Pathfinding with disappearing platforms - is it solvable?

Question

This is a puzzle but might not have a solution. So "is it possible?" is a proper question.

The puzzle idea was inspired by the question logic problem/puzzle solving, and if the puzzle here turns out to be solvable, this question would be an answer to the linked question. (Hence the cross-link.)

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Imagine the following situation:

There is a grid of 6x10 rectangular stones of fields of which some contain 'magical' stones which are solid under specific circumstances (defined below, shown in grey) and 'vacuum' otherwise. The other fields are 'vacuum' all the time.

This field is extended by a top and a bottom row of (6) always solid fields (shown in brown on the bottom, omitted on the top.)

There are 6 "players" which want to get across the field. They can only 'walk' on solid fields and (-this is due to the original puzzle question-) along a given path:

While a player stands still, he sends off six straight rays defined by the diagonals of the fields and a vertical line. (The centre of the rays is always the centre of the field he's standing in, regardless where on the field the player stands.):

Now a stone becomes solid (black) if, and only if, at least two different rays cross over its centre, while rays of a player standing on the platform do not count.

Note that 2 parallel rays are not considered crossing each other.

So the starting position is:

Now the puzzle-question is:

Can you bring all players safely across the field (along the defined path)?

and if not,

Can you bring at least one player safely across the field (along the defined path)?

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Notation: Assuming only one player moves at a time, you can note its colour and final position (assuming the path to that one is 'walkable'. See my example answer below.

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Hints

When testing a valid move, do it by the following 3 steps:

1. "Remove" a player and his colour and check which platforms are (still) solid.

2. Move player along valid, solid path to any position. (Multiple players are allowed on the same field!)

3. Add player-rays and check which additional platforms are solid.

4. This is the new "next" starting position. Note down the move and restart from 1.

Thanks to EFrog there is now also a helper tool for checking the situation. (See his answer below for details and instructions.) In accordance to the 4 points above:

Select a colour (click the big square to change colour) to "pick up" a player. Now check which 'path' is available and move the player to the end-position you want. (The tool does not allow going diagonal directly, but the rules allow it where indicated in the pre-given path.) To set-down a player, simply select another player/colour. Using 'black' mean that all players are set-down.

Answer 1

I stumbled over this question by chance and it caught my interest. I am surprised nobody wrote a program to solve it yet.

One question that isn't clear is what happens to a player who is standing on a tile when the tile stops to be solid. One would expect that the player falls and dies, but that isn't stated explicitly.

Anyway, I wrote a program using its own notation. I just gave names to the tiles instead of the chess-like notation. So the board looks like this:

V W X Y Z [
. . U . . .
. . . T . .
. Q R S . .
. P O . . .
. . N . . .
. . . M L .
. . . J K .
. . I . . .
. . H . . .
. G . . . .
F E D C B A

(Yes, '[' is the 27th letter of the English alphabet!).

I wrote a program to search all possible position accessible from the starting position. If it is correct, it is not possible to cross the whole path even with a single player. The furthest anyone can go is square Q.

solution: ABCDEF.....................
  C -> H: AB.DEF.H...................
  D -> I: AB..EF.HI..................
  F -> H: AB..E..HI..................
  E -> C: ABC....HI..................
  H -> L: ABC....HI..L...............
  A -> D: .BCD...HI..L...............
  C -> H: .B.D...HI..L...............
  D -> C: .BC....HI..L...............
  H -> K: .BC....HI.KL...............
  B -> D: ..CD...HI.KL...............
  C -> H: ...D...HI.KL...............
  D -> C: ..C....HI.KL...............
  H -> N: ..C....HI.KL.N.............
  C -> D: ...D...HI.KL.N.............
  H -> G: ...D..G.I.KL.N.............
  N -> Q: ...D..G.I.KL....Q..........
  I -> G: ...D..G...KL....Q..........
  D -> G: ......G...KL....Q..........

