Plant trees on a golf course!

Question

_{This challenge is inspired by this app.}

---

This is a much easier version of this challenge. This challenge is code-golf, while the other one is fastest-code.

---

You'll be given a square input grid, of dimensions 6-by-6 which is divided into 6 areas, where the cells of each area have a unique identifier (I'll use lower case letters from a-f in the the text here, but you may choose whatever you like, for instance integers 1-6).

The input may look like this (optional input format):

aabbbb
aabbbb
aaccbb
acccdb
ecccdb
eeefff

Or, easier to visualize:

Challenge:

You are to place 6 trees in this park, according to the following rules:

• There shall be exactly 1 tree per column, and 1 tree per row
• All areas shall have exactly 1 tree.
• No trees can be adjacent to another tree, vertically, horizontally or diagonally

The solution to the layout above is:

Note: There is only one solution to each puzzle

Additional rules:

• The input and output formats are optional
  • The output might for instance be a list of indices, a grid with 1/0 indicating if there's a tree in that position, or a modified version of the input where the trees are indicated
• The execution time must be deterministic
• The program must finish within 1 minute on a reasonable modern laptop
• Brownie points if you don't brute force!

Test cases:

aabbbb
aabbbb
aaccbb
acccdb
ecccdb
eeefff
---
aAbbbb
aabbbB
aaCcbb
acccDb
Ecccdb
eeeFff
----------
aabccc
aacccc
aaddce
aeeeee
aeeefe
eeeeee
---
aaBccc
aacccC
aadDce
Aeeeee
aeeeFe
eEeeee
----------
aaaabb
aacbbb
aadddb
addeef
ddddee
dddeee
---
aaaaBb
aaCbbb
Aadddb
addeeF
dDddee
dddEee
----------
abbbcd
abebcd
addddd
dddddd
effdff
eeffff
---
abBbcd
abebCd
Addddd
dddDdd
effdfF
eEffff

Same test cases on a format that's a bit easier to parse:

Test case 1:
[1,1,2,2,2,2;1,1,2,2,2,2;1,1,3,3,2,2;1,3,3,3,4,2;5,3,3,3,4,2;5,5,5,6,6,6]
Test case 2:
[1,1,2,3,3,3;1,1,3,3,3,3;1,1,4,4,3,5;1,5,5,5,5,5;1,5,5,5,6,5;5,5,5,5,5,5]
Test case 3:
[1,1,1,1,2,2;1,1,3,2,2,2;1,1,4,4,4,2;1,4,4,5,5,6;4,4,4,4,5,5;4,4,4,5,5,5]
Test case 4:
[1,2,2,2,3,4;1,2,5,2,3,4;1,4,4,4,4,4;4,4,4,4,4,4;5,6,6,4,6,6;5,5,6,6,6,6]

Answer 1

C, 223 182 bytes

O[15],U;main(y,v)char**v;{if(y>7)for(;y-->2;printf("%06o\n",O[y]));else for(int*r,x=1,X=8;X<14;U&x|*r|O[10-y]*9&x*9?0:(U^=O[9-y]=*r=x,*r=main(y+1,v),U^=x),x*=8)r=O+v[1][y*7-++X]-88;}

Takes input as an argument in the format given in the question. Writes output to stdout as a grid of 0s with 1s where the trees go.

./TreesMin 'aabbbb
aabbbb
aaccbb
acccdb
ecccdb
eeefff'

Sample output:

010000
000001
001000
000010
100000
000100

Breakdown

O[15],                                  // Tree positions & region usage
U;                                      // Column usage (bitmask)
main(y,v)char**v;{                      // Recursive main function
  if(y>7)                               // Finished grid?
    for(;y-->2;printf("%06o\n",O[y]));  //  Print it (rows are padded octal)
  else                                  // Not finished:
    for(int*r,x=1,X=8;X<14;             //  Loop over columns
      U&x|*r|O[10-y]*9&x*9              //   Current cell violates rules?
        ?0                              //    Do nothing
        :(U^=O[9-y]=*r=x,               //   Else: mark cell
          *r=main(y+1,v),               //    Recurse
          U^=x)                         //    Unmark cell
      ,x*=8)                            //   Advance to next column
      r=O+v[1][y*7-++X]-88;             //   Region pointer for current iteration
}

It's an adaptation of my answer to the fastest-code version of this question. Doesn't have as much short-circuiting, but it's plenty fast enough for 6x6 grids.

Answer 2

Clingo, 66 bytes

1{t(X,Y):c(X,Y,Z)}:-Z=1..n.:-t(X,Y),2{t(X,J;I,Y;X-1..X+1,Y..Y+1)}.

Run with clingo plant.lp - -c n=<n> where <n> is the grid size. The input format is a list of c(X,Y,Z). statements for each cell (X, Y) colored Z, with 1 ≤ X, Y, Z ≤ n, separated by optional whitespace. The output includes t(X,Y) for each tree at (X, Y).

Demo

$ clingo plant.lp - -c n=6 <<EOF
> c(1,1,1). c(2,1,1). c(3,1,2). c(4,1,2). c(5,1,2). c(6,1,2).
> c(1,2,1). c(2,2,1). c(3,2,2). c(4,2,2). c(5,2,2). c(6,2,2).
> c(1,3,1). c(2,3,1). c(3,3,3). c(4,3,3). c(5,3,2). c(6,3,2).
> c(1,4,1). c(2,4,3). c(3,4,3). c(4,4,3). c(5,4,4). c(6,4,2).
> c(1,5,5). c(2,5,3). c(3,5,3). c(4,5,3). c(5,5,4). c(6,5,2).
> c(1,6,5). c(2,6,5). c(3,6,5). c(4,6,6). c(5,6,6). c(6,6,6).
> EOF
clingo version 5.1.0
Reading from plant.lp ...
Solving...
Answer: 1
c(1,1,1) c(2,1,1) c(3,1,2) c(4,1,2) c(5,1,2) c(6,1,2) c(1,2,1) c(2,2,1) c(3,2,2) c(4,2,2) c(5,2,2) c(6,2,2) c(1,3,1) c(2,3,1) c(3,3,3) c(4,3,3) c(5,3,2) c(6,3,2) c(1,4,1) c(2,4,3) c(3,4,3) c(4,4,3) c(5,4,4) c(6,4,2) c(1,5,5) c(2,5,3) c(3,5,3) c(4,5,3) c(5,5,4) c(6,5,2) c(1,6,5) c(2,6,5) c(3,6,5) c(4,6,6) c(5,6,6) c(6,6,6) t(1,5) t(2,1) t(6,2) t(3,3) t(5,4) t(4,6)
SATISFIABLE

Models       : 1+
Calls        : 1
Time         : 0.045s (Solving: 0.00s 1st Model: 0.00s Unsat: 0.00s)
CPU Time     : 0.000s

To make the input/output format easier to deal with, here are Python programs to convert from and to the format given in the challenge.

Input

import sys
print(' '.join("c({},{},{}).".format(x + 1, y + 1, ord(cell) - ord('a') + 1) for y, row in enumerate(sys.stdin.read().splitlines()) for x, cell in enumerate(row)))

Output

import re
import sys
for line in sys.stdin:
    c = {(int(x), int(y)): int(z) for x, y, z in re.findall(r'\bc\((\d+),(\d+),(\d+)\)', line)}
    if c:
        t = {(int(x), int(y)) for x, y in re.findall(r'\bt\((\d+),(\d+)\)', line)}
        n, n = max(c)
        for y in range(1, n + 1):
            print(''.join(chr(ord('aA'[(x, y) in t]) + c[x, y] - 1) for x in range(1, n + 1)))
        print()