Shadows of the Knight - Episode 2 - Puzzle discussion

Hi! Thanks for your reply.

The bomb change of spot each time to avoid hard-coding.

Hmm. It also says, “The tests provided are similar to the validation tests used to compute the final score but remain different.” That’s how most puzzles prevent hard-coding.

Its position is computed using your first move and then stay fix until the end of the game.

Maybe something like that, but not exactly. I hardcoded up to 3 sets of coordinates based on how it moved, and looped those 3, and it moved the bomb to a 4th position. So it isn’t just the first move. I suspect it looks for any hard-coded coordinates and avoids them.

The difficulty increase come from the fact that you don’t know the direction of the bomb, but only if you get “warmer”.

Well, that is the difficulty increase between the first puzzle and the second. I understood that the moving bomb was to make the second puzzle (this one) more challenging than it would be otherwise.
I find the wording confusing:

For some tests, the bombs’ location may change from one execution to the other : the goal is to help you find the best algorithm in all cases.

That implies that an algorithm that solves a non-moving bomb won’t work on a moving one.
From what others are saying, it sounds like you can simply solve it using binary search, the way it seems. The fact that the bomb may move sounds irrelevant.

I actually meant to ask only one specific question:
When the bomb moves, does it respect the previous feedback of COLDER, WARMER and SAME? Or could it move the bomb somewhere that makes one of them no longer true? I am guessing it does respect the previous messages, since debug mode actually shows me visually what areas are left. But I wanted to ask around before working it out!

You don’t understand. The bomb position is fixed for one entire test. If you replay the same test, the bomb could be in another place, but it doesn’t move during the game.

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You don’t understand. The bomb position is fixed for one entire test. If you replay the same test, the bomb could be in another place, but it doesn’t move during the game.

Ah, I see. That does answer my question. Thank you!

I did it!
A couple of observations:

  1. If you do the dimensions simultaneously, you have to deal with Pythagoras, and you’ll rarely get SAME.
  2. If you do the dimensions separately, there’s no reason you need to make set-up jumps separately for the two dimensions.
  3. It is then possible to guarantee yourself two halving jumps per dimension for each reset. Just enough for test 8.

I figured out the algorithm on paper, sitting in my car waiting for my daughter at gymnastics. That wasn’t so difficult. Turning that list of instructions into C++ took some struggle, but… well, I did it!


Solved it!
Not so difficult after all. I simply used binary search on x and y separately until I go out of the grid, at which point I make a useless move to get back in the area of interest.
I also did a tiny optimization to finish last test with 3 remaining jumps (though not a really robust one).
Good luck!

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it just starts offf with highteck coding. an i dont know how to code i wanted to learn

I’m curious: how did you end up in the hard puzzles section?
You should start here:

Also, if you’re really new to programming, I suggest taking a tutorial online before trying to tackle puzzles.

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Props to everyone who tackled this one with centroids and polygons, but I’m pretty sure that’s the hard way. I did a separated x and y binary search, and I am happy to confirm it is able to solve every validator with moves to spare.

If you find you’re running out of jumps on the big levels, look closely at what your search is doing when jumping to and from the edges of the building. Maybe there’s a way to set up Bats so that his next jump will eliminate a larger part of the unsearched area. Maybe even bisect it…

Tried and failed to solve it using polygons. It was very hard for me to compute best-cut… So I tried it solving both axis separately

My solution is based on 3 steps:

  1. step - jump so that you split searched area in half. Do not forget, you cannot cross board boundaries - 50% score
  2. step - if jump computed in step 1 won’t improve your solution (new centroid outside actual search area) - jump either to start or end of actual search area/interval - 87% score
  3. step - if you see repeating steps around boundaries (0/width/height) try to search just around the boundaries - 100% score with huge reserve

it is possible to find the bomb within 16 rounds as I managed to
to it with my code, I confirm that the test is not broken.

Hi all,
my stategy is to list all the possible nodes from the beginning
and then remove the nodes where the bomb can’t be. I confirm that
at the beginning, moving batman to the opposit of the centroid is a good
idea because it removes half of the buliding in one round. After that I
usually move to the centroid of the remaining nodes and it works well.
This works well up to test 6. After that (7 and 8) it doesn’t work because
making the list of all possible nodes is to slow (several seconds) compared
to what is expected (<150ms). One can easily imagine that for example
with the case of 8000x8000, the list will be quite long…64Millions of terms
I’m looking for a new strategy taking into account what is said in this forum
about working on x and y separately.

My algorithm is the following:

  1. define a rectangular search window of the size of the board
  2. Mesh the search window to have a reasonable number of points (~300, python is slow)
  3. Check if each point is compatible with the results obtained during the previous moves (closer to point A than to point B, equidistant between A and B). Keep only the valid points.
  4. Compute the new search window from the valid points.
  5. Compute the centroid of the known valid points
  6. Move Batman to the known valid point closest to batman central symmetry to the centroid (If you aim for the centroid you are not discriminating enough points).
  7. Repeat from step two.

Why i get WARMER , while it’s COLDER
d = distance(0,6,4,10)
d2 = distance(0,5,4,10)
d = 5.656854249492381 d2 = 6.4031242374328485
I get WARMER with coordinate (0,5) while (0,6) is nearest of (4,10)

So i suppose the optimal solution is one where you make your polygon of n sides from ideal slices using 2d cartesian geometry and bresenham’s and keep slicing down till you only have 1 point left…

but the binary search on individual dimensions slicing down rectangles is so much easier.

it just feels icky to feel like i lucked out on some of the bigger test cases.

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I used dichotomy to solve one axis and then the other. Splitting in half makes you waste a lot of moves if the target is close to the edge, because if you get COLDER, you cannot split the remaining zone anymore and you basically lose a move. My strategy to mitigate those scenario was to split unevenly (i used 1/4 - 3/4) if you’re against an edge, so if you “lose a move” you at least reduced the zone more, and take much less moves, at a small cost, because if the bomb is in the 3/4 zone, you can immediately start splitting equally again.

In order to debug your code, I recommand you to decompose it as much as possible.

In codingame you have a few errors indicated, but the best option stay to create your own debug’s error.
Just put as much as possible error lines.

I see you’re working with VB.Net, this is how you can create a error lines : Console.Error.WriteLine("Debug messages...")

For example put some before and after a condition or loop :

Console.Error.WriteLine("I'm before my condition.")
   Console.Error.WriteLine("My condition is true.")
   Console.WriteLine("A is bigger than B");
} else {
   Console.Error.WriteLine("My condition is false.")
Console.Error.WriteLine("I'm after my condition.")

When you find where is the problem, it’s more easy to search for a solution :wink: