# ETOOBUSY đźš€ minimal blogging for the impatient

# Autobiographical numbers constraints - last is zero

**TL;DR**

Remember Autobiographical numbers? We will go on looking at constraints for it!

Letâ€™s move on to a more *fixed* and *theoretical* one, i.e. that the last
slot MUST be $0$. Code is in stage 2.

# A preliminary note

Letâ€™s first note one thing:

if slot

`x`

contains the value $y$, then there MUST be at least $x \cdot y$ slots in the whole array.

Letâ€™s see why, assuming that slot `x`

contains value $y$:

- by definition, value $x$ is contained in exactly $y$ slots
- assume that
`z`

slot is one of these $y$ slots, it means that it contains value $x$ - as a consequence, there are exactly $x$ slots that contain value $z$
- because there are $y$ such slots, each bearing a different value, then we
need to accomodate
*at least*$x * y$ slots.

The condition is actually stronger than this, as a simple extension of the reasoning above and observing that each slot can only contain a single value:

\[N = \sum_{i = 0}^{N-1} i \cdot v_i\]where $v_i$ represents the value contained in slot `i`

.

# One more thing: N > 3

The autobiographical numbers puzzle canâ€™t be solved for $N \leq 3$:

- $N = 0$ means that there is no slot.
- $N = 1$ means that there is only slot
`0`

. It cannot contain $0$ because otherwise it would have to contain $1$ (there would be $1$ value $0$ in it, right?!?), and of course it cannot contain $1$ because otherwise it would not contain any $0$. -
$N = 2$ means that there are only slots

`0`

and`1`

. Itâ€™s easy to see that no value combination is possible:- both slots can only contain $0$ or $1$, because of what we discussed in the previous section;
- slot
`0`

cannot contain $0$ for the same reasons as the previous case where $N = 1$, so it can only contain a $1$ (if anything); `10`

is not a solution because there is one $1$ and the slot for`1`

is $0$`11`

is not a solution because there are two values $1$, but slot`1`

contains a $1$.

- $N = 3$ is equally impossible. Allowed values for quantities are $0$, $1$
and $2$ because there are only slots
`0`

,`1`

and`2`

.- We already established that slot
`0`

cannot contain $0$ - in the previous section, we saw that slot
`2`

cannot contain $2$ (it would mean that we need to have 4 slots, but we have only three) - so we are left with:

- We already established that slot

```
10* not a solution, the value of slot 1 cannot be less than 1
11* not a solution, the value of slot 1 cannot be less than 2
120 not a solution, slot 1 should be 1
121 not a solution, there is no 0
2*0 not a solution, the value of slot 2 cannot be less than 1
201 not a solution, the value of slot 1 should be 1
211 not a solution, the value of slot 1 should be 2
22* not a solution, the value of slot 2 should be 2 but it can't
```

Hence, it only makes sense to consider cases where $N > 3$.

# Last slot MUST be 0?

Suppose you have $N$ slots, numbered from `0`

to `N-1`

and letâ€™s focus on
the last slot. Remember also that $N > 3$.

Can it be greater than 1? Letâ€™s remember the note in the previous section, and observe that $ k \cdot (N - 1) $ is greater than $N$ (i.e. the total number of available slots!) for $k > 1$ and $N > 2$. So, for $N > 3$ (as we are considering) we MUST have that $k \leq 1$.

Our next question is: can slot `N - 1`

actually take value $1$? Wellâ€¦ no
again. If it were true, then it would mean that the value $N - 1$ is written
in some slot, which MUST be one of the first $N - 1$ slots (the last one is
already occupied by the $1$ and we already know that $N - 1 > 1$).

We know that $N - 1$ cannot be written in *any* slot from `2`

on, again
because of the constraints discussed in the previous section. Hence, it
could only be either slot `0`

or slot `1`

.

Can it be slot `0`

? No it canâ€™t, because we would need to accomodate $N - 1$
slots with $0$ inside, but we only have $N - 2$ left (remember that slot `0`

is occupied by value $N - 1$, and slot `N - 1`

is occuped by value $1$, so
they cannot accomodate a $0$ at the same time).

Can it be slot `1`

? No again, because if we take the equation in the
previous section, we would end up with *at least* $1 \cdot (N - 1) + (N - 1) \cdot 1
= 2(N - 1) > N$ needed slots, which is impossible.

Letâ€™s visualize this latter case explicitly:

```
0 1 2 3
+---+---+---+---+
| 1 | 3 | 1 | 1 |
+---+---+---+---+
...
0 1 2 3 4 N-2 N-1
+---+-----+---+---+---+ ... +---+---+
| 1 | N-1 | 1 | 1 | 1 | | 1 | 1 |
+---+-----+---+---+---+ ... +---+---+
```

All slots show either $1$ or $N - 1$, and other values are absent. Which is
a violation of the main constraint about the game rule: value $0$ is
supposed to appear once (there is a $1$ in slot `0`

) but it does not appear
at all.

Hence, the very last value in a suitable solution MUST be $0$.

# Coding

Now that we know it MUST be $0$, itâ€™s easy to code it - we will do this directly upon initialization.

```
1 sub autobiographical_numbers ($n) {
2 my $solution = [
3 map {
4 +{map { $_ => 1 } 0 .. $n - 2} # "n-1" is always 0
5 } 1 .. $n -1
6 ];
7 push $solution->@*, {0 => 1}; # "n-1" is always 0
8 my @constraints = map { main->can('constraint_' . $_) }
9 qw< basic >;
10 my $state = solve_by_constraints(
11 constraints => \@constraints,
12 is_done => \&is_done,
13 search_factory => \&explore,
14 start => {solution => $solution},
15 logger => ($ENV{VERBOSE} ? \&printout : undef),
16 );
17 } ## end sub autobiographical_numbers ($n)
```

There are two places where this insight is useful:

- the obvious one is thatâ€¦ the last slot only allows for $0$, which is what line 7 is about;
- then, we can also get rid of value $N-1$ from all other slots (line 4, the range goes up to $N-2$ for this reason).

# Is it of help?

Letâ€™s see how it goes:

```
$ time ./run.sh 01-basic/ 30
solution => [26,2,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0]
real 0m6.449s
user 0m6.404s
sys 0m0.032s
$ time ./run.sh 02-last-is-zero/ 30
solution => [26,2,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0]
real 0m5.679s
user 0m5.648s
sys 0m0.012s
```

Not bad!

# So longâ€¦

Curious about the whole series? Here it is:

- Autobiographical numbers
- Autobiographical numbers constraints - basic
- Autobiographical numbers constraints - last is zero
- Autobiographical numbers constraints - weighted sum
- Autobiographical numbers constraints - luckier weighted sum
- Autobiographical numbers - step up
- Code repository

Comments? Please comment below!

*Comments? Octodon, , GitHub, Reddit, or drop me a line!*