package main

// The five settings derived from the desktop nodes' profiles and proposed for the operator's Approve, one layer
// at a time (novox/hq ADR 0329 §10, §12).
//
// betterbird_derive asks every desktop node that holds Betterbird for what its profile holds (its own derive with
// here), and lays the answers out per layer: an entry found on every such node goes into the mesh-wide layer, with
// the value of the node the derive was called on, and the answer lists every field another node holds differently,
// which then shows as drift there; an entry found on some nodes only goes into each of those nodes' layers, so
// nothing is added to another node until the operator moves it. With propose it proposes the next layer through
// the controller's settings verb: first the mesh-wide one, then each node's that needs one, each only once no
// proposal of the module's is open, against the layer as it then stands, which it carries whole beside the derived
// kinds (a layer is replaced whole). The ask shows the values in their shape (ADR 0277 §6); the operator reads them
// whole with `settings proposals <id>` at their desk.

import (
	"encoding/json"
	"errors"
	"fmt"
	"regexp"
	"sort"
	"strings"
)

// derivedEntry is one entry a node's profile holds, in the setting's shape. Match is what makes two nodes' entries
// the same one: an identity's account and address, a calendar's or an address book's address, a tag's key, an
// account's name for its filters.
type derivedEntry struct {
	Match string          `json:"match"`
	Name  string          `json:"name"`
	Value json.RawMessage `json:"value"`
}

// nodeDerivation is what one node's profile holds of the five kinds.
type nodeDerivation struct {
	Node  string                    `json:"node"`
	Kinds map[string][]derivedEntry `json:"kinds"`
	// NotDerived names what a setting does not hold, by name and why: Owl's, an ICS calendar, a local address
	// book, an address carrying a password, a filter a setting cannot say.
	NotDerived []string `json:"not_derived,omitempty"`
}

var nameChars = regexp.MustCompile(`[^a-z0-9]`)

// entryNameFor is a name of the operator's kind for an entry: lower-case letters and digits from its words,
// unique among those used.
func entryNameFor(words string, fallback string, used map[string]bool) string {
	base := nameChars.ReplaceAllString(strings.ToLower(words), "")
	if base == "" || base[0] < 'a' || base[0] > 'z' {
		base = fallback + base
	}
	if len(base) > 28 {
		base = base[:28]
	}
	name := base
	for i := 2; used[name]; i++ {
		name = fmt.Sprintf("%s%d", base, i)
	}
	used[name] = true
	return name
}

