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Question

So, here's a map of, let's say, a dungeon...

##########
#    #####
#    #####
##########
##########
##########
##########
####    ##
####    ##
##########

Let's say that the hero is in Room A (at the top left) and their goal (a prince in distress?) is in Room B (to the bottom right). Our map does not allow the hero to progress to their goal.

We need to add a passageway...

##########
#    #####
#    #####
####.#####
####.#####
####.#####
####.#####
####    ##
####    ##
##########

There, much better!

---

Rules

• A program or function which accepts a dungeon map (made up of hashes and spaces, with rows separated by new line characters).
• It will output a map with dots added to denote passages in all spaces which are on a direct path between the space characters.
• It will not change the line length, or number of lines.
• Passages are all in a direct line from spaces to spaces.
  • Passages can not turn around corners
  • They will not be between spaces and the edge of the map.
• Use any language.
• Attempt to perform the conversion in the fewest bytes.
• If no passageways can be drawn, return the map, unchanged.
• The map should always have hashes around all edges (You do not need to handle spaces at the edge).
• Input maps are always rectangular, each row should be the same width.

Test cases

####       ####
#  #   =>  #  #
#  #       #  #
####       ####

##########        ##########
#    #####        #    #####
#    #####        #    #####
##########        ####.#####
##########    =>  ####.#####
##########        ####.##### 
##########        ####.#####
####    ##        ####    ##
####    ##        ####    ##
##########        ##########

##########        ##########
#    #####        #    #####
#    #####        #    #####
##########        ##########
##########    =>  ##########
##########        ########## 
##########        ##########
######  ##        ######  ##
######  ##        ######  ##
##########        ##########

##########        ##########
#    #####        #    #####
#    #####        #    #####
##########        ####.#####
##########    =>  ####.#####
####   ###        ####   ### 
##########        ######.###
######  ##        ######  ##
######  ##        ######  ##
##########        ##########

##########        ##########
#    #####        #    #####
#    #####        #    #####
##########        ##..######
##########    =>  ##..######
##########        ##..###### 
##########        ##..######
## #######        ## .######
##  ######        ##  ######
##########        ##########

##########        ##########
#    #####        #    #####
#    #####        #    #####
##########        #.########
##########    =>  #.########
##########        #.######## 
#######  #        #.#####  #
#######  #        #.#####  #
# #####  #        # .....  #
##########        ##########

##########        ##########
#    #####        #    #####
#    #####        #    #####
##########        #.########
#####  ###    =>  #.###  ###
#####  ###        #.###  ### 
#######  #        #.#####  #
#######  #        #.#####  #
# #####  #        # .....  #
##########        ##########

##########        ##########
##       #        ##       #
##########        ##......##
##########        ##......##
##########    =>  ##......##
##########        ##......## 
##########        ##......##
##########        ##......##
#       ##        #       ##
##########        ##########

##########        ##########
####  ####        ####  ####
####### ##        ####..# ##
###### ###        ####.. ###
# ### ## #    =>  # ... .. #
# ## ### #        # .. ... # 
### ######        ### ..####
## #######        ## #..####
####  ####        ####  ####
##########        ##########

Answer 1

Jelly, 17 bytes

ỴḲaLḊṖƊ¦”.KƊ€Z$⁺Y

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Tricky -1 thanks to user202729.

Explanation:

ỴḲaLḊṖƊ¦”.KƊ€Z$⁺Y Arguments: S
Ỵ                 Split S on newlines
 ḲaLḊṖƊ¦”.KƊ€Z$   Monadic link
 ḲaLḊṖƊ¦”.KƊ€      Map over left argument
 ḲaLḊṖƊ¦”.KƊ        Monadic link
 Ḳ                   Split on spaces
  aLḊṖƊ¦”.           Dyadic link with right argument '.'
  aLḊṖƊ¦              Apply at specific indices
  a                    Logical AND (vectorizes)
   LḊṖƊ                Monadic link
   L                    Length
    Ḋ                   Range [2..n]
     Ṗ                  Remove last element
          K          Join with spaces
             Z     Zip
               ⁺  Previous link
                Y Join with newlines

