!YS-v0

defn main(n=nil):
  if n:
    say: next-highest-int-from-digits(n)
    test:

defn next-highest-int-from-digits(n):
  ds =: n:digits
  i =:
    sub ds.#.--:
      count:
        ? for [i range(ds.#.-- 0 -1)
              :while (ds.nth(i.--) >= ds.$i)]
        : 1
  if i > 0:
    then:
      a c =: ds.split-at(i)
      a b =: a.split-at(a.#.--)
      c d =: c:sort.split-with(\(b.0 >= _))
      d e =: d.split-at(1)
      e =: sort(b + c + e)
      read-string: concat(a d e).map(str):join
    else: 0

# Test code:
defn test():
  tests =::
    0:        0
    9:        0
    12:       21
    21:       0
    12453:    12534
    738440:   740348
    45072010: 45072100
    95322020: 95322200
    9589776899767587796600: 9589776899767587900667

  reduce-kv _ nil tests:
    fn(_ k v):
      n =: next-highest-int-from-digits(k)
      if n == v:
        say: "PASS - $k"
        say: "FAIL - $n != $v"