encoding/jsonschema/ref.go - cue - Git at Google

 // Copyright 2020 CUE Authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.

 package jsonschema

 import (
 	"net/url"
 	"path"
 	"strconv"
 	"strings"

 	"cuelang.org/go/cue/ast"
 	"cuelang.org/go/cue/errors"
 	"cuelang.org/go/cue/token"
 	"cuelang.org/go/internal"
 	"cuelang.org/go/internal/legacy/cue"
 )

 func (d *decoder) parseRef(p token.Pos, str string) []string {
 	u, err := url.Parse(str)
 	if err != nil {
 		d.addErr(errors.Newf(p, "invalid JSON reference: %s", err))
 		return nil
 	}

 	if u.Host != "" || u.Path != "" {
 		d.addErr(errors.Newf(p, "external references (%s) not supported", str))
 		// TODO: handle
 		//    host:
 		//      If the host corresponds to a package known to cue,
 		//      load it from there. It would prefer schema converted to
 		//      CUE, although we could consider loading raw JSON schema
 		//      if present.
 		//      If not present, advise the user to run cue get.
 		//    path:
 		//      Look up on file system or relatively to authority location.
 		return nil
 	}

 	if !path.IsAbs(u.Fragment) {
 		d.addErr(errors.Newf(p, "anchors (%s) not supported", u.Fragment))
 		// TODO: support anchors
 		return nil
 	}

 	// NOTE: Go bug?: url.URL has no raw representation of the fragment. This
 	// means that %2F gets translated to `/` before it can be split. This, in
 	// turn, means that field names cannot have a `/` as name.

 	return splitFragment(u)
 }

 // resolveURI parses a URI from n and resolves it in the current context.
 // To resolve it in the current context, it looks for the closest URI from
 // an $id in the parent scopes and the uses the URI resolution to get the
 // new URI.
 //
 // This method is used to resolve any URI, including those from $id and $ref.
 func (s *state) resolveURI(n cue.Value) *url.URL {
 	str, ok := s.strValue(n)
 	if !ok {
 		return nil
 	}

 	u, err := url.Parse(str)
 	if err != nil {
 		s.addErr(errors.Newf(n.Pos(), "invalid JSON reference: %s", err))
 		return nil
 	}

 	for {
 		if s.id != nil {
 			u = s.id.ResolveReference(u)
 			break
 		}
 		if s.up == nil {
 			break
 		}
 		s = s.up
 	}

 	return u
 }

 const topSchema = "_schema"

 // makeCUERef converts a URI into a CUE reference for the current location.
 // The returned identifier (or first expression in a selection chain), is
 // hardwired to point to the resolved value. This will allow astutil.Sanitize
 // to automatically unshadow any shadowed variables.
 func (s *state) makeCUERef(n cue.Value, u *url.URL) ast.Expr {
 	a := splitFragment(u)

 	switch fn := s.cfg.Map; {
 	case fn != nil:
 		// TODO: This block is only used in case s.cfg.Map is set, which is
 		// currently only used for OpenAPI. Handling should be brought more in
 		// line with JSON schema.
 		a, err := fn(n.Pos(), a)
 		if err != nil {
 			s.addErr(errors.Newf(n.Pos(), "invalid reference %q: %v", u, err))
 			return nil
 		}
 		if len(a) == 0 {
 			// TODO: should we allow inserting at root level?
 			s.addErr(errors.Newf(n.Pos(),
 				"invalid empty reference returned by map for %q", u))
 			return nil
 		}
 		sel, ok := a[0].(ast.Expr)
 		if !ok {
 			sel = &ast.BadExpr{}
 		}
 		for _, l := range a[1:] {
 			switch x := l.(type) {
 			case *ast.Ident:
 				sel = &ast.SelectorExpr{X: sel, Sel: x}

 			case *ast.BasicLit:
 				sel = &ast.IndexExpr{X: sel, Index: x}
 			}
 		}
 		return sel
 	}

 	var ident *ast.Ident

 	for ; ; s = s.up {
 		if s.up == nil {
 			switch {
 			case u.Host == "" && u.Path == "",
 				s.id != nil && s.id.Host == u.Host && s.id.Path == u.Path:
 				if len(a) == 0 {
 					// refers to the top of the file. We will allow this by
 					// creating a helper schema as such:
 					//   _schema: {...}
 					//   _schema
 					// This is created at the finalization stage if
 					// hasSelfReference is set.
 					s.hasSelfReference = true

 					ident = ast.NewIdent(topSchema)
 					ident.Node = s.obj
 					return ident
 				}

 				ident, a = s.getNextIdent(n, a)

 			case u.Host != "":
 				// Reference not found within scope. Create an import reference.