func raw(v any) json.RawMessage {
	b, _ := json.Marshal(v)
	return b
}

// deriveHere reads this node's profile into the five kinds.
func (m *Machine) deriveHere(kinds []string) (nodeDerivation, error) {
	d := nodeDerivation{Node: nodeName(), Kinds: map[string][]derivedEntry{}}
	p, err := m.profile()
	if err != nil {
		return d, err
	}
	prefs := mergedPrefs(p.path)
	accounts, _, err := m.readAccounts()
	if err != nil {
		return d, err
	}
	nameOf := map[string]string{}
	serverOf := map[string]string{}
	for _, r := range resolveAccounts(accounts, prefs, m.readRecord(p.Dir)) {
		if r.How != refused {
			nameOf[r.Keys.Account], serverOf[r.name] = r.name, r.Keys.Server
		}
	}
	want := map[string]bool{}
	for _, k := range kinds {
		want[k] = true
	}
	// Identities, by their account's name; the default carries no name or address (the setting accounts' own).
	identityName := map[string]string{}
	used := map[string]bool{}
	byAccount := map[string]bool{}
	for _, pi := range m.profileIdentities(prefs) {
		account, declared := nameOf[pi.AccountKey]
		switch {
		case pi.Owl:
			if pi.Default {
				d.NotDerived = append(d.NotDerived, fmt.Sprintf("identities: %s <%s> is Owl's (novox/hq ADR 0316 §13)",
					pi.Value.Name, pi.Value.Address))
			}
			continue
		case !declared:
			if !byAccount[pi.AccountKey] {
				byAccount[pi.AccountKey] = true
				d.NotDerived = append(d.NotDerived, fmt.Sprintf("identities: those of the profile's account %s, which the "+
					"setting accounts does not hold", pi.AccountKey))
			}
			continue
		}
		v := pi.Value
		v.Account = account
		e := derivedEntry{}
		if pi.Default {
			v.Address, v.Name = "", ""
			e.Match, e.Name = account+"|", entryNameFor(account, "id", used)
		} else {
			local, _, _ := strings.Cut(v.Address, "@")
			e.Match, e.Name = account+"|"+strings.ToLower(v.Address), entryNameFor(account+local, "id", used)
		}
		identityName[pi.Key] = e.Name
		e.Value = raw(v)
		if want[kindIdentities] {
			d.Kinds[kindIdentities] = append(d.Kinds[kindIdentities], e)
		}
	}
	// Calendars: CalDAV only; Owl's are listed, an ICS one by its name only, a local one by its name (§13).
	used = map[string]bool{}
	for _, pc := range profileCalendars(prefs) {
		if !want[kindCalendars] {
			break
		}
		switch {
		case pc.Kind == "owl" || strings.HasPrefix(pc.Kind, "owl"):
			d.NotDerived = append(d.NotDerived, fmt.Sprintf("calendars: %s (%s) is Owl's", pc.Value.Name, pc.Value.URL))
			continue
		case pc.Kind == "ics":
			d.NotDerived = append(d.NotDerived, fmt.Sprintf("calendars: %s is subscribed by an ICS address, which is "+
				"often itself the secret: listed by its name only, never declared (novox/hq ADR 0329 §13)", pc.Value.Name))
			continue
		case pc.Kind != "caldav":
			d.NotDerived = append(d.NotDerived, fmt.Sprintf("calendars: %s is %s, kept in the profile: its events are never declared",
				pc.Value.Name, pc.Kind))
			continue
		case addressPassword(pc.Value.URL):
			d.NotDerived = append(d.NotDerived, fmt.Sprintf("calendars: %s at %s carries a password in its address: refused "+
				"(novox/hq ADR 0329 §2)", pc.Value.Name, withoutPassword(pc.Value.URL)))
			continue
		}
		v := pc.Value
		v.Invitations = identityName[pc.IdentityKey]
		d.Kinds[kindCalendars] = append(d.Kinds[kindCalendars], derivedEntry{Match: strings.ToLower(strings.TrimRight(v.URL, "/")),
			Name: entryNameFor(v.Name, "calendar", used), Value: raw(v)})
	}
	// Address books: CardDAV only.
	used = map[string]bool{}
	for _, b := range profileAddressBooks(prefs) {
		if !want[kindAddressBooks] {
			break
		}
		switch {
		case !b.CardDAV:
			d.NotDerived = append(d.NotDerived, fmt.Sprintf("address-books: %s is %s: its contacts are never declared", b.Value.Name, b.Kind))
			continue
		case addressPassword(b.Value.URL):
			d.NotDerived = append(d.NotDerived, fmt.Sprintf("address-books: %s at %s carries a password in its address: "+
				"refused (novox/hq ADR 0329 §2)", b.Value.Name, withoutPassword(b.Value.URL)))
			continue
		}
		d.Kinds[kindAddressBooks] = append(d.Kinds[kindAddressBooks], derivedEntry{
			Match: strings.ToLower(strings.TrimRight(b.Value.URL, "/")), Name: entryNameFor(b.Value.Name, "book", used), Value: raw(b.Value)})
	}
	// Tags, by their own key.
	if want[kindTags] {
		for _, t := range profileTags(prefs) {
			d.Kinds[kindTags] = append(d.Kinds[kindTags], derivedEntry{Match: t.Key, Name: t.Key, Value: raw(t.Value)})
		}
	}
	// Filters of each declared account whose file holds one or more: an account with none keeps its file as the
	// person's until a setting names it.
	if want[kindFilters] {
		for _, name := range sortedKeys(serverOf) {
			dir := m.accountDirectory(p.path, prefs, serverOf[name])
			if dir == "" {
				continue
			}
			text, err := readBounded(dir + "/" + filterFileName)
			if err != nil {
				continue
			}
			parsed := parseFilters(string(text), folderBase(prefs, serverOf[name]))
			for _, why := range parsed.NotDeclarable {
				d.NotDerived = append(d.NotDerived, "filters of "+name+": "+why)
			}
			if len(parsed.Filters) > 0 {
				d.Kinds[kindFilters] = append(d.Kinds[kindFilters], derivedEntry{Match: name, Name: name, Value: raw(parsed.Filters)})
			}
		}
	}
	return d, nil
}

