There are two solutions for 7x7 grids
(not counting reflections and rotations)
I found these by brute force. Each grid can be abstracted as a permutation of 1234567, where each digit represents the column a black pearl is placed on the row correlating to the digit's position. I used this javascript page to generate a list of all 5040 permutations in a space delimited block of text. I used only the permutations that began with 1-4, because 5-7 would all be reflections of a previous permutation (in retrospect, I could have cut it off at 1-3).
As my coding skill is only slightly past the stone age, it took an embarrassingly long time to hack together code snippets from stack exchange answers to make a batch script that would write a text file for every permutation that would be properly formatted to be evaluated by Naoyuki Tamura's Masyu solver. I had to convert the space delimiters in my permutations list (permutations.txt) to newlines, which was easy enough with Notepad++.
Here is the laughable code for the batch script I used to write the puzzle files:
echo off
setlocal enableextensions enabledelayedexpansion
set /a countraw=0
for /F "tokens=*" %%a in (permutations.txt) do (
set /a countraw+=1
set count=000!countraw!
set count=!count:~-4!
set tstring=%%a
echo !count!: !tstring!
echo 7 > mgrid!count!.txt
echo 7 >> mgrid!count!.txt
set q=!tstring:~0,1!
if !q! EQU 1 echo b - - - - - - >> mgrid!count!.txt
if !q! EQU 2 echo - b - - - - - >> mgrid!count!.txt
if !q! EQU 3 echo - - b - - - - >> mgrid!count!.txt
if !q! EQU 4 echo - - - b - - - >> mgrid!count!.txt
if !q! EQU 5 echo - - - - b - - >> mgrid!count!.txt
if !q! EQU 6 echo - - - - - b - >> mgrid!count!.txt
if !q! EQU 7 echo - - - - - - b >> mgrid!count!.txt
set q=!tstring:~1,1!
if !q! EQU 1 echo b - - - - - - >> mgrid!count!.txt
if !q! EQU 2 echo - b - - - - - >> mgrid!count!.txt
if !q! EQU 3 echo - - b - - - - >> mgrid!count!.txt
if !q! EQU 4 echo - - - b - - - >> mgrid!count!.txt
if !q! EQU 5 echo - - - - b - - >> mgrid!count!.txt
if !q! EQU 6 echo - - - - - b - >> mgrid!count!.txt
if !q! EQU 7 echo - - - - - - b >> mgrid!count!.txt
set q=!tstring:~2,1!
if !q! EQU 1 echo b - - - - - - >> mgrid!count!.txt
if !q! EQU 2 echo - b - - - - - >> mgrid!count!.txt
if !q! EQU 3 echo - - b - - - - >> mgrid!count!.txt
if !q! EQU 4 echo - - - b - - - >> mgrid!count!.txt
if !q! EQU 5 echo - - - - b - - >> mgrid!count!.txt
if !q! EQU 6 echo - - - - - b - >> mgrid!count!.txt
if !q! EQU 7 echo - - - - - - b >> mgrid!count!.txt
set q=!tstring:~3,1!
if !q! EQU 1 echo b - - - - - - >> mgrid!count!.txt
if !q! EQU 2 echo - b - - - - - >> mgrid!count!.txt
if !q! EQU 3 echo - - b - - - - >> mgrid!count!.txt
if !q! EQU 4 echo - - - b - - - >> mgrid!count!.txt
if !q! EQU 5 echo - - - - b - - >> mgrid!count!.txt
if !q! EQU 6 echo - - - - - b - >> mgrid!count!.txt
if !q! EQU 7 echo - - - - - - b >> mgrid!count!.txt
set q=!tstring:~4,1!
if !q! EQU 1 echo b - - - - - - >> mgrid!count!.txt
if !q! EQU 2 echo - b - - - - - >> mgrid!count!.txt
if !q! EQU 3 echo - - b - - - - >> mgrid!count!.txt
if !q! EQU 4 echo - - - b - - - >> mgrid!count!.txt
if !q! EQU 5 echo - - - - b - - >> mgrid!count!.txt
if !q! EQU 6 echo - - - - - b - >> mgrid!count!.txt
if !q! EQU 7 echo - - - - - - b >> mgrid!count!.txt
set q=!tstring:~5,1!
if !q! EQU 1 echo b - - - - - - >> mgrid!count!.txt
if !q! EQU 2 echo - b - - - - - >> mgrid!count!.txt
if !q! EQU 3 echo - - b - - - - >> mgrid!count!.txt
if !q! EQU 4 echo - - - b - - - >> mgrid!count!.txt
if !q! EQU 5 echo - - - - b - - >> mgrid!count!.txt
if !q! EQU 6 echo - - - - - b - >> mgrid!count!.txt
if !q! EQU 7 echo - - - - - - b >> mgrid!count!.txt
set q=!tstring:~6,1!
if !q! EQU 1 echo b - - - - - - >> mgrid!count!.txt
if !q! EQU 2 echo - b - - - - - >> mgrid!count!.txt
if !q! EQU 3 echo - - b - - - - >> mgrid!count!.txt
if !q! EQU 4 echo - - - b - - - >> mgrid!count!.txt
if !q! EQU 5 echo - - - - b - - >> mgrid!count!.txt
if !q! EQU 6 echo - - - - - b - >> mgrid!count!.txt
if !q! EQU 7 echo - - - - - - b >> mgrid!count!.txt
)
Obviously, it would have been ideal to structure it as nested for's, but batch is weird about variables in for loops and I could never get any variable references to work. Perhaps someone who knows more about batch scripting can come up with a better solution, or suggest what coding environment I should have used.
In any case, I then put all the text files into a folder with the solver and another batch script, whose code follows:
echo off
echo BEGIN REPORT>report.txt
for %%a in (mgrid*.txt) do (
echo %%a
echo %%a >> report.txt
scala -cp copris-masyu-v1-1.jar masyu.Solver -o multi %%a >> report.txt
echo. >>report.txt
)
I then searched the report for the string "NumOfSolutions = 1" and rendered all of the results that weren't reflections and rotations of previous results, of which there were the two above.
By modifying the code and testing, I was able to determine that
There are no solutions for 6x6, 5x5, or 4x4 grids. N=7
EDIT:
I decided to try white pearls as well. For them, N=4, as there is one solution, not counting its reflection:
I tried 5x5, but found there were no grids that would produce a unique solution, which leads me to believe that 4x4 is the only grid size that could produce a unique solution with white pearls.
I'm curious about the upper limit of the grid size for black pearls. This would certainly require that I figure out some way to pare out all reflections and rotations from the permutation list, which would then leave me with 45360 permutations for a 9x9 grid (as 8x8 has already been proven). The solver usually finishes smaller grids in about 1 second. If this is true for 9x9 grids, it would take at least 12 hours to go through all unique 9x9, but I estimate it could easily take double that.
