internal/mcp/jsonschema/resolve.go - tools - Git at Google

 // Copyright 2025 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // This file deals with preparing a schema for validation, including various checks,
 // optimizations, and the resolution of cross-schema references.

 package jsonschema

 import (
 	"errors"
 	"fmt"
 	"net/url"
 	"regexp"
 	"strings"
 )

 // A Resolved consists of a [Schema] along with associated information needed to
 // validate documents against it.
 // A Resolved has been validated against its meta-schema, and all its references
 // (the $ref and $dynamicRef keywords) have been resolved to their referenced Schemas.
 // Call [Schema.Resolve] to obtain a Resolved from a Schema.
 type Resolved struct {
 	root *Schema
 	// map from $ids to their schemas
 	resolvedURIs map[string]*Schema
 }

 // A Loader reads and unmarshals the schema at uri, if any.
 type Loader func(uri *url.URL) (*Schema, error)

 // Resolve resolves all references within the schema and performs other tasks that
 // prepare the schema for validation.
 //
 // baseURI can be empty, or an absolute URI (one that starts with a scheme).
 // It is resolved (in the URI sense; see [url.ResolveReference]) with root's $id property.
 // If the resulting URI is not absolute, then the schema cannot not contain relative URI references.
 //
 // loader loads schemas that are referred to by a $ref but not under root (a remote reference).
 // If nil, remote references will return an error.
 func (root *Schema) Resolve(baseURI string, loader Loader) (*Resolved, error) {
 	// There are four steps involved in preparing a schema to validate.
 	// 1. Load: read the schema from somewhere and unmarshal it.
 	//    This schema (root) may have been loaded or created in memory, but other schemas that
 	//    come into the picture in step 4 will be loaded by the given loader.
 	// 2. Check: validate the schema against a meta-schema, and perform other well-formedness checks.
 	//    Precompute some values along the way.
 	// 3. Resolve URIs: determine the base URI of the root and all its subschemas, and
 	//    resolve (in the URI sense) all identifiers and anchors with their bases. This step results
 	//    in a map from URIs to schemas within root.
 	// These three steps are idempotent. They may occur a several times on a schema, if
 	// it is loaded from several places.
 	// 4. Resolve references: all refs in the schemas are replaced with the schema they refer to.
 	var base *url.URL
 	if baseURI == "" {
 		base = &url.URL{} // so we can call ResolveReference on it
 	} else {
 		var err error
 		base, err = url.Parse(baseURI)
 		if err != nil {
 			return nil, fmt.Errorf("parsing base URI: %w", err)
 		}
 	}

 	if loader == nil {
 		loader = func(uri *url.URL) (*Schema, error) {
 			return nil, errors.New("cannot resolve remote schemas: no loader passed to Schema.Resolve")
 		}
 	}
 	r := &resolver{
 		loader: loader,
 		loaded: map[string]*Resolved{},
 	}

 	return r.resolve(root, base)
 	// TODO: before we return, throw away anything we don't need for validation.
 }

 // A resolver holds the state for resolution.
 type resolver struct {
 	loader Loader
 	// A cache of loaded and partly resolved schemas. (They may not have had their
 	// refs resolved.) The cache ensures that the loader will never be called more
 	// than once with the same URI, and that reference cycles are handled properly.
 	loaded map[string]*Resolved
 }

 func (r *resolver) resolve(s *Schema, baseURI *url.URL) (*Resolved, error) {
 	if baseURI.Fragment != "" {
 		return nil, fmt.Errorf("base URI %s must not have a fragment", baseURI)
 	}
 	if err := s.check(); err != nil {
 		return nil, err
 	}

 	m, err := resolveURIs(s, baseURI)
 	if err != nil {
 		return nil, err
 	}
 	rs := &Resolved{root: s, resolvedURIs: m}
 	// Remember the schema by both the URI we loaded it from and its canonical name,
 	// which may differ if the schema has an $id.
 	// We must set the map before calling resolveRefs, or ref cycles will cause unbounded recursion.
 	r.loaded[baseURI.String()] = rs
 	r.loaded[s.uri.String()] = rs