// difference is one field another node holds differently from the value the mesh-wide layer gets.
type difference struct {
	Kind     string `json:"kind"`
	Entry    string `json:"entry"`
	Node     string `json:"node"`
	Field    string `json:"field"`
	MeshWide any    `json:"mesh_wide"`
	There    any    `json:"there"`
}

// DeriveAnswer is what betterbird_derive answers its caller: the values it derived, which ADR 0329 §10 lets it show.
type DeriveAnswer struct {
	// Here is this node's own derivation, when asked with here.
	Here *nodeDerivation `json:"here,omitempty"`
	// From are the desktop nodes derived from, and NotHeard those holding Betterbird that did not answer.
	From     []string `json:"derived_from,omitempty"`
	NotHeard []string `json:"not_heard,omitempty"`
	// Layers are the derived values per layer: "mesh-wide", then each node's own.
	Layers      map[string]map[string]any `json:"layers,omitempty"`
	Differences []difference              `json:"differences,omitempty"`
	NotDerived  map[string][]string       `json:"not_derived,omitempty"`
	// Next is the layer a propose proposes now, or why none.
	Next     string `json:"next,omitempty"`
	Proposed string `json:"proposed,omitempty"`
	Note     string `json:"note,omitempty"`
}

const meshWide = "mesh-wide"

