json: ContextString implementation

This JSON-flavored ContextString implementation returns a chain of
property names using JavaScript-style attribute access syntax.

zcl/json/navigation.go

@@ -0,0 +1,41 @@
+package json
+
+import (
+	"strings"
+)
+
+type navigation struct {
+	root *objectVal
+}
+
+// Implementation of zcled.ContextString
+func (n navigation) ContextString(offset int) string {
+	steps := navigationStepsRev(n.root, offset)
+	if steps == nil {
+		return ""
+	}
+
+	// We built our slice backwards, so we'll reverse it in-place now.
+	half := len(steps) / 2 // integer division
+	for i := 0; i < half; i++ {
+		steps[i], steps[len(steps)-1-i] = steps[len(steps)-1-i], steps[i]
+	}
+
+	return strings.Join(steps, ".")
+}
+
+func navigationStepsRev(obj *objectVal, offset int) []string {
+	// Do any of our properties have an object that contains the target
+	// offset?
+	for k, attr := range obj.Attrs {
+		ov, ok := attr.Value.(*objectVal)
+		if !ok {
+			continue
+		}
+
+		if ov.SrcRange.ContainsOffset(offset) {
+			return append(navigationStepsRev(ov, offset), k)
+		}
+	}
+	return nil
+}

zcl/json/navigation_test.go

@@ -0,0 +1,50 @@
+package json
+
+import (
+	"strconv"
+	"testing"
+)
+
+func TestNavigationContextString(t *testing.T) {
+	src := `
+{
+  "version": 1,
+  "resource": {
+    "null_resource": {
+      "baz": {
+        "id": "foo"
+      }
+    }
+  }
+}
+`
+	file, diags := Parse([]byte(src), "test.json")
+	if len(diags) != 0 {
+		t.Errorf("Unexpected diagnostics: %#v", diags)
+	}
+	if file == nil {
+		t.Fatalf("Got nil file")
+	}
+	nav := file.Nav.(navigation)
+
+	tests := []struct {
+		Offset int
+		Want   string
+	}{
+		{0, ``},
+		{8, ``},
+		{36, `resource`},
+		{60, `resource.null_resource`},
+		{89, `resource.null_resource.baz`},
+	}
+
+	for _, test := range tests {
+		t.Run(strconv.Itoa(test.Offset), func(t *testing.T) {
+			got := nav.ContextString(test.Offset)
+
+			if got != test.Want {
+				t.Errorf("wrong result\ngot:  %s\nwant: %s", got, test.Want)
+			}
+		})
+	}
+}

zcl/json/public.go

@@ -34,6 +34,7 @@ func Parse(src []byte, filename string) (*zcl.File, zcl.Diagnostics) {
 				obj: rootNode.(*objectVal),
 			},
 			Bytes: src,
+			Nav:   navigation{rootNode.(*objectVal)},
 		}
 	}
 	return file, diags