# Advent of Code 2017

## December 3rd

You come across an experimental new kind of memory stored on an infinite two-dimensional grid.

Each square on the grid is allocated in a spiral pattern starting at a location marked 1 and then counting up while spiraling outward. For example, the first few squares are allocated like this:

    17  16  15  14  13
    18   5   4   3  12
    19   6   1   2  11
    20   7   8   9  10
    21  22  23---> ...

While this is very space-efficient (no squares are skipped), requested data must be carried back to square 1 (the location of the only access port for this memory system) by programs that can only move up, down, left, or right. They always take the shortest path: the Manhattan Distance between the location of the data and square 1.

For example:

* Data from square 1 is carried 0 steps, since it's at the access port.
* Data from square 12 is carried 3 steps, such as: down, left, left.
* Data from square 23 is carried only 2 steps: up twice.
* Data from square 1024 must be carried 31 steps.

How many steps are required to carry the data from the square identified in your puzzle input all the way to the access port?

### Analysis

I freely admit that I worked out the program I wanted to write using graph paper and some Python doodles.  There's no point in trying to write a Joy program until I'm sure I understand the problem well enough.

The first thing I did was to write a column of numbers from 1 to n (32 as it happens) and next to them the desired output number, to look for patterns directly:

    1  0
    2  1
    3  2
    4  1
    5  2
    6  1
    7  2
    8  1
    9  2
    10 3
    11 2
    12 3
    13 4
    14 3
    15 2
    16 3
    17 4
    18 3
    19 2
    20 3
    21 4
    22 3
    23 2
    24 3
    25 4
    26 5
    27 4
    28 3
    29 4
    30 5
    31 6
    32 5

There are four groups repeating for a given "rank", then the pattern enlarges and four groups repeat again, etc.

            1 2
          3 2 3 4
        5 4 3 4 5 6
      7 6 5 4 5 6 7 8
    9 8 7 6 5 6 7 8 9 10

Four of this pyramid interlock to tile the plane extending from the initial "1" square.


             2   3   |    4  5   |    6  7   |    8  9
          10 11 12 13|14 15 16 17|18 19 20 21|22 23 24 25

And so on.

We can figure out the pattern for a row of the pyramid at a given "rank" $k$:

$2k - 1, 2k - 2, ..., k, k + 1, k + 2, ..., 2k$

or

$k + (k - 1), k + (k - 2), ..., k, k + 1, k + 2, ..., k + k$

This shows that the series consists at each place of $k$ plus some number that begins at $k - 1$, decreases to zero, then increases to $k$.  Each row has $2k$ members.

Let's figure out how, given an index into a row, we can calculate the value there.  The index will be from 0 to $k - 1$. 

 Let's look at an example, with $k = 4$:

    0 1 2 3 4 5 6 7
    7 6 5 4 5 6 7 8


```python
k = 4
```

Subtract $k$ from the index and take the absolute value:


```python
for n in range(2 * k):
    print(abs(n - k),)
```

    4
    3
    2
    1
    0
    1
    2
    3


Not quite. Subtract $k - 1$ from the index and take the absolute value:


```python
for n in range(2 * k):
    print(abs(n - (k - 1)), end=' ')
```

    3 2 1 0 1 2 3 4 

Great, now add $k$...


```python
for n in range(2 * k):
    print(abs(n - (k - 1)) + k, end=' ')
```

    7 6 5 4 5 6 7 8 

So to write a function that can give us the value of a row at a given index:


```python
def row_value(k, i):
    i %= (2 * k)  # wrap the index at the row boundary.
    return abs(i - (k - 1)) + k
```


```python
k = 5
for i in range(2 * k):
    print(row_value(k, i), end=' ')
```

    9 8 7 6 5 6 7 8 9 10 

(I'm leaving out details of how I figured this all out and just giving the relevent bits.  It took a little while to zero in of the aspects of the pattern that were important for the task.)

### Finding the rank and offset of a number.
Now that we can compute the desired output value for a given rank and the offset (index) into that rank, we need to determine how to find the rank and offset of a number.

