function [ success, trail ] = digraph_arc_euler ( nnode, nedge, inode, jnode )
%*****************************************************************************80
%
%% digraph_arc_euler() returns an Euler circuit in a digraph.
%
% Description:
%
% An Euler circuit of a digraph is a path which starts and ends at
% the same node and uses each directed edge exactly once. A digraph is
% eulerian if it has an Euler circuit. The problem is to decide whether
% a given digraph is eulerian and to find an Euler circuit if the
% answer is affirmative.
%
% A digraph has an Euler circuit if and only if the number of incoming
% edges is equal to the number of outgoing edges at each node.
%
% This characterization gives a straightforward procedure to decide whether
% a digraph is eulerian. Furthermore, an Euler circuit in an eulerian
% digraph G of NEDGE edges can be determined by the following method:
%
% STEP 1: Choose any node U as the starting node, and traverse any edge
% ( U, V ) incident to node U, and than traverse any unused edge incident
% to node U. Repeat this process of traversing unused edges until the
% starting node U is reached. Let P be the resulting walk consisting of
% all used edges. If all edges of G are in P, than stop.
%
% STEP 2: Choose any unused edge ( X, Y) in G such that X is
% in P and Y is not in P. Use node X as the starting node and
% find another walk Q using all unused edges as in step 1.
%
% STEP 3: Walk P and walk Q share a common node X, they can be merged
% to form a walk R by starting at any node S of P and to traverse P
% until node X is reached; than, detour and traverse all edges of Q
% until node X is reached and continue to traverse the edges of P until
% the starting node S is reached. Set P = R.
%
% STEP 4: Repeat steps 2 and 3 until all edges are used.
%
% The running time of the algorithm is O ( NEDGE ).
%
% The digraph is assumed to be connected.
%
% Licensing:
%
% This code is distributed under the MIT license.
%
% Modified:
%
% 19 July 2004
%
% Author:
%
% Original FORTRAN77 version by Hang Tong Lau.
% This version by John Burkardt
%
% Reference:
%
% Hang Tong Lau,
% Algorithms on Graphs,
% Tab Books, 1989.
%
% Input:
%
% integer NNODE, the number of nodes.
%
% integer NEDGE, the number of edges.
%
% integer INODE(NEDGE), JNODE(NEDGE); the I-th edge starts at node
% INODE(I) and ends at node JNODE(I).
%
% Output:
%
% logical SUCCESS, is TRUE if an Euler circuit was found,
% and FALSE otherwise.
%
% integer TRAIL(NEDGE). TRAIL(I) is the edge number of the I-th
% edge in the Euler circuit.
%
%
% Check if the digraph is eulerian.
%
trail(1:nedge) = 0;
endnod(1:nedge) = 0;
for i = 1 : nedge
j = inode(i);
trail(j) = trail(j) + 1;
j = jnode(i);
endnod(j) = endnod(j) + 1;
end
for i = 1 : nnode
if ( trail(i) ~= endnod(i) )
success = 0;
return
end
end
%
% The digraph is eulerian; find an Euler circuit.
%
success = 1;
lensol = 1;
lenstk = 0;
%
% Find the next edge.
%
stacks = zeros ( 100, 1 );
while ( true )
if ( lensol == 1 )
endnod(1) = inode(1);
stacks(1) = 1;
stacks(2) = 1;
lenstk = 2;
else
l = lensol - 1;
if ( lensol ~= 2 )
endnod(l) = inode(trail(l)) + jnode(trail(l)) - endnod(l-1);
end
k = endnod(l);
for i = 1 : nedge
candid(i) = ( k == jnode(i) );
end
for i = 1 : l
candid(trail(i)) = 0;
end
len = lenstk;
for i = 1 : nedge
if ( candid(i) )
len = len + 1;
stacks(len) = i;
end
end
stacks(len+1) = len - lenstk;
lenstk = len + 1;
end
while ( true )
istak = stacks(lenstk);
lenstk = lenstk - 1;
if ( istak ~= 0 )
break
end
lensol = lensol - 1;
if ( lensol == 0 )
trail = i4vec_reverse ( nedge, trail );
return
end
end
trail(lensol) = stacks(lenstk);
stacks(lenstk) = istak - 1;
if ( lensol == nedge )
break
end
lensol = lensol + 1;
end
trail = i4vec_reverse ( nedge, trail );
return
end