 				// TODO: allow the configuration to specify a map from
 				// URI domain+paths to CUE packages.

 				// TODO: currently only $ids that are in scope can be
 				// referenced. We could consider doing an extra pass to record
 				// all '$id's in a file to be able to link to them even if they
 				// are not in scope.
 				p := u.Path

 				base := path.Base(p)
 				if !ast.IsValidIdent(base) {
 					if strings.HasSuffix(base, ".json") {
 						base = base[:len(base)-len(".json")]
 					}
 					if !ast.IsValidIdent(base) {
 						// Find something more clever to do there. For now just
 						// pick "schema" as the package name.
 						base = "schema"
 					}
 					p += ":" + base
 				}

 				ident = ast.NewIdent(base)
 				ident.Node = &ast.ImportSpec{Path: ast.NewString(u.Host + p)}

 			default:
 				// Just a path, not sure what that means.
 				s.errf(n, "unknown domain for reference %q", u)
 				return nil
 			}
 			break
 		}

 		if s.id == nil {
 			continue
 		}

 		if s.id.Host == u.Host && s.id.Path == u.Path {
 			if len(a) == 0 {
 				if len(s.idRef) == 0 {
 					// This is a reference to either root or a schema for which
 					// we do not yet support references. See Issue #386.
 					if s.up.up != nil {
 						s.errf(n, "cannot refer to internal schema %q", u)
 						return nil
 					}

 					// This is referring to the root scope. There is a dummy
 					// state above the root state that we need to update.
 					s = s.up

 					// refers to the top of the file. We will allow this by
 					// creating a helper schema as such:
 					//   _schema: {...}
 					//   _schema
 					// This is created at the finalization stage if
 					// hasSelfReference is set.
 					s.hasSelfReference = true
 					ident = ast.NewIdent(topSchema)
 					ident.Node = s.obj
 					return ident
 				}

 				x := s.idRef[0]
 				if !x.isDef && !ast.IsValidIdent(x.name) {
 					s.errf(n, "referring to field %q not supported", x.name)
 					return nil
 				}
 				e := ast.NewIdent(x.name)
 				if len(s.idRef) == 1 {
 					return e
 				}
 				return newSel(e, s.idRef[1])
 			}
 			ident, a = s.getNextIdent(n, a)
 			ident.Node = s.obj
 			break
 		}
 	}

 	return s.newSel(ident, n, a)
 }

 // getNextSelector translates a JSON Reference path into a CUE path by consuming
 // the first path elements and returning the corresponding CUE label.
 func (s *state) getNextSelector(v cue.Value, a []string) (l label, tail []string) {
 	switch elem := a[0]; elem {
 	case "$defs", "definitions":
 		if len(a) == 1 {
 			s.errf(v, "cannot refer to %s section: must refer to one of its elements", a[0])
 			return label{}, nil
 		}

 		if name := "#" + a[1]; ast.IsValidIdent(name) {
 			return label{name, true}, a[2:]
 		}

 		return label{"#", true}, a[1:]

 	case "properties":
 		if len(a) == 1 {
 			s.errf(v, "cannot refer to %s section: must refer to one of its elements", a[0])
 			return label{}, nil
 		}

 		return label{a[1], false}, a[2:]

 	default:
 		return label{elem, false}, a[1:]

 	case "additionalProperties",
 		"patternProperties",
 		"items",
 		"additionalItems":
 		// TODO: as a temporary workaround, include the schema verbatim.
 		// TODO: provide definitions for these in CUE.
 		s.errf(v, "referring to field %q not yet supported", elem)

 		// Other known fields cannot be supported.
 		return label{}, nil
 	}
 }

 // newSel converts a JSON Reference path and initial CUE identifier to
 // a CUE selection path.
 func (s *state) newSel(e ast.Expr, v cue.Value, a []string) ast.Expr {
 	for len(a) > 0 {
 		var label label
 		label, a = s.getNextSelector(v, a)
 		e = newSel(e, label)
 	}
 	return e
 }

 // newSel converts label to a CUE index and creates an expression to index
 // into e.
 func newSel(e ast.Expr, label label) ast.Expr {
 	if label.isDef {
 		return ast.NewSel(e, label.name)

