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# Nine Prisoners

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An article in Quanta magazine mentions the following puzzle by Dudeney and popularized by Martin Gardner:

"So in the spirit of Kirkman, we will leave the gentle reader with another brainteaser, a slight variation on the schoolgirl puzzle devised in 1917 by the British puzzle aficionado Henry Ernest Dudeney and later popularized by Martin Gardner: Nine prisoners are taken outdoors for exercise in rows of three, with each adjacent pair of prisoners linked by handcuffs, on each of the six weekdays (back in Dudeney’s less enlightened times, Saturday was still a weekday). Can the prisoners be arranged over the course of the six days so that each pair of prisoners shares handcuffs exactly once?"

Here is the specification

```MACHINE NinePrisoners
DEFINITIONS
Prisoners == 1..9;
Days == 6;
Cuffs == {1,2,  4,5, 7,8 };
share(day,cuff) == {arrange(day)(cuff),arrange(day)(cuff+1)}
CONSTANTS arrange
PROPERTIES
/* typing + permutation requirement */
arrange : (1..Days) --> perm(Prisoners) &

/* symmetry breaking */
arrange(1) = %i.(i:Prisoners|i) &
!d.(d:1..Days =>
!c.(c:Prisoners & c mod 3 = 1 => arrange(d)(c) < arrange(d)(c+2))) &
!d.(d:1..Days =>
!c.(c:{1,3} => arrange(d)(c) < arrange(d)(c+3)))
&

/* the constraint proper */
!(c,d).(c:Cuffs & d:2..Days =>
!(c1,d1).(d1<d & d1>0 & c1:Cuffs =>
share(d,c) /= share(d1,c1))
)
END
```

Solving time for Days == 4: 0.08 seconds, for Days==5: 0.20 seconds, for Days == 6: 80 seconds.

## Simple Graphical Visualization

``` ANIMATION_FUNCTION ==
{r,c,i| r:1..Days & c:1..3 & i = arrange(r)(c) } \/
{r,c,i| r:1..Days & c:5..7 & i = arrange(r)(c-1) } \/
{r,c,i| r:1..Days & c:9..11 & i = arrange(r)(c-2) };
ANIMATION_FUNCTION_DEFAULT == {r,c,i| r:1..Days & c:1..11 & i = "  " }
```

## Adding graphical visualization

To add a better graphical visualization one needs to generate 10 pictures (in GIF format). I generate these using OmniGraffle. In the DEFINITIONS section of the B machine above, you then simply have to add the following (and remove the definition of ANIMATION_FUNCTION_DEFAULT from above):

``` ANIMATION_FUNCTION_DEFAULT == {r,c,i| r:1..Days & c:1..11 & i = 0 };
ANIMATION_IMG0 == "images/Prisoner_0.gif";
ANIMATION_IMG1 == "images/Prisoner_1.gif";
ANIMATION_IMG2 == "images/Prisoner_2.gif";
ANIMATION_IMG3 == "images/Prisoner_3.gif";
ANIMATION_IMG4 == "images/Prisoner_4.gif";
ANIMATION_IMG5 == "images/Prisoner_5.gif";
ANIMATION_IMG6 == "images/Prisoner_6.gif";
ANIMATION_IMG7 == "images/Prisoner_7.gif";
ANIMATION_IMG8 == "images/Prisoner_8.gif";
ANIMATION_IMG9 == "images/Prisoner_9.gif"
```

Details about the use of this feature can be found in the Graphical_Visualization page of the manual. With the above, ProB will display the variables x, y and dist in a graphical way, and by animation one can gain insights into why it is impossible not to generate four quartets.