Answer 2

Perl 5 -p0, 56 bytes

#!/usr/bin/perl -p0
/
/;$n="(.{@+})*";s%#%/ #*\G#+ |(?= )$n\G$n /s?".":$&%eg

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Answer 3

APL+WIN, 87 bytes

Prompts for character matrix:

n←(' '=m←⎕)⋄c←(∨⍀n)+⊖∨⍀⊖n⋄r←(∨\n)+⌽∨\⌽n⋄((,c>1)/,m)←'.'⋄((,r>1)/,m)←'.'⋄((,n)/,m)←' '⋄m

Answer 4

Haskell, 209 165 162 bytes.

import Data.List
t=transpose
k=concat
j a=(foldr1 max<$>)<$>t<$>t[a,f<$>a,t$f<$>t a]
f b|(e:g:d@(h:_:_))<-group b=k[f$e++g,'.'<$h,drop(length h)$f$k d]|1>0=' '<$b

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Not the most efficient way of doing it in Haskell I'm sure. It's got too many parentheses for my liking but I'm not sure how to remove any more.

Answer 5

Python 2, 173 148 bytes

m=input().split('\n')
exec"m=zip(*[[c*(c!='#')or'#.'[(' 'in r[i:])*(' 'in r[:i])]for i,c in enumerate(r)]for r in m]);"*2
for r in m:print''.join(r)

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Answer 6

Retina 0.8.2, 95 bytes

+`(?<=(.)*)#(?=.*¶(?>(?<-1>.)*)[ .])
.
+`\.(?=(.)*)(?<![ .](?>(?<-1>.)*)¶.*)
#
 (\S+) 
 $.1$*. 

Try it online! Explanation:

+`(?<=(.)*)#(?=.*¶(?>(?<-1>.)*)[ .])
.

This looks for # signs that are above spaces or .s and turns them into dots until there are none left. The lookbehind finds the #'s column and then the lookahead skips to the next line and atomically to the same column below so that the space or . can only match if it's exactly below the #.

+`\.(?=(.)*)(?<![ .](?>(?<-1>.)*)¶.*)
#

This looks for .s that are not below spaces or .s and turns them back into #s until there are none left. The lookahead finds the .'s column and then the lookbehind skips to the previous line and atomically to the same column above in much the same way so that the space or . can only match if it's exactly above the #. A negative lookbehind is used so that this also works for .s in the top row.

 (\S+) 
 $.1$*. 

(Note trailing space on both lines) This simply looks for all runs of non-whitespace characters between spaces and ensures that they are all .s.

Answer 7

Ruby, 104 bytes

->s{2.times{s=((0...s=~/\n/).map{|i|s.lines.map{|b|b[i]}*""}*"\n").gsub(/ [#.]+(?= )/){$&.tr(?#,?.)}};s}

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Well, it's not great, but at least it's convoluted. I'm sure it can be improved.

Answer 8

JavaScript (Node.js),205 193 190 186 181 175 172 bytes

r=>r.split`
`.map(x=>[...x]).map((R,y,r)=>R.map((c,x)=>{for(D=2;c<"#"&&D--;){for(;(T=(r[y+=D]||0)[x+=!D])>" ";);for(;r[y-=D][x-=!D]>c;)T?r[y][x]=".":0}})&&R.join``).join`
`

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Commented

f=r=>r.split`
` ->                                     //getting as string with lines
.map(x=>[...x])                          //to 2d string array
  .map((R,y,r)=>                         //r - the new 2d string array
    R.map((c,x)=>{                       //
      for(D=2;c<"#"&&D--;)              //instead of using if joining c==" " with the loop,D=1/0
        {for(;                           //
         (T=(r[y+=D]||0)[x+=!D])>" ";);  //0[num] = undefined. checking for a path - consisting of # or .(or not consisting of space or undefined), we dont need temp (X,Y) because in the next loop we will return to our original position regardless of the correctness of the path
           for(;T&&r[y-=D][x-=!D]>c;)    //again instead of if(T) combine with loop. if T is not undefined it will be a space because the array can return .#(space). and we then go back to the source(x,y)
                                         //remeber that c==" "
             r[y][x]="."                 //and just putting . where weve been
     }})&&R.join``                       //instead of return r as string at the end , we know that we cant change a row at a smaller index(due to D-0/1) so we can return R.join`` already
    ).join`
`

Answer 9

Stax, 19 bytes

╛XA╟φkôα`æbπ┐w↨╙j≥☺

Run and debug it