 	}
 	if ast.IsValidIdent(label.name) && !internal.IsDefOrHidden(label.name) {
 		return ast.NewSel(e, label.name)
 	}
 	return &ast.IndexExpr{X: e, Index: ast.NewString(label.name)}
 }

 func (s *state) setField(lab label, f *ast.Field) {
 	x := s.getRef(lab)
 	x.field = f
 	s.setRef(lab, x)
 	x = s.getRef(lab)
 }

 func (s *state) getRef(lab label) refs {
 	if s.fieldRefs == nil {
 		s.fieldRefs = make(map[label]refs)
 	}
 	x, ok := s.fieldRefs[lab]
 	if !ok {
 		if lab.isDef ||
 			(ast.IsValidIdent(lab.name) && !internal.IsDefOrHidden(lab.name)) {
 			x.ident = lab.name
 		} else {
 			x.ident = "_X" + strconv.Itoa(s.decoder.numID)
 			s.decoder.numID++
 		}
 		s.fieldRefs[lab] = x
 	}
 	return x
 }

 func (s *state) setRef(lab label, r refs) {
 	s.fieldRefs[lab] = r
 }

 // getNextIdent gets the first CUE reference from a JSON Reference path and
 // converts it to a CUE identifier.
 func (s *state) getNextIdent(v cue.Value, a []string) (resolved *ast.Ident, tail []string) {
 	lab, a := s.getNextSelector(v, a)

 	x := s.getRef(lab)
 	ident := ast.NewIdent(x.ident)
 	x.refs = append(x.refs, ident)
 	s.setRef(lab, x)

 	return ident, a
 }

 // linkReferences resolves identifiers to relevant nodes. This allows
 // astutil.Sanitize to unshadow nodes if necessary.
 func (s *state) linkReferences() {
 	for _, r := range s.fieldRefs {
 		if r.field == nil {
 			// TODO: improve error message.
 			s.errf(cue.Value{}, "reference to non-existing value %q", r.ident)
 			continue
 		}

 		// link resembles the link value. See astutil.Resolve.
 		var link ast.Node

 		ident, ok := r.field.Label.(*ast.Ident)
 		if ok && ident.Name == r.ident {
 			link = r.field.Value
 		} else if len(r.refs) > 0 {
 			r.field.Label = &ast.Alias{
 				Ident: ast.NewIdent(r.ident),
 				Expr:  r.field.Label.(ast.Expr),
 			}
 			link = r.field
 		}

 		for _, i := range r.refs {
 			i.Node = link
 		}
 	}
 }

 // splitFragment splits the fragment part of a URI into path components. The
 // result may be an empty slice.
 //
 // TODO: this requires RawFragment introduced in go1.15 to function properly.
 // As for now, CUE still uses go1.12.
 func splitFragment(u *url.URL) []string {
 	if u.Fragment == "" {
 		return nil
 	}
 	s := strings.TrimRight(u.Fragment[1:], "/")
 	if s == "" {
 		return nil
 	}
 	return strings.Split(s, "/")
 }

 func (d *decoder) mapRef(p token.Pos, str string, ref []string) []ast.Label {
 	fn := d.cfg.Map
 	if fn == nil {
 		fn = jsonSchemaRef
 	}
 	a, err := fn(p, ref)
 	if err != nil {
 		if str == "" {
 			str = "#/" + strings.Join(ref, "/")
 		}
 		d.addErr(errors.Newf(p, "invalid reference %q: %v", str, err))
 		return nil
 	}
 	if len(a) == 0 {
 		// TODO: should we allow inserting at root level?
 		if str == "" {
 			str = "#/" + strings.Join(ref, "/")
 		}
 		d.addErr(errors.Newf(p,
 			"invalid empty reference returned by map for %q", str))
 		return nil
 	}
 	return a
 }

 func jsonSchemaRef(p token.Pos, a []string) ([]ast.Label, error) {
 	// TODO: technically, references could reference a
 	// non-definition. We disallow this case for the standard
 	// JSON Schema interpretation. We could detect cases that
 	// are not definitions and then resolve those as literal
 	// values.
 	if len(a) != 2 || (a[0] != "definitions" && a[0] != "$defs") {
 		return nil, errors.Newf(p,
 			// Don't mention the ability to use $defs, as this definition seems
 			// to already have been withdrawn from the JSON Schema spec.
 			"$ref must be of the form #/definitions/...")
 	}
 	name := a[1]
 	if ast.IsValidIdent(name) &&
 		name != rootDefs[1:] &&
 		!internal.IsDefOrHidden(name) {
 		return []ast.Label{ast.NewIdent("#" + name)}, nil
 	}
 	return []ast.Label{ast.NewIdent(rootDefs), ast.NewString(name)}, nil
 }
	// Copyright 2020 CUE Authors
	//
	// Licensed under the Apache License, Version 2.0 (the "License");
	// you may not use this file except in compliance with the License.
	// You may obtain a copy of the License at
	//
	// http://www.apache.org/licenses/LICENSE-2.0
	//
	// Unless required by applicable law or agreed to in writing, software
	// distributed under the License is distributed on an "AS IS" BASIS,
	// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	// See the License for the specific language governing permissions and
	// limitations under the License.

