
    if (n < 2) { return(0) }
    if (n % 2 == 0) { return(n == 2) }
    if (n % 3 == 0) { return(n == 3) }
    while (d*d <= n) {
      if (n % d == 0) { return(0) }
      d += 2
      if (n % d == 0) { return(0) }
      d += 4
    }
    return(1)
}