Mauch MIREX 2010

function [Gs, op, nodes] = mk_nbrs_of_digraph2(G0)

% MK_NBRS_OF_DIGRAPH Make all digraphs that differ from G0 by a single edge deletion, addition or reversal

% [Gs, op, nodes] = mk_nbrs_of_digraph(G0)

% op{i} = 'add', 'del', or 'rev' is the operation used to create the i'th neighbor.

% nodes(i,1:2) are the head and tail of the operated-on arc.

[I,J] = find(G0);

G0bar = setdiag(~G0, 0); % exclude self loops in graph complement

[Ibar,Jbar] = find(G0bar);

nnbrs = 2*length(I) + length(Ibar);

Gs = cell(1, nnbrs);

op = cell(1, nnbrs);

nodes = zeros(nnbrs, 2);

nbr = 1;

% all single edge deletions

for e=1:length(I)

  i = I(e); j = J(e);

  G = G0;

  G(i,j) = 0;

  Gs{nbr} = G;

  op{nbr} = 'del';

  nodes(nbr, :) = [i j];

  nbr = nbr + 1;

end

% all single edge reversals

for e=1:length(I)

  i = I(e); j = J(e);

  G = G0;

  G(i,j) = 0;

  G(j,i) = 1;

  Gs{nbr} = G;

  op{nbr} = 'rev';

  nodes(nbr, :) = [i j];

  nbr = nbr + 1;

end

[I,J] = find(~G0);

% all single edge additions

for e=1:length(I)

  i = I(e); j = J(e);

  G = G0;

  if i ~= j % don't add self loops

    G(i,j) = 1;

    Gs{nbr} = G;

    op{nbr} = 'add';

    nodes(nbr, :) = [i j];

    nbr = nbr + 1;

  end

end

assert(nnbrs == nbr-1);