Tidy up of how group IDs are generated.

Author: simonbrowndotje

src/main/java/com/structurizr/export/mermaid/MermaidDiagramExporter.java

@@ -32,6 +32,7 @@ public MermaidDiagramExporter() {
 
     @Override
     protected void writeHeader(ModelView view, IndentingWriter writer) {
+        groupId = 0;
         String direction = "TB";
 
         if (view.getAutomaticLayout() != null) {

src/test/java/com/structurizr/export/mermaid/MermaidDiagramExporterTests.java

@@ -261,22 +261,22 @@ public void test_renderGroupStyles() {
                 "  subgraph diagram [System Landscape]\n" +
                 "    style diagram fill:#ffffff,stroke:#ffffff\n" +
                 "\n" +
-                "    subgraph group4 [Group 1]\n" +
-                "      style group4 fill:#ffffff,stroke:#111111,color:#111111\n" +
+                "    subgraph group1 [Group 1]\n" +
+                "      style group1 fill:#ffffff,stroke:#111111,color:#111111\n" +
                 "\n" +
                 "      1[\"<div style='font-weight: bold'>User 1</div><div style='font-size: 70%; margin-top: 0px'>[Person]</div>\"]\n" +
                 "      style 1 fill:#dddddd,stroke:#9a9a9a,color:#000000\n" +
                 "    end\n" +
                 "\n" +
-                "    subgraph group5 [Group 2]\n" +
-                "      style group5 fill:#ffffff,stroke:#222222,color:#222222\n" +
+                "    subgraph group2 [Group 2]\n" +
+                "      style group2 fill:#ffffff,stroke:#222222,color:#222222\n" +
                 "\n" +
                 "      2[\"<div style='font-weight: bold'>User 2</div><div style='font-size: 70%; margin-top: 0px'>[Person]</div>\"]\n" +
                 "      style 2 fill:#dddddd,stroke:#9a9a9a,color:#000000\n" +
                 "    end\n" +
                 "\n" +
-                "    subgraph group6 [Group 3]\n" +
-                "      style group6 fill:#ffffff,stroke:#aabbcc,color:#aabbcc\n" +
+                "    subgraph group3 [Group 3]\n" +
+                "      style group3 fill:#ffffff,stroke:#aabbcc,color:#aabbcc\n" +
                 "\n" +
                 "      3[\"<div style='font-weight: bold'>User 3</div><div style='font-size: 70%; margin-top: 0px'>[Person]</div>\"]\n" +
                 "      style 3 fill:#dddddd,stroke:#9a9a9a,color:#000000\n" +

src/test/java/com/structurizr/export/mermaid/groups-Components.mmd

@@ -10,8 +10,8 @@ graph TB
     subgraph 6 [F]
       style 6 fill:#ffffff,stroke:#444444,color:#444444
 
-      subgraph group4 [Group 4]
-        style group4 fill:#ffffff,stroke:#cccccc,color:#cccccc
+      subgraph group1 [Group 4]
+        style group1 fill:#ffffff,stroke:#cccccc,color:#cccccc
 
         8["<div style='font-weight: bold'>H</div><div style='font-size: 70%; margin-top: 0px'>[Component]</div>"]
         style 8 fill:#dddddd,stroke:#9a9a9a,color:#000000

src/test/java/com/structurizr/export/mermaid/groups-Containers.mmd

@@ -10,8 +10,8 @@ graph TB
     subgraph 4 [D]
       style 4 fill:#ffffff,stroke:#444444,color:#444444
 
-      subgraph group3 [Group 3]
-        style group3 fill:#ffffff,stroke:#cccccc,color:#cccccc
+      subgraph group1 [Group 3]
+        style group1 fill:#ffffff,stroke:#cccccc,color:#cccccc
 
         6["<div style='font-weight: bold'>F</div><div style='font-size: 70%; margin-top: 0px'>[Container]</div>"]
         style 6 fill:#dddddd,stroke:#9a9a9a,color:#000000