 	if err := r.resolveRefs(rs); err != nil {
 		return nil, err
 	}
 	return rs, nil
 }

 func (root *Schema) check() error {
 	if root == nil {
 		return errors.New("nil schema")
 	}
 	var errs []error
 	report := func(err error) { errs = append(errs, err) }

 	seen := map[*Schema]bool{}
 	for ss := range root.all() {
 		if seen[ss] {
 			// The schema graph rooted at s is not a tree, but it needs to
 			// be because we assume a unique parent when we store a schema's base
 			// in the Schema. A cycle would also put Schema.all into an infinite
 			// recursion.
 			return fmt.Errorf("schemas rooted at %s do not form a tree (saw %s twice)", root, ss)
 		}
 		seen[ss] = true
 		ss.checkLocal(report)
 	}
 	return errors.Join(errs...)
 }

 // checkLocal checks s for validity, independently of other schemas it may refer to.
 // Since checking a regexp involves compiling it, checkLocal saves those compiled regexps
 // in the schema for later use.
 // It appends the errors it finds to errs.
 func (s *Schema) checkLocal(report func(error)) {
 	addf := func(format string, args ...any) {
 		report(fmt.Errorf("jsonschema.Schema: "+format, args...))
 	}

 	if s == nil {
 		addf("nil subschema")
 		return
 	}
 	if err := s.basicChecks(); err != nil {
 		report(err)
 		return
 	}

 	// TODO: validate the schema's properties,
 	// ideally by jsonschema-validating it against the meta-schema.

 	// Some properties are present so that Schemas can round-trip, but we do not
 	// validate them.
 	// Currently, it's just the $vocabulary property.
 	// As a special case, we can validate the 2020-12 meta-schema.
 	if s.Vocabulary != nil && s.Schema != draft202012 {
 		addf("cannot validate a schema with $vocabulary")
 	}

 	// Check and compile regexps.
 	if s.Pattern != "" {
 		re, err := regexp.Compile(s.Pattern)
 		if err != nil {
 			addf("pattern: %w", err)
 		} else {
 			s.pattern = re
 		}
 	}
 	if len(s.PatternProperties) > 0 {
 		s.patternProperties = map[*regexp.Regexp]*Schema{}
 		for reString, subschema := range s.PatternProperties {
 			re, err := regexp.Compile(reString)
 			if err != nil {
 				addf("patternProperties[%q]: %w", reString, err)
 				continue
 			}
 			s.patternProperties[re] = subschema
 		}
 	}

 	// Build a set of required properties, to avoid quadratic behavior when validating
 	// a struct.
 	if len(s.Required) > 0 {
 		s.isRequired = map[string]bool{}
 		for _, r := range s.Required {
 			s.isRequired[r] = true
 		}
 	}
 }

 // resolveURIs resolves the ids and anchors in all the schemas of root, relative
 // to baseURI.
 // See https://json-schema.org/draft/2020-12/json-schema-core#section-8.2, section
 // 8.2.1.

 // TODO(jba): dynamicAnchors (§8.2.2)
 //
 // Every schema has a base URI and a parent base URI.
 //
 // The parent base URI is the base URI of the lexically enclosing schema, or for
 // a root schema, the URI it was loaded from or the one supplied to [Schema.Resolve].
 //
 // If the schema has no $id property, the base URI of a schema is that of its parent.
 // If the schema does have an $id, it must be a URI, possibly relative. The schema's
 // base URI is the $id resolved (in the sense of [url.URL.ResolveReference]) against
 // the parent base.
 //
 // As an example, consider this schema loaded from http://a.com/root.json (quotes omitted):
 //
 //	{
 //	    allOf: [
 //	        {$id: "sub1.json", minLength: 5},
 //	        {$id: "http://b.com", minimum: 10},
 //	        {not: {maximum: 20}}
 //	    ]
 //	}
 //
 // The base URIs are as follows. Schema locations are expressed in the JSON Pointer notation.
 //
 //	schema         base URI
 //	root           http://a.com/root.json
 //	allOf/0        http://a.com/sub1.json
 //	allOf/1        http://b.com (absolute $id; doesn't matter that it's not under the loaded URI)
 //	allOf/2        http://a.com/root.json (inherited from parent)
 //	allOf/2/not    http://a.com/root.json (inherited from parent)
 func resolveURIs(root *Schema, baseURI *url.URL) (map[string]*Schema, error) {
 	resolvedURIs := map[string]*Schema{}

