[p1-greedy] Add option to control determinism of greedy cycle breaker (…

…fixes #12)

autolayout_options.go

@@ -32,6 +32,7 @@ var defaultOptions = options{
 	p4: positioning.SinkColoring,
 	p5: routing.Polyline,
 	params: graph.Params{
+		GreedyCycleBreakerRandomNodeChoice:       false,
 		NetworkSimplexThoroughness:               28,
 		NetworkSimplexMaxIterFactor:              0,
 		NetworkSimplexBalance:                    graph.OptionNsBalanceV,

autolayout_options_funcs.go

@@ -6,6 +6,12 @@ import (
 	imonitor "github.com/nulab/autog/internal/monitor"
 )
 
+func WithNonDeterministicGreedyCycleBreaker() Option {
+	return func(o *options) {
+		o.params.GreedyCycleBreakerRandomNodeChoice = true
+	}
+}
+
 func WithNetworkSimplexThoroughness(thoroughness uint) Option {
 	return func(o *options) {
 		o.params.NetworkSimplexThoroughness = thoroughness

autolayout_options_test.go

@@ -25,6 +25,7 @@ func TestOptions(t *testing.T) {
 		WithBrandesKoepfLayout(2),
 		WithNodeFixedSize(100.0, 100.0),
 		WithNodeSize(map[string]graph.Size{"N1": {W: 20, H: 20}}),
+		WithNonDeterministicGreedyCycleBreaker(),
 	)
 
 	assert.Equal(t, phase1.DepthFirst, opts.p1)
@@ -39,6 +40,7 @@ func TestOptions(t *testing.T) {
 	assert.Nil(t, opts.monitor)
 	assert.NotNil(t, opts.params.NodeFixedSizeFunc)
 	assert.NotNil(t, opts.params.NodeSizeFunc)
+	assert.True(t, opts.params.GreedyCycleBreakerRandomNodeChoice)
 
 	assert.Equal(t, CycleBreakingGreedy, phase1.Greedy)
 	assert.Equal(t, CycleBreakingDepthFirst, phase1.DepthFirst)

internal/graph/params.go

@@ -20,6 +20,12 @@ type Params struct {
 	// Sets a width and height to individual non-virtual nodes
 	NodeSizeFunc func(n *Node)
 
+	// ---- phase1 options ---
+
+	// Controls whether the next maximum outflow node in the greedy cycle breaker is picked at random.
+	// When this option is true, the greedy cycle breaker behaves non-deterministically.
+	GreedyCycleBreakerRandomNodeChoice bool
+
 	// ---- phase2 options ---
 
 	// Factor used in to determine the maximum number of iterations.

internal/phase1/alg_process.go

@@ -6,7 +6,7 @@ import (
 )
 
 // Process runs this cycle breaking algorithm on the input graph. The graph must be connected.
-func (alg Alg) Process(g *graph.DGraph, _ graph.Params) {
+func (alg Alg) Process(g *graph.DGraph, params graph.Params) {
 	imonitor.PrefixFor(alg)
 
 	// self-loop edges are removed from the edge list in a preprocessing step
@@ -23,7 +23,7 @@ func (alg Alg) Process(g *graph.DGraph, _ graph.Params) {
 	}
 	switch alg {
 	case Greedy:
-		execGreedy(g)
+		execGreedy(g, params)
 	case DepthFirst:
 		execDepthFirst(g)
 	default:

internal/phase1/greedy.go

@@ -27,7 +27,7 @@ type greedyProcessor struct {
 //
 // The algorithm arranges the nodes of G in an arc diagram, with source nodes to the right and sink nodes to the left.
 // Then it reverses edges that point right.
-func execGreedy(g *graph.DGraph) {
+func execGreedy(g *graph.DGraph, params graph.Params) {
 	p := greedyProcessor{
 		rnd:     rand.New(rand.NewSource(time.Now().UnixNano())),
 		arcdiag: graph.NodeIntMap{},
@@ -97,7 +97,7 @@ func execGreedy(g *graph.DGraph) {
 			}
 
 			// randomly select a node from the ones with maximal outflow and put it left
-			n := p.pickRandom(maxOutflowNodes)
+			n := p.pickNode(maxOutflowNodes, params.GreedyCycleBreakerRandomNodeChoice)
 			p.arcdiag[n] = nextLeft
 			nextLeft++
 			p.updateNeighbors(n, &sources, &sinks)
@@ -123,9 +123,11 @@ func execGreedy(g *graph.DGraph) {
 	}
 }
 
-func (p *greedyProcessor) pickRandom(nodes []*graph.Node) *graph.Node {
-	return nodes[len(nodes)/2] // todo: deterministic for debugging
-	// return nodes[p.rnd.Intn(len(nodes))]
+func (p *greedyProcessor) pickNode(nodes []*graph.Node, random bool) *graph.Node {
+	if random {
+		return nodes[p.rnd.Intn(len(nodes))]
+	}
+	return nodes[len(nodes)/2] // arbitrary deterministic choice
 }
 
 // Updates indegree and outdegree values of the neighbors of the given node,