River crossing puzzle

thugs_suitcase.py

import itertools as it

NTHUGS = 3
NCASES = 2
BOATCAP = 3

thugs = set()
cases = set()
owner = dict()
nresults = 0

for i in range(NTHUGS):
  t = "thug"+str(i)
  thugs.add(t)
  for j in range(NCASES):
    c = "case"+str(i)+str(j)
    cases.add(c)
    owner[c] = t

def is_valid(config):
  cs = cases & config
  ts = thugs & config
  if not ts:
    return True
  else:
    for c in cs:
      if owner[c] not in ts:
        return False
    return True
  
def is_end(config):
  return len(config) == NTHUGS*(1 + NCASES)

def print_result(steps):
  global nresults
  nresults +=1
  print("\nResult {}".format(nresults))
  print("----------------")
  for s in steps:
    print(s)

def solve_further(configs_seen,steps,boat_side,off_side,where):
  if where=="right" and is_end(boat_side):
    print_result(steps)
    return
  nxt = "left" if where=="right" else "right"
  for n in range(1,BOATCAP+1):
    for p in it.combinations(boat_side,n):
      passengers = set(p)
      
      # Must have at least one thug in the boat
      if not (passengers & thugs):
        continue
      
      off_side2 = boat_side - passengers
      boat_side2 = off_side | passengers
      if not (is_valid(boat_side2) and is_valid(off_side2)):
        continue
      nconfig = (boat_side2,off_side2)

      # Do not repeat configurations
      if nconfig in configs_seen:
        continue

      solve_further(configs_seen+[nconfig],steps+[passengers],
                    boat_side2,off_side2,nxt)

start = thugs|cases
solve_further([],[],start,set(),"left")