 	var resolve func(s, base *Schema) error
 	resolve = func(s, base *Schema) error {
 		// ids are scoped to the root.
 		if s.ID != "" {
 			// A non-empty ID establishes a new base.
 			idURI, err := url.Parse(s.ID)
 			if err != nil {
 				return err
 			}
 			if idURI.Fragment != "" {
 				return fmt.Errorf("$id %s must not have a fragment", s.ID)
 			}
 			// The base URI for this schema is its $id resolved against the parent base.
 			s.uri = base.uri.ResolveReference(idURI)
 			if !s.uri.IsAbs() {
 				return fmt.Errorf("$id %s does not resolve to an absolute URI (base is %s)", s.ID, s.base.uri)
 			}
 			resolvedURIs[s.uri.String()] = s
 			base = s // needed for anchors
 		}
 		s.base = base

 		// Anchors and dynamic anchors are URI fragments that are scoped to their base.
 		// We treat them as keys in a map stored within the schema.
 		setAnchor := func(anchor string, dynamic bool) error {
 			if anchor != "" {
 				if _, ok := base.anchors[anchor]; ok {
 					return fmt.Errorf("duplicate anchor %q in %s", anchor, base.uri)
 				}
 				if base.anchors == nil {
 					base.anchors = map[string]anchorInfo{}
 				}
 				base.anchors[anchor] = anchorInfo{s, dynamic}
 			}
 			return nil
 		}

 		setAnchor(s.Anchor, false)
 		setAnchor(s.DynamicAnchor, true)

 		for c := range s.children() {
 			if err := resolve(c, base); err != nil {
 				return err
 			}
 		}
 		return nil
 	}

 	// Set the root URI to the base for now. If the root has an $id, this will change.
 	root.uri = baseURI
 	// The original base, even if changed, is still a valid way to refer to the root.
 	resolvedURIs[baseURI.String()] = root
 	if err := resolve(root, root); err != nil {
 		return nil, err
 	}
 	return resolvedURIs, nil
 }

 // resolveRefs replaces every ref in the schemas with the schema it refers to.
 // A reference that doesn't resolve within the schema may refer to some other schema
 // that needs to be loaded.
 func (r *resolver) resolveRefs(rs *Resolved) error {
 	for s := range rs.root.all() {
 		if s.Ref != "" {
 			refSchema, _, err := r.resolveRef(rs, s, s.Ref)
 			if err != nil {
 				return err
 			}
 			// Whether or not the anchor referred to by $ref fragment is dynamic,
 			// the ref still treats it lexically.
 			s.resolvedRef = refSchema
 		}
 		if s.DynamicRef != "" {
 			refSchema, frag, err := r.resolveRef(rs, s, s.DynamicRef)
 			if err != nil {
 				return err
 			}
 			if frag != "" {
 				// The dynamic ref's fragment points to a dynamic anchor.
 				// We must resolve the fragment at validation time.
 				s.dynamicRefAnchor = frag
 			} else {
 				// There is no dynamic anchor in the lexically referenced schema,
 				// so the dynamic ref behaves like a lexical ref.
 				s.resolvedDynamicRef = refSchema
 			}
 		}
 	}
 	return nil
 }