	package jsonschema

	import (
	"net/url"
	"path"
	"strconv"
	"strings"

	"cuelang.org/go/cue/ast"
	"cuelang.org/go/cue/errors"
	"cuelang.org/go/cue/token"
	"cuelang.org/go/internal"
	"cuelang.org/go/internal/legacy/cue"
	)

	func (d *decoder) parseRef(p token.Pos, str string) []string {
	u, err := url.Parse(str)
	if err != nil {
	d.addErr(errors.Newf(p, "invalid JSON reference: %s", err))
	return nil
	}

	if u.Host != "" \|\| u.Path != "" {
	d.addErr(errors.Newf(p, "external references (%s) not supported", str))
	// TODO: handle
	// host:
	// If the host corresponds to a package known to cue,
	// load it from there. It would prefer schema converted to
	// CUE, although we could consider loading raw JSON schema
	// if present.
	// If not present, advise the user to run cue get.
	// path:
	// Look up on file system or relatively to authority location.
	return nil
	}

	if !path.IsAbs(u.Fragment) {
	d.addErr(errors.Newf(p, "anchors (%s) not supported", u.Fragment))
	// TODO: support anchors
	return nil
	}

	// NOTE: Go bug?: url.URL has no raw representation of the fragment. This
	// means that %2F gets translated to `/` before it can be split. This, in
	// turn, means that field names cannot have a `/` as name.

	return splitFragment(u)
	}

	// resolveURI parses a URI from n and resolves it in the current context.
	// To resolve it in the current context, it looks for the closest URI from
	// an $id in the parent scopes and the uses the URI resolution to get the
	// new URI.
	//
	// This method is used to resolve any URI, including those from $id and $ref.
	func (s state) resolveURI(n cue.Value) url.URL {
	str, ok := s.strValue(n)
	if !ok {
	return nil
	}

	u, err := url.Parse(str)
	if err != nil {
	s.addErr(errors.Newf(n.Pos(), "invalid JSON reference: %s", err))
	return nil
	}

	for {
	if s.id != nil {
	u = s.id.ResolveReference(u)
	break
	}
	if s.up == nil {
	break
	}
	s = s.up
	}

	return u
	}

	const topSchema = "_schema"

	// makeCUERef converts a URI into a CUE reference for the current location.
	// The returned identifier (or first expression in a selection chain), is
	// hardwired to point to the resolved value. This will allow astutil.Sanitize
	// to automatically unshadow any shadowed variables.
	func (s state) makeCUERef(n cue.Value, u url.URL) ast.Expr {
	a := splitFragment(u)

	switch fn := s.cfg.Map; {
	case fn != nil:
	// TODO: This block is only used in case s.cfg.Map is set, which is
	// currently only used for OpenAPI. Handling should be brought more in
	// line with JSON schema.
	a, err := fn(n.Pos(), a)
	if err != nil {
	s.addErr(errors.Newf(n.Pos(), "invalid reference %q: %v", u, err))
	return nil
	}
	if len(a) == 0 {
	// TODO: should we allow inserting at root level?
	s.addErr(errors.Newf(n.Pos(),
	"invalid empty reference returned by map for %q", u))
	return nil
	}
	sel, ok := a[0].(ast.Expr)
	if !ok {
	sel = &ast.BadExpr{}
	}
	for _, l := range a[1:] {
	switch x := l.(type) {
	case *ast.Ident:
	sel = &ast.SelectorExpr{X: sel, Sel: x}

	case *ast.BasicLit:
	sel = &ast.IndexExpr{X: sel, Index: x}
	}
	}
	return sel
	}

	var ident *ast.Ident

	for ; ; s = s.up {
	if s.up == nil {
	switch {
	case u.Host == "" && u.Path == "",
	s.id != nil && s.id.Host == u.Host && s.id.Path == u.Path:
	if len(a) == 0 {
	// refers to the top of the file. We will allow this by
	// creating a helper schema as such:
	// _schema: {...}
	// _schema
	// This is created at the finalization stage if
	// hasSelfReference is set.
	s.hasSelfReference = true

	ident = ast.NewIdent(topSchema)
	ident.Node = s.obj
	return ident
	}

	ident, a = s.getNextIdent(n, a)

	case u.Host != "":
	// Reference not found within scope. Create an import reference.