The rank is easy to find by iteratively stripping off the amount already covered by previous ranks until you find the one that brackets the target number.  Because each row is $2k$ places and there are $4$ per rank each rank contains $8k$ places.  Counting the initial square we have:

$corner_k = 1 + \sum_{n=1}^k 8n$

I'm not mathematically sophisticated enough to turn this directly into a formula (but Sympy is, see below.)  I'm going to write a simple Python function to iterate and search:


```python
def rank_and_offset(n):
    assert n >= 2  # Guard the domain.
    n -= 2  # Subtract two,
            # one for the initial square,
            # and one because we are counting from 1 instead of 0.
    k = 1
    while True:
        m = 8 * k  # The number of places total in this rank, 4(2k).
        if n < m:
            return k, n % (2 * k)
        n -= m  # Remove this rank's worth.
        k += 1
```


```python
for n in range(2, 51):
    print(n, rank_and_offset(n))
```

    2 (1, 0)
    3 (1, 1)
    4 (1, 0)
    5 (1, 1)
    6 (1, 0)
    7 (1, 1)
    8 (1, 0)
    9 (1, 1)
    10 (2, 0)
    11 (2, 1)
    12 (2, 2)
    13 (2, 3)
    14 (2, 0)
    15 (2, 1)
    16 (2, 2)
    17 (2, 3)
    18 (2, 0)
    19 (2, 1)
    20 (2, 2)
    21 (2, 3)
    22 (2, 0)
    23 (2, 1)
    24 (2, 2)
    25 (2, 3)
    26 (3, 0)
    27 (3, 1)
    28 (3, 2)
    29 (3, 3)
    30 (3, 4)
    31 (3, 5)
    32 (3, 0)
    33 (3, 1)
    34 (3, 2)
    35 (3, 3)
    36 (3, 4)
    37 (3, 5)
    38 (3, 0)
    39 (3, 1)
    40 (3, 2)
    41 (3, 3)
    42 (3, 4)
    43 (3, 5)
    44 (3, 0)
    45 (3, 1)
    46 (3, 2)
    47 (3, 3)
    48 (3, 4)
    49 (3, 5)
    50 (4, 0)



```python
for n in range(2, 51):
    k, i = rank_and_offset(n)
    print(n, row_value(k, i))
```

    2 1
    3 2
    4 1
    5 2
    6 1
    7 2
    8 1
    9 2
    10 3
    11 2
    12 3
    13 4
    14 3
    15 2
    16 3
    17 4
    18 3
    19 2
    20 3
    21 4
    22 3
    23 2
    24 3
    25 4
    26 5
    27 4
    28 3
    29 4
    30 5
    31 6
    32 5
    33 4
    34 3
    35 4
    36 5
    37 6
    38 5
    39 4
    40 3
    41 4
    42 5
    43 6
    44 5
    45 4
    46 3
    47 4
    48 5
    49 6
    50 7


### Putting it all together


```python
def row_value(k, i):
    return abs(i - (k - 1)) + k


def rank_and_offset(n):
    n -= 2  # Subtract two,
            # one for the initial square,
            # and one because we are counting from 1 instead of 0.
    k = 1
    while True:
        m = 8 * k  # The number of places total in this rank, 4(2k).
        if n < m:
            return k, n % (2 * k)
        n -= m  # Remove this rank's worth.
        k += 1


def aoc20173(n):
    if n <= 1:
        return 0
    k, i = rank_and_offset(n)
    return row_value(k, i)
```


```python
aoc20173(23)
```




    2




```python
aoc20173(23000)
```




    105




```python
aoc20173(23000000000000)
```




    4572225



# Sympy to the Rescue
### Find the rank for large numbers
Using e.g. Sympy we can find the rank directly by solving for the roots of an equation.  For large numbers this will (eventually) be faster than iterating as `rank_and_offset()` does.


```python
from sympy import floor, lambdify, solve, symbols
from sympy import init_printing
init_printing() 
```


```python
k = symbols('k')
```

Since

$1 + 2 + 3 + ... + N = \frac{N(N + 1)}{2}$

and

$\sum_{n=1}^k 8n = 8(\sum_{n=1}^k n) = 8\frac{k(k + 1)}{2}$

We want:


```python
E = 2 + 8 * k * (k + 1) / 2  # For the reason for adding 2 see above.

E
```




$\displaystyle 4 k \left(k + 1\right) + 2$



We can write a function to solve for $k$ given some $n$...


```python
def rank_of(n):
    return floor(max(solve(E - n, k))) + 1
```

First `solve()` for $E - n = 0$ which has two solutions (because the equation is quadratic so it has two roots) and since we only care about the larger one we use `max()` to select it.  It will generally not be a nice integer (unless $n$ is the number of an end-corner of a rank) so we take the `floor()` and add 1 to get the integer rank of $n$.  (Taking the `ceiling()` gives off-by-one errors on the rank boundaries.  I don't know why.  I'm basically like a monkey doing math here.)  =-D

It gives correct answers:


```python
for n in (9, 10, 25, 26, 49, 50):
    print(n, rank_of(n))
```

    9 1
    10 2
    25 2
    26 3
    49 3
    50 4


And it runs much faster (at least for large numbers):


```python
%time rank_of(23000000000000)  # Compare runtime with rank_and_offset()!
```

    CPU times: user 27.8 ms, sys: 5 µs, total: 27.8 ms
    Wall time: 27.3 ms





$\displaystyle 2397916$




```python
%time rank_and_offset(23000000000000)
```

    CPU times: user 216 ms, sys: 89 µs, total: 216 ms
    Wall time: 215 ms





$\displaystyle \left( 2397916, \  223606\right)$



After finding the rank you would still have to find the actual value of the rank's first corner and subtract it (plus 2) from the number and compute the offset as above and then the final output, but this overhead is partially shared by the other method, and overshadowed by the time it (the other iterative method) would take for really big inputs.