 // resolveRef resolves the reference ref, which is either s.Ref or s.DynamicRef.
 func (r *resolver) resolveRef(rs *Resolved, s *Schema, ref string) (_ *Schema, dynamicFragment string, err error) {
 	refURI, err := url.Parse(ref)
 	if err != nil {
 		return nil, "", err
 	}
 	// URI-resolve the ref against the current base URI to get a complete URI.
 	refURI = s.base.uri.ResolveReference(refURI)
 	// The non-fragment part of a ref URI refers to the base URI of some schema.
 	// This part is the same for dynamic refs too: their non-fragment part resolves
 	// lexically.
 	u := *refURI
 	u.Fragment = ""
 	fraglessRefURI := &u
 	// Look it up locally.
 	referencedSchema := rs.resolvedURIs[fraglessRefURI.String()]
 	if referencedSchema == nil {
 		// The schema is remote. Maybe we've already loaded it.
 		// We assume that the non-fragment part of refURI refers to a top-level schema
 		// document. That is, we don't support the case exemplified by
 		// http://foo.com/bar.json/baz, where the document is in bar.json and
 		// the reference points to a subschema within it.
 		// TODO: support that case.
 		if lrs := r.loaded[fraglessRefURI.String()]; lrs != nil {
 			referencedSchema = lrs.root
 		} else {
 			// Try to load the schema.
 			ls, err := r.loader(fraglessRefURI)
 			if err != nil {
 				return nil, "", fmt.Errorf("loading %s: %w", fraglessRefURI, err)
 			}
 			lrs, err := r.resolve(ls, fraglessRefURI)
 			if err != nil {
 				return nil, "", err
 			}
 			referencedSchema = lrs.root
 			assert(referencedSchema != nil, "nil referenced schema")
 		}
 	}

 	frag := refURI.Fragment
 	// Look up frag in refSchema.
 	// frag is either a JSON Pointer or the name of an anchor.
 	// A JSON Pointer is either the empty string or begins with a '/',
 	// whereas anchors are always non-empty strings that don't contain slashes.
 	if frag != "" && !strings.HasPrefix(frag, "/") {
 		info, found := referencedSchema.anchors[frag]
 		if !found {
 			return nil, "", fmt.Errorf("no anchor %q in %s", frag, s)
 		}
 		if info.dynamic {
 			dynamicFragment = frag
 		}
 		return info.schema, dynamicFragment, nil
 	}
 	// frag is a JSON Pointer.
 	s, err = dereferenceJSONPointer(referencedSchema, frag)
 	return s, "", err
 }
	// Copyright 2025 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// This file deals with preparing a schema for validation, including various checks,
	// optimizations, and the resolution of cross-schema references.

	package jsonschema

	import (
	"errors"
	"fmt"
	"net/url"
	"regexp"
	"strings"
	)

	// A Resolved consists of a [Schema] along with associated information needed to
	// validate documents against it.
	// A Resolved has been validated against its meta-schema, and all its references
	// (the $ref and $dynamicRef keywords) have been resolved to their referenced Schemas.
	// Call [Schema.Resolve] to obtain a Resolved from a Schema.
	type Resolved struct {
	root *Schema
	// map from $ids to their schemas
	resolvedURIs map[string]*Schema
	}

	// A Loader reads and unmarshals the schema at uri, if any.
	type Loader func(uri url.URL) (Schema, error)