	// TODO: allow the configuration to specify a map from
	// URI domain+paths to CUE packages.

	// TODO: currently only $ids that are in scope can be
	// referenced. We could consider doing an extra pass to record
	// all '$id's in a file to be able to link to them even if they
	// are not in scope.
	p := u.Path

	base := path.Base(p)
	if !ast.IsValidIdent(base) {
	if strings.HasSuffix(base, ".json") {
	base = base[:len(base)-len(".json")]
	}
	if !ast.IsValidIdent(base) {
	// Find something more clever to do there. For now just
	// pick "schema" as the package name.
	base = "schema"
	}
	p += ":" + base
	}

	ident = ast.NewIdent(base)
	ident.Node = &ast.ImportSpec{Path: ast.NewString(u.Host + p)}

	default:
	// Just a path, not sure what that means.
	s.errf(n, "unknown domain for reference %q", u)
	return nil
	}
	break
	}

	if s.id == nil {
	continue
	}

	if s.id.Host == u.Host && s.id.Path == u.Path {
	if len(a) == 0 {
	if len(s.idRef) == 0 {
	// This is a reference to either root or a schema for which
	// we do not yet support references. See Issue #386.
	if s.up.up != nil {
	s.errf(n, "cannot refer to internal schema %q", u)
	return nil
	}

	// This is referring to the root scope. There is a dummy
	// state above the root state that we need to update.
	s = s.up

	// refers to the top of the file. We will allow this by
	// creating a helper schema as such:
	// _schema: {...}
	// _schema
	// This is created at the finalization stage if
	// hasSelfReference is set.
	s.hasSelfReference = true
	ident = ast.NewIdent(topSchema)
	ident.Node = s.obj
	return ident
	}

	x := s.idRef[0]
	if !x.isDef && !ast.IsValidIdent(x.name) {
	s.errf(n, "referring to field %q not supported", x.name)
	return nil
	}
	e := ast.NewIdent(x.name)
	if len(s.idRef) == 1 {
	return e
	}
	return newSel(e, s.idRef[1])
	}
	ident, a = s.getNextIdent(n, a)
	ident.Node = s.obj
	break
	}
	}

	return s.newSel(ident, n, a)
	}

	// getNextSelector translates a JSON Reference path into a CUE path by consuming
	// the first path elements and returning the corresponding CUE label.
	func (s *state) getNextSelector(v cue.Value, a []string) (l label, tail []string) {
	switch elem := a[0]; elem {
	case "$defs", "definitions":
	if len(a) == 1 {
	s.errf(v, "cannot refer to %s section: must refer to one of its elements", a[0])
	return label{}, nil
	}

	if name := "#" + a[1]; ast.IsValidIdent(name) {
	return label{name, true}, a[2:]
	}

	return label{"#", true}, a[1:]

	case "properties":
	if len(a) == 1 {
	s.errf(v, "cannot refer to %s section: must refer to one of its elements", a[0])
	return label{}, nil
	}

	return label{a[1], false}, a[2:]

	default:
	return label{elem, false}, a[1:]

	case "additionalProperties",
	"patternProperties",
	"items",
	"additionalItems":
	// TODO: as a temporary workaround, include the schema verbatim.
	// TODO: provide definitions for these in CUE.
	s.errf(v, "referring to field %q not yet supported", elem)

	// Other known fields cannot be supported.
	return label{}, nil
	}
	}

	// newSel converts a JSON Reference path and initial CUE identifier to
	// a CUE selection path.
	func (s *state) newSel(e ast.Expr, v cue.Value, a []string) ast.Expr {
	for len(a) > 0 {
	var label label
	label, a = s.getNextSelector(v, a)
	e = newSel(e, label)
	}
	return e
	}

	// newSel converts label to a CUE index and creates an expression to index
	// into e.
	func newSel(e ast.Expr, label label) ast.Expr {
	if label.isDef {
	return ast.NewSel(e, label.name)

