jsonschema: resolve anchors

Change-Id: Ibe7d8df0a46d3e3a8b5d0651b1113a54e756c7df
Reviewed-on: https://go-review.googlesource.com/c/tools/+/669998
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

jsonschema/resolve.go

@@ -122,8 +122,7 @@ func (s *Schema) checkLocal(report func(error)) {
 // See https://json-schema.org/draft/2020-12/json-schema-core#section-8.2, section
 // 8.2.1.
 
-// TODO(jba): anchors (section 8.2.2)
-// TODO(jba): dynamicAnchors (ditto)
+// TODO(jba): dynamicAnchors (§8.2.2)
 //
 // Every schema has a base URI and a parent base URI.
 //
@@ -180,6 +179,18 @@ func resolveURIs(root *Schema, baseURI *url.URL) (map[string]*Schema, error) {
 			base = s // needed for anchors
 		}
 
+		// Anchors are URI fragments that are scoped to their base.
+		// We treat them as keys in a map stored within the schema.
+		if s.Anchor != "" {
+			if base.anchors[s.Anchor] != nil {
+				return fmt.Errorf("duplicate anchor %q in %s", s.Anchor, base.baseURI)
+			}
+			if base.anchors == nil {
+				base.anchors = map[string]*Schema{}
+			}
+			base.anchors[s.Anchor] = s
+		}
+
 		for c := range s.children() {
 			if err := resolve(c, base); err != nil {
 				return err

jsonschema/resolve_test.go

@@ -70,23 +70,36 @@ func TestResolveURIs(t *testing.T) {
 				t.Fatal(err)
 			}
 
-			want := map[string]*Schema{
+			wantIDs := map[string]*Schema{
 				baseURI:                       root,
 				"http://b.com/foo.json":       root.Items,
 				"http://b.com/bar.json":       root.Contains,
 				"http://b.com/bar.json?items": root.Contains.Items.Items,
 			}
 			if baseURI != root.ID {
-				want[root.ID] = root
+				wantIDs[root.ID] = root
+			}
+			wantAnchors := map[*Schema]map[string]*Schema{
+				root.Contains:             {"a": root.Contains, "b": root.Contains.Items},
+				root.Contains.Items.Items: {"c": root.Contains.Items.Items},
 			}
 
 			gotKeys := slices.Sorted(maps.Keys(got))
-			wantKeys := slices.Sorted(maps.Keys(want))
+			wantKeys := slices.Sorted(maps.Keys(wantIDs))
 			if !slices.Equal(gotKeys, wantKeys) {
 				t.Errorf("ID keys:\ngot  %q\nwant %q", gotKeys, wantKeys)
 			}
-			if !maps.Equal(got, want) {
-				t.Errorf("IDs:\ngot  %+v\n\nwant %+v", got, want)
+			if !maps.Equal(got, wantIDs) {
+				t.Errorf("IDs:\ngot  %+v\n\nwant %+v", got, wantIDs)
+			}
+			for s := range root.all() {
+				if want := wantAnchors[s]; want != nil {
+					if got := s.anchors; !maps.Equal(got, want) {
+						t.Errorf("anchors:\ngot  %+v\n\nwant %+v", got, want)
+					}
+				} else if s.anchors != nil {
+					t.Errorf("non-nil anchors for %s", s)
+				}
 			}
 		})
 	}

jsonschema/schema.go

@@ -114,7 +114,9 @@ type Schema struct {
 	// Otherwise, the base URI is the ID resolved against the parent's baseURI.
 	// The parent base URI at top level is where the schema was loaded from, or
 	// if not loaded, then it should be provided to Schema.Resolve.
-	baseURI           *url.URL
+	baseURI *url.URL
+	// map from anchors to subschemas
+	anchors           map[string]*Schema
 	pattern           *regexp.Regexp
 	patternProperties map[*regexp.Regexp]*Schema
 }