subgraphs in mermaid

README.md

@@ -119,6 +119,12 @@ g := dot.NewGraph(dot.Directed)
 fmt.Println(dot.MermaidGraph(g, dot.MermaidTopToBottom))
 ```
 
+### subgraphs in mermaid
+
+```mermaid
+flowchart LR;n8-->n3;subgraph one;n2("a1");n3("a2");n2-->n3;end;subgraph three;n8("c1");n9("c2");n8-->n9;end;subgraph two;n5("b1");n6("b2");n5-->n6;end;
+```
+
 ## extensions
 
 See also package `dot/dotx` for types that can help in constructing complex graphs.

mermaid.go

@@ -60,6 +60,17 @@ func diagram(g *Graph, diagramType string, orientation int) string {
 		sb.WriteString("TD")
 	}
 	writeEnd(sb)
+	diagramGraph(g, sb)
+	for _, id := range g.sortedSubgraphsKeys() {
+		each := g.subgraphs[id]
+		fmt.Fprintf(sb, "subgraph %s;\n", id)
+		diagramGraph(each, sb)
+		fmt.Fprintln(sb, "end;")
+	}
+	return sb.String()
+}
+
+func diagramGraph(g *Graph, sb *strings.Builder) {
 	// graph nodes
 	for _, key := range g.sortedNodesKeys() {
 		nodeShape := MermaidShapeRound
@@ -92,7 +103,6 @@ func diagram(g *Graph, diagramType string, orientation int) string {
 			}
 		}
 	}
-	return sb.String()
 }
 
 func writeEnd(sb *strings.Builder) {

mermaid_test.go

@@ -79,3 +79,21 @@ func TestUndirectedMermaid(t *testing.T) {
 		t.Errorf("got [%v]:%T want [%v]:%T", got, got, want, want)
 	}
 }
+
+// example from https://mermaid.js.org/syntax/flowchart.html
+// note that c1 and a2 are nodes created in their subgraphs to make the diagram match with the example.
+func TestMermaidSubgraph(t *testing.T) {
+	di := NewGraph(Directed)
+	sub1 := di.Subgraph("one")
+	sub1.Node("a1").Edge(sub1.Node("a2"))
+	sub2 := di.Subgraph("two")
+	sub2.Node("b1").Edge(sub2.Node("b2"))
+	sub3 := di.Subgraph("three")
+	sub3.Node("c1").Edge(sub3.Node("c2"))
+
+	sub3.Node("c1").Edge(sub1.Node("a2"))
+	mf := MermaidFlowchart(di, MermaidLeftToRight)
+	if got, want := flatten(mf), `flowchart LR;n8-->n3;subgraph one;n2("a1");n3("a2");n2-->n3;end;subgraph three;n8("c1");n9("c2");n8-->n9;end;subgraph two;n5("b1");n6("b2");n5-->n6;end;`; got != want {
+		t.Errorf("got [%[1]v:%[1]T] want [%[2]v:%[2]T]", got, want)
+	}
+}