求助，这个程序为什么停不下来？

a=int(input("请输入四位不完全相同的整数："))
d=[]

def fjsz(n):
      i=0
      del d[:]
      while i<4:
            
            
            c=int((n[i]))
            i+=1
            d.append(c)
      
      return(d)
def xzpx(x):
      
      x=x.sort(reverse=True)
      
      return(x)

def qds():
      
      global dashu
      dashu=''
      i=0
      while i<4:
            dashu=str(dashu)+str((d[i]))
            i+=1
            dashu=int(dashu)
      return(dashu)
def qxs():
      global xiaoshu
      xiaoshu=''
      i=3
      while i>=0:
            xiaoshu=str(xiaoshu)+str((d[i]))
            i-=1
            xiaoshu=int(xiaoshu)
      return(xiaoshu)
def szhd6174(c):
      print(c)
      while c!=6174:
            print(c)
            fjsz(str(c))
            xzpx(d)
            print(d)
            qds()
            print(dashu)
            qxs()
            print(xiaoshu)
            z=dashu-xiaoshu
            print(z)
            print('-----')
            szhd6174(z)
szhd6174(a)