	// Resolve resolves all references within the schema and performs other tasks that
	// prepare the schema for validation.
	//
	// baseURI can be empty, or an absolute URI (one that starts with a scheme).
	// It is resolved (in the URI sense; see [url.ResolveReference]) with root's $id property.
	// If the resulting URI is not absolute, then the schema cannot not contain relative URI references.
	//
	// loader loads schemas that are referred to by a $ref but not under root (a remote reference).
	// If nil, remote references will return an error.
	func (root Schema) Resolve(baseURI string, loader Loader) (Resolved, error) {
	// There are four steps involved in preparing a schema to validate.
	// 1. Load: read the schema from somewhere and unmarshal it.
	// This schema (root) may have been loaded or created in memory, but other schemas that
	// come into the picture in step 4 will be loaded by the given loader.
	// 2. Check: validate the schema against a meta-schema, and perform other well-formedness checks.
	// Precompute some values along the way.
	// 3. Resolve URIs: determine the base URI of the root and all its subschemas, and
	// resolve (in the URI sense) all identifiers and anchors with their bases. This step results
	// in a map from URIs to schemas within root.
	// These three steps are idempotent. They may occur a several times on a schema, if
	// it is loaded from several places.
	// 4. Resolve references: all refs in the schemas are replaced with the schema they refer to.
	var base *url.URL
	if baseURI == "" {
	base = &url.URL{} // so we can call ResolveReference on it
	} else {
	var err error
	base, err = url.Parse(baseURI)
	if err != nil {
	return nil, fmt.Errorf("parsing base URI: %w", err)
	}
	}

	if loader == nil {
	loader = func(uri url.URL) (Schema, error) {
	return nil, errors.New("cannot resolve remote schemas: no loader passed to Schema.Resolve")
	}
	}
	r := &resolver{
	loader: loader,
	loaded: map[string]*Resolved{},
	}

	return r.resolve(root, base)
	// TODO: before we return, throw away anything we don't need for validation.
	}

	// A resolver holds the state for resolution.
	type resolver struct {
	loader Loader
	// A cache of loaded and partly resolved schemas. (They may not have had their
	// refs resolved.) The cache ensures that the loader will never be called more
	// than once with the same URI, and that reference cycles are handled properly.
	loaded map[string]*Resolved
	}

	func (r resolver) resolve(s Schema, baseURI url.URL) (Resolved, error) {
	if baseURI.Fragment != "" {
	return nil, fmt.Errorf("base URI %s must not have a fragment", baseURI)
	}
	if err := s.check(); err != nil {
	return nil, err
	}

	m, err := resolveURIs(s, baseURI)
	if err != nil {
	return nil, err
	}
	rs := &Resolved{root: s, resolvedURIs: m}
	// Remember the schema by both the URI we loaded it from and its canonical name,
	// which may differ if the schema has an $id.
	// We must set the map before calling resolveRefs, or ref cycles will cause unbounded recursion.
	r.loaded[baseURI.String()] = rs
	r.loaded[s.uri.String()] = rs

	if err := r.resolveRefs(rs); err != nil {
	return nil, err
	}
	return rs, nil
	}

	func (root *Schema) check() error {
	if root == nil {
	return errors.New("nil schema")
	}
	var errs []error
	report := func(err error) { errs = append(errs, err) }

	seen := map[*Schema]bool{}
	for ss := range root.all() {
	if seen[ss] {
	// The schema graph rooted at s is not a tree, but it needs to
	// be because we assume a unique parent when we store a schema's base
	// in the Schema. A cycle would also put Schema.all into an infinite
	// recursion.
	return fmt.Errorf("schemas rooted at %s do not form a tree (saw %s twice)", root, ss)
	}
	seen[ss] = true
	ss.checkLocal(report)
	}
	return errors.Join(errs...)
	}

	// checkLocal checks s for validity, independently of other schemas it may refer to.
	// Since checking a regexp involves compiling it, checkLocal saves those compiled regexps
	// in the schema for later use.
	// It appends the errors it finds to errs.
	func (s *Schema) checkLocal(report func(error)) {
	addf := func(format string, args ...any) {
	report(fmt.Errorf("jsonschema.Schema: "+format, args...))
	}

	if s == nil {
	addf("nil subschema")
	return
	}
	if err := s.basicChecks(); err != nil {
	report(err)
	return
	}

	// TODO: validate the schema's properties,
	// ideally by jsonschema-validating it against the meta-schema.

