Source code in the bbc programming language

      REM The names (only used for printing the results):
      DIM Drink$(4), Nation$(4), Colr$(4), Smoke$(4), Animal$(4)
      Drink$()  = "Beer", "Coffee", "Milk", "Tea", "Water"
      Nation$() = "Denmark", "England", "Germany", "Norway", "Sweden"
      Colr$()   = "Blue", "Green", "Red", "White", "Yellow"
      Smoke$()  = "Blend", "BlueMaster", "Dunhill", "PallMall", "Prince"
      Animal$() = "Birds", "Cats", "Dog", "Horse", "Zebra"
      
      REM Some single-character tags:
      a$ = "A" : b$ = "B" : c$ = "C" : d$ = "D" : e$ = "E"
      
      REM BBC BASIC Doesn't have enumerations!
      Beer$=a$    : Coffee$=b$     : Milk$=c$    : Tea$=d$      : Water$=e$
      Denmark$=a$ : England$=b$    : Germany$=c$ : Norway$=d$   : Sweden$=e$
      Blue$=a$    : Green$=b$      : Red$=c$     : White$=d$    : Yellow$=e$
      Blend$=a$   : BlueMaster$=b$ : Dunhill$=c$ : PallMall$=d$ : Prince$=e$
      Birds$=a$   : Cats$=b$       : Dog$=c$     : Horse$=d$    : Zebra$=e$
      
      REM Create the 120 permutations of 5 objects:
      DIM perm$(120), x$(4) : x$() = a$, b$, c$, d$, e$
      REPEAT
        p% += 1
        perm$(p%) = x$(0)+x$(1)+x$(2)+x$(3)+x$(4)
      UNTIL NOT FNperm(x$())
      
      REM Express the statements as conditional expressions:
      ex2$ = "INSTR(Nation$,England$) = INSTR(Colr$,Red$)"
      ex3$ = "INSTR(Nation$,Sweden$) = INSTR(Animal$,Dog$)"
      ex4$ = "INSTR(Nation$,Denmark$) = INSTR(Drink$,Tea$)"
      ex5$ = "INSTR(Colr$,Green$+White$) <> 0"
      ex6$ = "INSTR(Drink$,Coffee$) = INSTR(Colr$,Green$)"
      ex7$ = "INSTR(Smoke$,PallMall$) = INSTR(Animal$,Birds$)"
      ex8$ = "INSTR(Smoke$,Dunhill$) = INSTR(Colr$,Yellow$)"
      ex9$ = "MID$(Drink$,3,1) = Milk$"
      ex10$ = "LEFT$(Nation$,1) = Norway$"
      ex11$ = "ABS(INSTR(Smoke$,Blend$)-INSTR(Animal$,Cats$)) = 1"
      ex12$ = "ABS(INSTR(Smoke$,Dunhill$)-INSTR(Animal$,Horse$)) = 1"
      ex13$ = "INSTR(Smoke$,BlueMaster$) = INSTR(Drink$,Beer$)"
      ex14$ = "INSTR(Nation$,Germany$) = INSTR(Smoke$,Prince$)"
      ex15$ = "ABS(INSTR(Nation$,Norway$)-INSTR(Colr$,Blue$)) = 1"
      ex16$ = "ABS(INSTR(Smoke$,Blend$)-INSTR(Drink$,Water$)) = 1"
      
      REM Solve:
      solutions% = 0
      TIME = 0
      FOR nation% = 1 TO 120
        Nation$ = perm$(nation%)
        IF EVAL(ex10$) THEN
          FOR colr% = 1 TO 120
            Colr$ = perm$(colr%)
            IF EVAL(ex5$) IF EVAL(ex2$) IF EVAL(ex15$) THEN
              FOR drink% = 1 TO 120
                Drink$ = perm$(drink%)
                IF EVAL(ex9$) IF EVAL(ex4$) IF EVAL(ex6$) THEN
                  FOR smoke% = 1 TO 120
                    Smoke$ = perm$(smoke%)
                    IF EVAL(ex14$) IF EVAL(ex13$) IF EVAL(ex16$) IF EVAL(ex8$) THEN
                      FOR animal% = 1 TO 120
                        Animal$ = perm$(animal%)
                        IF EVAL(ex3$) IF EVAL(ex7$) IF EVAL(ex11$) IF EVAL(ex12$) THEN
                          PRINT "House     Drink     Nation    Colour    Smoke     Animal"
                          FOR house% = 1 TO 5
                            PRINT ; house% ,;
                            PRINT Drink$(ASCMID$(Drink$,house%)-65),;
                            PRINT Nation$(ASCMID$(Nation$,house%)-65),;
                            PRINT Colr$(ASCMID$(Colr$,house%)-65),;
                            PRINT Smoke$(ASCMID$(Smoke$,house%)-65),;
                            PRINT Animal$(ASCMID$(Animal$,house%)-65)
                          NEXT
                          solutions% += 1
                        ENDIF
                      NEXT animal%
                    ENDIF
                  NEXT smoke%
                ENDIF
              NEXT drink%
            ENDIF
          NEXT colr%
        ENDIF
      NEXT nation%
      PRINT '"Number of solutions = "; solutions%
      PRINT "Solved in " ; TIME/100 " seconds"
      END
      
      DEF FNperm(x$())
      LOCAL i%, j%
      FOR i% = DIM(x$(),1)-1 TO 0 STEP -1
        IF x$(i%) < x$(i%+1) EXIT FOR
      NEXT
      IF i% < 0 THEN = FALSE
      j% = DIM(x$(),1)
      WHILE x$(j%) <= x$(i%) j% -= 1 : ENDWHILE
      SWAP x$(i%), x$(j%)
      i% += 1
      j% = DIM(x$(),1)
      WHILE i% < j%
        SWAP x$(i%), x$(j%)
        i% += 1
        j% -= 1
      ENDWHILE
      = TRUE