terraform: add TransitiveReductionTransformer

terraform/test-fixtures/transform-trans-reduce-basic/main.tf

@@ -0,0 +1,10 @@
+resource "aws_instance" "A" {}
+
+resource "aws_instance" "B" {
+    A = "${aws_instance.A.id}"
+}
+
+resource "aws_instance" "C" {
+    A = "${aws_instance.A.id}"
+    B = "${aws_instance.B.id}"
+}

terraform/transform_transitive_reduction.go

@@ -0,0 +1,20 @@
+package terraform
+
+// TransitiveReductionTransformer is a GraphTransformer that performs
+// finds the transitive reduction of the graph. For a definition of
+// transitive reduction, see Wikipedia.
+type TransitiveReductionTransformer struct{}
+
+func (t *TransitiveReductionTransformer) Transform(g *Graph) error {
+	// If the graph isn't valid, skip the transitive reduction.
+	// We don't error here because Terraform itself handles graph
+	// validation in a better way, or we assume it does.
+	if err := g.Validate(); err != nil {
+		return nil
+	}
+
+	// Do it
+	g.TransitiveReduction()
+
+	return nil
+}

terraform/transform_transitive_reduction_test.go

@@ -0,0 +1,39 @@
+package terraform
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestTransitiveReductionTransformer(t *testing.T) {
+	mod := testModule(t, "transform-trans-reduce-basic")
+
+	g := Graph{Path: RootModulePath}
+	{
+		tf := &ConfigTransformer{Module: mod}
+		if err := tf.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	{
+		transform := &TransitiveReductionTransformer{}
+		if err := transform.Transform(&g); err != nil {
+			t.Fatalf("err: %s", err)
+		}
+	}
+
+	actual := strings.TrimSpace(g.String())
+	expected := strings.TrimSpace(testTransformTransReduceBasicStr)
+	if actual != expected {
+		t.Fatalf("bad:\n\n%s", actual)
+	}
+}
+
+const testTransformTransReduceBasicStr = `
+aws_instance.A
+aws_instance.B
+  aws_instance.A
+aws_instance.C
+  aws_instance.B
+`