The fun thing to do here would be to graph the actual runtime of both methods against each other to find the trade-off point.

### It took me a second to realize I could do this...
Sympy is a *symbolic* math library, and it supports symbolic manipulation of equations.  I can put in $y$ (instead of a value) and ask it to solve for $k$.


```python
y = symbols('y')
```


```python
g, f = solve(E - y, k)
```

The equation is quadratic so there are two roots, we are interested in the greater one...


```python
g
```




$\displaystyle - \frac{\sqrt{y - 1}}{2} - \frac{1}{2}$




```python
f
```




$\displaystyle \frac{\sqrt{y - 1}}{2} - \frac{1}{2}$



Now we can take the `floor()`, add 1, and `lambdify()` the equation to get a Python function that calculates the rank directly.


```python
floor(f) + 1
```




$\displaystyle \left\lfloor{\frac{\sqrt{y - 1}}{2} - \frac{1}{2}}\right\rfloor + 1$




```python
F = lambdify(y, floor(f) + 1)
```


```python
for n in (9, 10, 25, 26, 49, 50):
    print(n, int(F(n)))
```

    9 1
    10 2
    25 2
    26 3
    49 3
    50 4


It's pretty fast.


```python
%time int(F(23000000000000))  # The clear winner.
```

    CPU times: user 60 µs, sys: 4 µs, total: 64 µs
    Wall time: 67 µs





$\displaystyle 2397916$



Knowing the equation we could write our own function manually, but the speed is no better.


```python
from math import floor as mfloor, sqrt

def mrank_of(n):
    return int(mfloor(sqrt(n - 1) / 2 - 0.5) + 1)
```


```python
%time mrank_of(23000000000000)
```

    CPU times: user 7 µs, sys: 1 µs, total: 8 µs
    Wall time: 10 µs





$\displaystyle 2397916$



### Given $n$ and a rank, compute the offset.

Now that we have a fast way to get the rank, we still need to use it to compute the offset into a pyramid row.


```python
def offset_of(n, k):
    return (n - 2 + 4 * k * (k - 1)) % (2 * k)
```

(Note the sneaky way the sign changes from $k(k + 1)$ to $k(k - 1)$.  This is because we want to subract the $(k - 1)$th rank's total places (its own and those of lesser rank) from our $n$ of rank $k$.  Substituting $k - 1$ for $k$ in $k(k + 1)$ gives $(k - 1)(k - 1 + 1)$, which of course simplifies to $k(k - 1)$.)


```python
offset_of(23000000000000, 2397916)
```




$\displaystyle 223606$



So, we can compute the rank, then the offset, then the row value.


```python
def rank_of(n):
    return int(mfloor(sqrt(n - 1) / 2 - 0.5) + 1)


def offset_of(n, k):
    return (n - 2 + 4 * k * (k - 1)) % (2 * k)


def row_value(k, i):
    return abs(i - (k - 1)) + k


def aoc20173(n):
    k = rank_of(n)
    i = offset_of(n, k)
    return row_value(k, i)
```


```python
aoc20173(23)
```




$\displaystyle 2$




```python
aoc20173(23000)
```




$\displaystyle 105$




```python
aoc20173(23000000000000)
```




$\displaystyle 4572225$




```python
%time aoc20173(23000000000000000000000000)  # Fast for large values.
```

    CPU times: user 22 µs, sys: 2 µs, total: 24 µs
    Wall time: 26.7 µs





$\displaystyle 2690062495969$



# A Joy Version
At this point I feel confident that I can implement a concise version of this code in Joy.  ;-)


```python
from notebook_preamble import J, V, define
```

### `rank_of`

       n rank_of
    ---------------
          k

The translation is straightforward.

    int(floor(sqrt(n - 1) / 2 - 0.5) + 1)

    rank_of == -- sqrt 2 / 0.5 - floor ++


```python
define('rank_of -- sqrt 2 / 0.5 - floor ++')
```

### `offset_of`

       n k offset_of
    -------------------
             i

    (n - 2 + 4 * k * (k - 1)) % (2 * k)

A little tricky...