	// Some properties are present so that Schemas can round-trip, but we do not
	// validate them.
	// Currently, it's just the $vocabulary property.
	// As a special case, we can validate the 2020-12 meta-schema.
	if s.Vocabulary != nil && s.Schema != draft202012 {
	addf("cannot validate a schema with $vocabulary")
	}

	// Check and compile regexps.
	if s.Pattern != "" {
	re, err := regexp.Compile(s.Pattern)
	if err != nil {
	addf("pattern: %w", err)
	} else {
	s.pattern = re
	}
	}
	if len(s.PatternProperties) > 0 {
	s.patternProperties = map[regexp.Regexp]Schema{}
	for reString, subschema := range s.PatternProperties {
	re, err := regexp.Compile(reString)
	if err != nil {
	addf("patternProperties[%q]: %w", reString, err)
	continue
	}
	s.patternProperties[re] = subschema
	}
	}

	// Build a set of required properties, to avoid quadratic behavior when validating
	// a struct.
	if len(s.Required) > 0 {
	s.isRequired = map[string]bool{}
	for _, r := range s.Required {
	s.isRequired[r] = true
	}
	}
	}

	// resolveURIs resolves the ids and anchors in all the schemas of root, relative
	// to baseURI.
	// See https://json-schema.org/draft/2020-12/json-schema-core#section-8.2, section
	// 8.2.1.

	// TODO(jba): dynamicAnchors (§8.2.2)
	//
	// Every schema has a base URI and a parent base URI.
	//
	// The parent base URI is the base URI of the lexically enclosing schema, or for
	// a root schema, the URI it was loaded from or the one supplied to [Schema.Resolve].
	//
	// If the schema has no $id property, the base URI of a schema is that of its parent.
	// If the schema does have an $id, it must be a URI, possibly relative. The schema's
	// base URI is the $id resolved (in the sense of [url.URL.ResolveReference]) against
	// the parent base.
	//
	// As an example, consider this schema loaded from http://a.com/root.json (quotes omitted):
	//
	// {
	// allOf: [
	// {$id: "sub1.json", minLength: 5},
	// {$id: "http://b.com", minimum: 10},
	// {not: {maximum: 20}}
	// ]
	// }
	//
	// The base URIs are as follows. Schema locations are expressed in the JSON Pointer notation.
	//
	// schema base URI
	// root http://a.com/root.json
	// allOf/0 http://a.com/sub1.json
	// allOf/1 http://b.com (absolute $id; doesn't matter that it's not under the loaded URI)
	// allOf/2 http://a.com/root.json (inherited from parent)
	// allOf/2/not http://a.com/root.json (inherited from parent)
	func resolveURIs(root Schema, baseURI url.URL) (map[string]*Schema, error) {
	resolvedURIs := map[string]*Schema{}

	var resolve func(s, base *Schema) error
	resolve = func(s, base *Schema) error {
	// ids are scoped to the root.
	if s.ID != "" {
	// A non-empty ID establishes a new base.
	idURI, err := url.Parse(s.ID)
	if err != nil {
	return err
	}
	if idURI.Fragment != "" {
	return fmt.Errorf("$id %s must not have a fragment", s.ID)
	}
	// The base URI for this schema is its $id resolved against the parent base.
	s.uri = base.uri.ResolveReference(idURI)
	if !s.uri.IsAbs() {
	return fmt.Errorf("$id %s does not resolve to an absolute URI (base is %s)", s.ID, s.base.uri)
	}
	resolvedURIs[s.uri.String()] = s
	base = s // needed for anchors
	}
	s.base = base

	// Anchors and dynamic anchors are URI fragments that are scoped to their base.
	// We treat them as keys in a map stored within the schema.
	setAnchor := func(anchor string, dynamic bool) error {
	if anchor != "" {
	if _, ok := base.anchors[anchor]; ok {
	return fmt.Errorf("duplicate anchor %q in %s", anchor, base.uri)
	}
	if base.anchors == nil {
	base.anchors = map[string]anchorInfo{}
	}
	base.anchors[anchor] = anchorInfo{s, dynamic}
	}
	return nil
	}

	setAnchor(s.Anchor, false)
	setAnchor(s.DynamicAnchor, true)

	for c := range s.children() {
	if err := resolve(c, base); err != nil {
	return err
	}
	}
	return nil
	}