Java source code: https://pastebin.com/kHv7uSEB

If however there are 7 players, 2 players starting on A, then all can cross to the top row. Here is how. Each move is displayed with the occupied squares along the path.

solution: ABCDEF.....................  
  A -> H: ABCDEF.H...................  
  D -> I: ABC.EF.HI..................  
  F -> L: ABC.E..HI..L...............  
  A -> K: .BC.E..HI.KL...............  
  B -> N: ..C.E..HI.KL.N.............  
  H -> O: ..C.E...I.KL.NO............  
  I -> Q: ..C.E.....KL.NO.Q..........  
  C -> G: ....E.G...KL.NO.Q..........  
  E -> C: ..C...G...KL.NO.Q..........  
  L -> P: ..C...G...K..NOPQ..........  
  C -> B: .B....G...K..NOPQ..........  
  G -> C: .BC.......K..NOPQ..........  
  K -> T: .BC..........NOPQ..T.......  
  B -> D: ..CD.........NOPQ..T.......  
  N -> W: ..CD..........OPQ..T..W....  
  P -> [: ..CD..........O.Q..T..W...[  
  T -> P: ..CD..........OPQ.....W...[  
  W -> Z: ..CD..........OPQ........Z[  
  O -> K: ..CD......K....PQ........Z[  
  C -> G: ...D..G...K....PQ........Z[  
  [ -> Y: ...D..G...K....PQ.......YZ.  
  P -> L: ...D..G...KL....Q.......YZ.  
  Z -> X: ...D..G...KL....Q......XY..  
  D -> P: ......G...KL...PQ......XY..  
  G -> N: ..........KL.N.PQ......XY..  
  Y -> Z: ..........KL.N.PQ......X.Z.  
  P -> O: ..........KL.NO.Q......X.Z.  
  X -> Y: ..........KL.NO.Q.......YZ.  
  L -> P: ..........K..NOPQ.......YZ.  
  K -> S: .............NOPQ.S.....YZ.  
  Z -> [: .............NOPQ.S.....Y.[  
  N -> W: ..............OPQ.S...W.Y.[  
  P -> X: ..............O.Q.S...WXY.[  
  Q -> V: ..............O...S..VWXY.[  
  O -> P: ...............P..S..VWXY.[  
  S -> Z: ...............P.....VWXYZ[  
  P -> V: .....................VWXYZ[

Another possibility is to remove the restriction that a stone must remain solid as long as a player stands on it. If you only need to solidify the stone when stepping on it, the problem becomes possible.

But the original problem taken strictly has no solution.

Answer 2

This isn't really an answer, but this is a program that I made that should allow you to better solve this puzzle (or create your own using the same premises).

BmyGuest's Puzzle

Instructions

This is a very clunky program that I put together, but it should be fairly user-friendly and easy to work with.

The grid: The grid consists of the same 6x10 layout presented in the OP (with one extra row at the bottom to start on and another at top for finishing). Each tile in the grid can be turned "on" or "off" by clicking. Tiles that are turned "on" are indicated by white triangles in the corners, while tiles that are "off" are indicated by black triangles. The tiles that are "on" represent the path -- only these tiles will respond to the characters' positions. (Note that it is set to the default path presented in the OP, but it can be changed if you like.)

The little arrow things: In the bottom-right of the screen is a large red box with an arrow on each side. If you click on the red box, it should cycle through the colors (Red > Blue > Green > Yellow > Purple > Orange > Blank/None > Red). Use this to choose which player you intend to control. By clicking on the appropriate arrow button near the selection box, you can then move that character in the indicated direction on the map.

Other: You may use the spacebar to place all of the players back in their starting positions. This does not reset the path to default, just the player positions.

Please Note:

As I said, this is a rather clunky program. So there are a couple of known issues.

First, the player dots are able to move off of the grid. I didn't feel the need to put in restrictions, as this is just a sandbox-style program.

Second, players aren't able to move diagonally in my program; however, this is allowed in the puzzle. Just be sure that you complete each diagonal move you intend and don't stop halfway. :D

I hope this helps and my instructions make sense.

Answer 3

This is a partial answer only, demonstrating the first few (potential) steps.

Each image depicts the situation after the named move. Remember during movement the rays of the moving player are off, so all platforms 'held' by his rays (and no other 2 crossing colours) will immediately disappear!

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Orange --> A2

Yellow --> C2

Cyan--> C3

Orange --> 4

Yellow --> D4

Purple --> B3

etc...