    n k dup 2 *
    n k k 2 *
    n k k*2 [Q] dip %
    n k Q k*2 %

    n k dup --
    n k k --
    n k k-1 4 * * 2 + -
    n k*k-1*4     2 + -
    n k*k-1*4+2       -
    n-k*k-1*4+2

    n-k*k-1*4+2 k*2 %
    n-k*k-1*4+2%k*2

Ergo:

    offset_of == dup 2 * [dup -- 4 * * 2 + -] dip %


```python
define('offset_of dup 2 * [dup -- 4 * * 2 + -] dip %')
```

### `row_value`

       k i row_value
    -------------------
            n

    abs(i - (k - 1)) + k

    k i over -- - abs +
    k i k    -- - abs +
    k i k-1     - abs +
    k i-k-1       abs +
    k |i-k-1|         +
    k+|i-k-1|


```python
define('row_value over -- - abs +')
```

### `aoc2017.3`

       n aoc2017.3
    -----------------
            m

    n dup rank_of
    n k [offset_of] dupdip
    n k offset_of k
    i             k swap row_value
    k i row_value
    m


```python
define('aoc2017.3 dup rank_of [offset_of] dupdip swap row_value')
```


```python
J('23 aoc2017.3')
```

    2



```python
J('23000 aoc2017.3')
```

    105



```python
V('23000000000000 aoc2017.3')
```

                                                        • 23000000000000 aoc2017.3
                                         23000000000000 • aoc2017.3
                                         23000000000000 • dup rank_of [offset_of] dupdip swap row_value
                          23000000000000 23000000000000 • rank_of [offset_of] dupdip swap row_value
                          23000000000000 23000000000000 • -- sqrt 2 / 0.5 - floor ++ [offset_of] dupdip swap row_value
                          23000000000000 22999999999999 • sqrt 2 / 0.5 - floor ++ [offset_of] dupdip swap row_value
                       23000000000000 4795831.523312615 • 2 / 0.5 - floor ++ [offset_of] dupdip swap row_value
                     23000000000000 4795831.523312615 2 • / 0.5 - floor ++ [offset_of] dupdip swap row_value
                      23000000000000 2397915.7616563076 • 0.5 - floor ++ [offset_of] dupdip swap row_value
                  23000000000000 2397915.7616563076 0.5 • - floor ++ [offset_of] dupdip swap row_value
                      23000000000000 2397915.2616563076 • floor ++ [offset_of] dupdip swap row_value
                                 23000000000000 2397915 • ++ [offset_of] dupdip swap row_value
                                 23000000000000 2397916 • [offset_of] dupdip swap row_value
                     23000000000000 2397916 [offset_of] • dupdip swap row_value
                                 23000000000000 2397916 • offset_of 2397916 swap row_value
                                 23000000000000 2397916 • dup 2 * [dup -- 4 * * 2 + -] dip % 2397916 swap row_value
                         23000000000000 2397916 2397916 • 2 * [dup -- 4 * * 2 + -] dip % 2397916 swap row_value
                       23000000000000 2397916 2397916 2 • * [dup -- 4 * * 2 + -] dip % 2397916 swap row_value
                         23000000000000 2397916 4795832 • [dup -- 4 * * 2 + -] dip % 2397916 swap row_value
    23000000000000 2397916 4795832 [dup -- 4 * * 2 + -] • dip % 2397916 swap row_value
                                 23000000000000 2397916 • dup -- 4 * * 2 + - 4795832 % 2397916 swap row_value
                         23000000000000 2397916 2397916 • -- 4 * * 2 + - 4795832 % 2397916 swap row_value
                         23000000000000 2397916 2397915 • 4 * * 2 + - 4795832 % 2397916 swap row_value
                       23000000000000 2397916 2397915 4 • * * 2 + - 4795832 % 2397916 swap row_value
                         23000000000000 2397916 9591660 • * 2 + - 4795832 % 2397916 swap row_value
                          23000000000000 22999994980560 • 2 + - 4795832 % 2397916 swap row_value
                        23000000000000 22999994980560 2 • + - 4795832 % 2397916 swap row_value
                          23000000000000 22999994980562 • - 4795832 % 2397916 swap row_value
                                                5019438 • 4795832 % 2397916 swap row_value
                                        5019438 4795832 • % 2397916 swap row_value
                                                 223606 • 2397916 swap row_value
                                         223606 2397916 • swap row_value
                                         2397916 223606 • row_value
                                         2397916 223606 • over -- - abs +
                                 2397916 223606 2397916 • -- - abs +
                                 2397916 223606 2397915 • - abs +
                                       2397916 -2174309 • abs +
                                        2397916 2174309 • +
                                                4572225 • 


      rank_of == -- sqrt 2 / 0.5 - floor ++
    offset_of == dup 2 * [dup -- 4 * * 2 + -] dip %
    row_value == over -- - abs +

    aoc2017.3 == dup rank_of [offset_of] dupdip swap row_value