	// Set the root URI to the base for now. If the root has an $id, this will change.
	root.uri = baseURI
	// The original base, even if changed, is still a valid way to refer to the root.
	resolvedURIs[baseURI.String()] = root
	if err := resolve(root, root); err != nil {
	return nil, err
	}
	return resolvedURIs, nil
	}

	// resolveRefs replaces every ref in the schemas with the schema it refers to.
	// A reference that doesn't resolve within the schema may refer to some other schema
	// that needs to be loaded.
	func (r resolver) resolveRefs(rs Resolved) error {
	for s := range rs.root.all() {
	if s.Ref != "" {
	refSchema, _, err := r.resolveRef(rs, s, s.Ref)
	if err != nil {
	return err
	}
	// Whether or not the anchor referred to by $ref fragment is dynamic,
	// the ref still treats it lexically.
	s.resolvedRef = refSchema
	}
	if s.DynamicRef != "" {
	refSchema, frag, err := r.resolveRef(rs, s, s.DynamicRef)
	if err != nil {
	return err
	}
	if frag != "" {
	// The dynamic ref's fragment points to a dynamic anchor.
	// We must resolve the fragment at validation time.
	s.dynamicRefAnchor = frag
	} else {
	// There is no dynamic anchor in the lexically referenced schema,
	// so the dynamic ref behaves like a lexical ref.
	s.resolvedDynamicRef = refSchema
	}
	}
	}
	return nil
	}

	// resolveRef resolves the reference ref, which is either s.Ref or s.DynamicRef.
	func (r resolver) resolveRef(rs Resolved, s Schema, ref string) (_ Schema, dynamicFragment string, err error) {
	refURI, err := url.Parse(ref)
	if err != nil {
	return nil, "", err
	}
	// URI-resolve the ref against the current base URI to get a complete URI.
	refURI = s.base.uri.ResolveReference(refURI)
	// The non-fragment part of a ref URI refers to the base URI of some schema.
	// This part is the same for dynamic refs too: their non-fragment part resolves
	// lexically.
	u := *refURI
	u.Fragment = ""
	fraglessRefURI := &u
	// Look it up locally.
	referencedSchema := rs.resolvedURIs[fraglessRefURI.String()]
	if referencedSchema == nil {
	// The schema is remote. Maybe we've already loaded it.
	// We assume that the non-fragment part of refURI refers to a top-level schema
	// document. That is, we don't support the case exemplified by
	// http://foo.com/bar.json/baz, where the document is in bar.json and
	// the reference points to a subschema within it.
	// TODO: support that case.
	if lrs := r.loaded[fraglessRefURI.String()]; lrs != nil {
	referencedSchema = lrs.root
	} else {
	// Try to load the schema.
	ls, err := r.loader(fraglessRefURI)
	if err != nil {
	return nil, "", fmt.Errorf("loading %s: %w", fraglessRefURI, err)
	}
	lrs, err := r.resolve(ls, fraglessRefURI)
	if err != nil {
	return nil, "", err
	}
	referencedSchema = lrs.root
	assert(referencedSchema != nil, "nil referenced schema")
	}
	}

	frag := refURI.Fragment
	// Look up frag in refSchema.
	// frag is either a JSON Pointer or the name of an anchor.
	// A JSON Pointer is either the empty string or begins with a '/',
	// whereas anchors are always non-empty strings that don't contain slashes.
	if frag != "" && !strings.HasPrefix(frag, "/") {
	info, found := referencedSchema.anchors[frag]
	if !found {
	return nil, "", fmt.Errorf("no anchor %q in %s", frag, s)
	}
	if info.dynamic {
	dynamicFragment = frag
	}
	return info.schema, dynamicFragment, nil
	}
	// frag is a JSON Pointer.
	s, err = dereferenceJSONPointer(referencedSchema, frag)
	return s, "", err
	}