// Derive derives the kinds asked for (all five when none) from the desktop nodes, and with propose proposes the
// next layer. prefs are values for the setting prefs to carry in the mesh-wide layer (issue 586: the operator's
// setup moved out of the catalogue); layer names the layer to propose instead of the next in order.
func (m *Machine) Derive(kinds []string, here, propose bool, prefs map[string]any, layer string) (DeriveAnswer, error) {
	var a DeriveAnswer
	if len(kinds) == 0 {
		kinds = allKinds
	}
	for _, k := range kinds {
		if !contains(allKinds, k) {
			return a, fmt.Errorf("kinds are %s; %q is not one", strings.Join(allKinds, ", "), k)
		}
	}
	if here {
		d, err := m.deriveHere(kinds)
		if err != nil {
			return a, err
		}
		a.Here = &d
		return a, nil
	}
	self := nodeName()
	if self == "" {
		return a, errors.New("this runtime does not say which machine it serves (MESH_NODE), so the node derived on is unknown")
	}
	holders, err := holdersOfBetterbird()
	if err != nil {
		return a, err
	}
	derived := map[string]nodeDerivation{}
	for _, n := range holders {
		if n == self {
			d, err := m.deriveHere(kinds)
			if err != nil {
				return a, fmt.Errorf("this node's profile: %v", err)
			}
			derived[n] = d
			continue
		}
		d, err := deriveOn(n, kinds)
		if err != nil {
			a.NotHeard = append(a.NotHeard, n+": "+err.Error())
			continue
		}
		derived[n] = d
	}
	if _, ok := derived[self]; !ok {
		return a, fmt.Errorf("%s does not hold Betterbird as the controller says: derive on a desktop node", self)
	}
	a.From = sortedKeys(derived)
	a.Layers, a.Differences = layOut(self, derived, kinds)
	a.NotDerived = map[string][]string{}
	for n, d := range derived {
		if len(d.NotDerived) > 0 {
			a.NotDerived[n] = d.NotDerived
		}
	}
	if len(a.NotDerived) == 0 {
		a.NotDerived = nil
	}
	if len(prefs) > 0 {
		a.Layers[meshWide]["prefs"] = prefs
	}
	a.Note = "the values each layer would get; an entry on every desktop node is mesh-wide with this node's value, one " +
		"on some nodes only is in those nodes' own layers. With propose, the next layer is proposed for the operator's " +
		"Approve, carrying the layer as it stands beside these kinds; nothing changes until then (novox/hq ADR 0329 §10)"
	if !propose {
		return a, nil
	}
	if len(a.NotHeard) > 0 {
		return a, fmt.Errorf("nothing was proposed: %d desktop node(s) holding Betterbird did not answer (%s), and a "+
			"layer derived without them would put on every node what only some hold; derive again when they are awake",
			len(a.NotHeard), strings.Join(a.NotHeard, "; "))
	}
	if open, err := openProposals(); err != nil {
		return a, fmt.Errorf("nothing was proposed: whether a proposal of the module's is open cannot be read: %v", err)
	} else if len(open) > 0 {
		return a, fmt.Errorf("nothing was proposed: the module's proposal %s waits for the operator; the next layer is "+
			"proposed once it is set or declined (novox/hq ADR 0329 §10, ADR 0277 §10)", strings.Join(open, ", "))
	}
	order := []string{meshWide}
	for _, n := range sortedKeys(a.Layers) {
		if n != meshWide {
			order = append(order, n)
		}
	}
	if layer != "" {
		if _, ok := a.Layers[layer]; !ok {
			return a, fmt.Errorf("nothing was proposed: layer is %s", strings.Join(order, " or "))
		}
		order = []string{layer}
	}
	for _, l := range order {
		node := l
		if l == meshWide {
			node = ""
		}
		current, err := readLayer(node)
		if err != nil {
			return a, fmt.Errorf("nothing was proposed: %v", err)
		}
		next := withKinds(current, a.Layers[l])
		if l == meshWide {
			if prefs, ok := next["prefs"]; ok {
				body, _ := json.Marshal(map[string]any{"prefs": prefs})
				if _, err := parseDeclaredPrefs(body); err != nil {
					return a, fmt.Errorf("nothing was proposed: %v", err)
				}
			}
		}
		if err := proposable(next, l == meshWide); err != nil {
			return a, fmt.Errorf("nothing was proposed for %s: %v", l, err)
		}
		if mustJSON(next) == mustJSON(current) {
			continue
		}
		a.Next = l
		said, err := proposeLayer(node, next, true)
		a.Proposed = m.maskValue(said)
		if err != nil {
			return a, err
		}
		a.Note = "proposed the " + l + " layer for the operator's Approve; the next layer is proposed by calling again " +
			"once this one is set or declined"
		return a, nil
	}
	a.Next = "none: every layer already holds what was derived"
	return a, nil
}

// proposable checks a layer's derived kinds as the module would read them, so the operator is never asked to set
// a value the module then refuses. A key to adopt names one node's profile, so the mesh-wide layer holds none (§4).
func proposable(layer map[string]any, mesh bool) error {
	conf := map[string]any{}
	for _, k := range []string{kindIdentities, kindCalendars, kindAddressBooks, kindFilters} {
		if v, ok := layer[k]; ok {
			conf[k] = v
		}
	}
	var c Configuration
	if err := decodeInto([]byte(mustJSON(conf)), configurationFile, &c); err != nil {
		return err
	}
	if v, ok := layer[kindTags]; ok {
		if err := decodeInto([]byte(mustJSON(map[string]any{kindTags: v})), tagsFile, &c); err != nil {
			return err
		}
	}
	// A node's layer may name an identity the mesh-wide layer holds for a calendar's invitations: that is judged
	// where the layers meet, on the node.
	var problems []string
	if mesh {
		for name, id := range c.Identities {
			if id != nil && id.Key != "" {
				problems = append(problems, "identities "+name+": a key is given in a node's own layer only")
			}
		}
		for name, cal := range c.Calendars {
			if cal != nil && cal.Key != "" {
				problems = append(problems, "calendars "+name+": a key is given in a node's own layer only")
			}
		}
		for name, ab := range c.AddressBooks {
			if ab != nil && ab.Key != "" {
				problems = append(problems, "address-books "+name+": a key is given in a node's own layer only")
			}
		}
	}
	for _, p := range configurationProblems(c) {
		if !strings.Contains(p, "invitations names") {
			problems = append(problems, p)
		}
	}
	if len(problems) > 0 {
		return errors.New(strings.Join(problems, "; "))
	}
	return nil
}