	}
	if ast.IsValidIdent(label.name) && !internal.IsDefOrHidden(label.name) {
	return ast.NewSel(e, label.name)
	}
	return &ast.IndexExpr{X: e, Index: ast.NewString(label.name)}
	}

	func (s state) setField(lab label, f ast.Field) {
	x := s.getRef(lab)
	x.field = f
	s.setRef(lab, x)
	x = s.getRef(lab)
	}

	func (s *state) getRef(lab label) refs {
	if s.fieldRefs == nil {
	s.fieldRefs = make(map[label]refs)
	}
	x, ok := s.fieldRefs[lab]
	if !ok {
	if lab.isDef \|\|
	(ast.IsValidIdent(lab.name) && !internal.IsDefOrHidden(lab.name)) {
	x.ident = lab.name
	} else {
	x.ident = "_X" + strconv.Itoa(s.decoder.numID)
	s.decoder.numID++
	}
	s.fieldRefs[lab] = x
	}
	return x
	}

	func (s *state) setRef(lab label, r refs) {
	s.fieldRefs[lab] = r
	}

	// getNextIdent gets the first CUE reference from a JSON Reference path and
	// converts it to a CUE identifier.
	func (s state) getNextIdent(v cue.Value, a []string) (resolved ast.Ident, tail []string) {
	lab, a := s.getNextSelector(v, a)

	x := s.getRef(lab)
	ident := ast.NewIdent(x.ident)
	x.refs = append(x.refs, ident)
	s.setRef(lab, x)

	return ident, a
	}

	// linkReferences resolves identifiers to relevant nodes. This allows
	// astutil.Sanitize to unshadow nodes if necessary.
	func (s *state) linkReferences() {
	for _, r := range s.fieldRefs {
	if r.field == nil {
	// TODO: improve error message.
	s.errf(cue.Value{}, "reference to non-existing value %q", r.ident)
	continue
	}

	// link resembles the link value. See astutil.Resolve.
	var link ast.Node

	ident, ok := r.field.Label.(*ast.Ident)
	if ok && ident.Name == r.ident {
	link = r.field.Value
	} else if len(r.refs) > 0 {
	r.field.Label = &ast.Alias{
	Ident: ast.NewIdent(r.ident),
	Expr: r.field.Label.(ast.Expr),
	}
	link = r.field
	}

	for _, i := range r.refs {
	i.Node = link
	}
	}
	}

	// splitFragment splits the fragment part of a URI into path components. The
	// result may be an empty slice.
	//
	// TODO: this requires RawFragment introduced in go1.15 to function properly.
	// As for now, CUE still uses go1.12.
	func splitFragment(u *url.URL) []string {
	if u.Fragment == "" {
	return nil
	}
	s := strings.TrimRight(u.Fragment[1:], "/")
	if s == "" {
	return nil
	}
	return strings.Split(s, "/")
	}

	func (d *decoder) mapRef(p token.Pos, str string, ref []string) []ast.Label {
	fn := d.cfg.Map
	if fn == nil {
	fn = jsonSchemaRef
	}
	a, err := fn(p, ref)
	if err != nil {
	if str == "" {
	str = "#/" + strings.Join(ref, "/")
	}
	d.addErr(errors.Newf(p, "invalid reference %q: %v", str, err))
	return nil
	}
	if len(a) == 0 {
	// TODO: should we allow inserting at root level?
	if str == "" {
	str = "#/" + strings.Join(ref, "/")
	}
	d.addErr(errors.Newf(p,
	"invalid empty reference returned by map for %q", str))
	return nil
	}
	return a
	}

	func jsonSchemaRef(p token.Pos, a []string) ([]ast.Label, error) {
	// TODO: technically, references could reference a
	// non-definition. We disallow this case for the standard
	// JSON Schema interpretation. We could detect cases that
	// are not definitions and then resolve those as literal
	// values.
	if len(a) != 2 \|\| (a[0] != "definitions" && a[0] != "$defs") {
	return nil, errors.Newf(p,
	// Don't mention the ability to use $defs, as this definition seems
	// to already have been withdrawn from the JSON Schema spec.
	"$ref must be of the form #/definitions/...")
	}
	name := a[1]
	if ast.IsValidIdent(name) &&
	name != rootDefs[1:] &&
	!internal.IsDefOrHidden(name) {
	return []ast.Label{ast.NewIdent("#" + name)}, nil
	}
	return []ast.Label{ast.NewIdent(rootDefs), ast.NewString(name)}, nil
	}