[Jprogramming] AoC Day 11 - something is wrong

[David Lambert]

In particular I failed to realize that the optimal step might not be 
unique.  Before I boxed

p=. p,<i#DIRS

the example problem returned 4 instead of 3.  All solutions are appended to path, and the tail is chosen by

   i=. (=<./)|position-,h+/{:path

The program worked by good luck rather than through my comprehension.  The length of p is correct, and the length of path is wrong, and could be fixed by choosing only one of i .
Thank you.

> Date: Mon, 18 Dec 2017 09:07:52 -0500
> From: Raul Miller<rauldmiller at gmail.com>
> To: Programming forum<programming at jsoftware.com>
> Subject: Re: [Jprogramming] AoC Day 11 - something is wrong
> Message-ID:
> 	<CAD2jOU8yqDZKd=D42v_AbA3owV+31UEo00BfT_hd-LgnY+Af6w at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
>
> If I read this right it works like this:
>
> (a) Use complex numbers to represent x,y coordinates
> (b) Find the location of the end of the path
> (c) Trace a path from the start to that location, by repeatedly
> picking a step that gets closer to that location.
>
> This works great for part 1, but is inefficient for part 2 (where you
> must find the greatest number of steps needed to reach any point along
> the path).
>
> That said, if you don't mind the wait, the inefficiency for part 2 is
> not a blocker.
>
> Thanks,
>
> -- Raul On Mon, Dec 18, 2017 at 8:18 AM, David Lambert 
> <b49p23tivg at gmail.com> wrote:
>> This step counting verb uses lengths of equilateral hexagonal grid, taking
>> that step or apparently quantum tunneling pair of optimal steps.  I have not
>> tried to understand it, but it works.
>>
>>
>> DIRS=:;:a=:'ne n nw sw s se'
>> ('a'~)=. 1ad30 1ad90 1ad150 1ad210 1ad270 1ad330
>>
>> count_steps=: 3 :0
>>   position=. +/(". ::])y
>>   h=. ne,n,nw,sw,s,se
>>   path=. 0j0
>>   p=. 0$<''
>>   while. 0.2<|position-{:path do.
>>    i=. (=<./)|position-,h+/{:path
>>    path=.path,({:path)+h#~i
>>    p=. p,<i#DIRS
>>   end.
>>   (;~#)p
>> )
>>
>>
>>     count_steps se,sw,se,sw,sw
>> ┌─┬───┬──────┬────┐
>> │3│┌─┐│┌──┬─┐│┌──┐│
>> │ ││s│││sw│s│││sw││
>> │ │└─┘│└──┴─┘│└──┘│
>> └─┴───┴──────┴────┘
>>