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REM nPr function
FUNCTION nPr(n,r)
   P = 1
   FOR x = n TO n-r STEP -1
      P = P * X
   NEXT
ENDFUNCTION P

REM ! function
FUNCTION factorial(n)
   fac = nPr(n,n)
ENDFUNCTION fac

REM nCr function
FUNCTION nCr(n,r)
   C = nPr(n,r) / factorial(n)
ENDFUNCTION

FUNCTION combinations_of_two_letter_string$(orig$)
   IF LEN(orig$) <> 2 THEN EXITFUNCTION ""
   ret$ = orig$ + " "
   retadd$ = MID$(orig$,2) + MID$(orig$,1)
   ret$ = ret$ + retadd$
ENDFUNCTION ret$

dim letters$[10] (a,b,c,d,e,f,g,h,i,j)
for i=0 to 9
for i2=0 to 9
for i3=0 to 9
print letters$[i], letters$[i2], letters$[i3]
next i
next i2
next i3

dim letters$[10] (a,b,c,d,e,f,g,h,i,j)
for i=0 to 9
for i2=i to 9
for i3=i2 to 9
print letters$[i], letters$[i2], letters$[i3]
next i
next i2
next i3

sync on:sync rate 0
Combinations("abcd")

do

   sync
loop

function Combinations(string$)
`Make array
   dim Letters$(len(string$))
   dim Combinations$(len(string$)^2)
   dim LetterCombOrder(len(string$))

`Set array
   for i=1 to len(string$)
      Letters$(i)=right$(left$(string$,i),1)
   next i

`Set beginning order
   for i=1 to len(string$)
      LetterOrder(i)=i
      LetterCombOrder(i)=i
   next i
   pos=1

`Find combinations
   repeat
      count=count+1
      for i=2 to len(string$)
         valpos=valpos+1
         Record_Combinations(valpos,len(string$))
         message Combinations$(valpos)
         Change_Order(i,len(string$))
      next i
   until count=len(string$)
endfunction

function FindCombPosition(pos,amount)
`Find letter in that position
   for i=1 to amount
      if LetterCombOrder(i)=pos then letter=i
   next
endfunction letter

function Record_Combinations(pos,amount)
   for i=1 to amount
      comb$=comb$+Letters$(FindCombPosition(i,amount))
   next i
   Combinations$(pos)=comb$
endfunction

function Change_Order(pos,amount)
`Change order
   LetterCombOrder(pos)=LetterCombOrder(pos)-1
   if LetterCombOrder(pos)=0 then LetterCombOrder(pos)=amount
endfunction

Do
   Input "Enter a 3 to 4 letter combination: ",combo$
   If len(combo$)>2 and len(combo$)<5 then exit
Loop
n=1
For a = 1 to len(combo$)
   For b = 1 to len(combo$)
      For c = 1 to len(combo$)
         if len(combo$)>3
            For d= 1 to len(combo$)
               Print mid$(combo$,a);
               Print mid$(combo$,b);
               Print mid$(combo$,c);
               Print mid$(combo$,d);
               inc n
               if n>4
                  Print " "
                  n=1
               Else
                  Print " ";
               Endif
            Next d
         Else
            Print mid$(combo$,a);
            Print mid$(combo$,b);
            Print mid$(combo$,c);
            inc n
            if n>3
               Print " "
               n=1
            Else
               Print " ";
            Endif
         Endif
      Next c
   Next b
Next a

Wait key

input "Enter a letter combination: ",string$
Combination(string$)

suspend for key

function Combination(string$)
`Arrays
   letters=len(string$)
   dim Counters(letters)
   dim Counting(letters):Counting(letters)=1

`Set counters
   for i=1 to letters
      Counters(i)=1
   next i

`Combinations
   repeat
      for i=letters to 1 step -1
         TempLetters$=TempLetters$+mid$(string$,Counters(i))
         if (i-1)=0 and Counting(i-1)=1 then done=1
         if Counting(i)=1 then inc Counters(i),1
         if Counters(i)>letters then Counters(i)=1:Counting(i-1)=1
      next i
      print TempLetters$:TempLetters$=""
   until done=1
endfunction