Adjacent table for 2D Ising model

ising2d.jl

function ising2d(N; BC="PBC")
   J = zeros(N^2, N^2)
   for i in 1:N^2-1
     i₁, i₂ = i%N, i÷N
     if i₁==0
       if BC == "PBC"
         J[1+(i₂-1)*N,i] = J[i,1+(i₂-1)*N] = 1.0
       end
       J[(i₂+1)*N,i] = J[i,(i₂+1)*N] = 1.0
       continue
     end
     if i₂==N-1
       if BC == "PBC"
         J[i₁,i] = J[i,i₁] = 1.0
       end
       J[i+1,i] = J[i,i+1] = 1.0
       continue
     end
     J[i,i+1] = J[i+1,i] = 1.0
     J[i,i₁+(i₂+1)*N] = J[i₁+(i₂+1)*N,i] = 1.0
   end
   if BC == "PBC"
     J[N,N^2] = J[N^2,N] = 1.0
     J[N^2-N+1,N^2] = J[N^2,N^2-N+1] = 1.0
   end
     return J
   end