// layOut puts each kind's entries into the layers (§10).
func layOut(self string, derived map[string]nodeDerivation, kinds []string) (map[string]map[string]any, []difference) {
	layers := map[string]map[string]any{meshWide: {}}
	var diffs []difference
	nodes := sortedKeys(derived)
	for _, kind := range kinds {
		type seen struct {
			by map[string]derivedEntry
		}
		groups := map[string]*seen{}
		var order []string
		for _, n := range append([]string{self}, nodes...) {
			for _, e := range derived[n].Kinds[kind] {
				g := groups[e.Match]
				if g == nil {
					g = &seen{by: map[string]derivedEntry{}}
					groups[e.Match] = g
					order = append(order, e.Match)
				}
				if _, already := g.by[n]; !already {
					g.by[n] = e
				}
			}
		}
		mesh := map[string]any{}
		used := map[string]bool{}
		for _, match := range order {
			g := groups[match]
			if len(g.by) == len(nodes) {
				mine := g.by[self]
				name := uniqueName(mine.Name, used)
				var v any
				_ = json.Unmarshal(mine.Value, &v)
				mesh[name] = v
				for _, n := range nodes {
					if n == self {
						continue
					}
					diffs = append(diffs, fieldDifferences(kind, name, n, mine.Value, g.by[n].Value)...)
				}
			}
		}
		for _, match := range order {
			g := groups[match]
			if len(g.by) == len(nodes) {
				continue
			}
			for _, n := range sortedKeys(g.by) {
				e := g.by[n]
				if layers[n] == nil {
					layers[n] = map[string]any{}
				}
				nodeKind, _ := layers[n][kind].(map[string]any)
				if nodeKind == nil {
					nodeKind = map[string]any{}
					layers[n][kind] = nodeKind
				}
				var v any
				_ = json.Unmarshal(e.Value, &v)
				nodeKind[uniqueName(e.Name, used)] = v
			}
		}
		if len(mesh) > 0 {
			layers[meshWide][kind] = mesh
		}
	}
	return layers, diffs
}

func uniqueName(name string, used map[string]bool) string {
	n := name
	for i := 2; used[n]; i++ {
		n = fmt.Sprintf("%s%d", name, i)
	}
	used[n] = true
	return n
}

// fieldDifferences are the fields a node holds differently from the mesh-wide value of one entry.
func fieldDifferences(kind, entry, node string, mesh, there json.RawMessage) []difference {
	var a, b map[string]any
	if json.Unmarshal(mesh, &a) != nil || json.Unmarshal(there, &b) != nil {
		if mustJSON(json.RawMessage(mesh)) != mustJSON(json.RawMessage(there)) {
			var x, y any
			_ = json.Unmarshal(mesh, &x)
			_ = json.Unmarshal(there, &y)
			return []difference{{Kind: kind, Entry: entry, Node: node, Field: "the whole list", MeshWide: x, There: y}}
		}
		return nil
	}
	var out []difference
	for _, f := range sortedKeys(mergeKeys(a, b)) {
		if mustJSON(a[f]) != mustJSON(b[f]) {
			out = append(out, difference{Kind: kind, Entry: entry, Node: node, Field: f, MeshWide: a[f], There: b[f]})
		}
	}
	return out
}

func mergeKeys(a, b map[string]any) map[string]bool {
	out := map[string]bool{}
	for k := range a {
		out[k] = true
	}
	for k := range b {
		out[k] = true
	}
	return out
}

// withKinds is a layer with the derived kinds in place of its own, every other key kept.
func withKinds(layer map[string]any, kinds map[string]any) map[string]any {
	out := map[string]any{}
	for k, v := range layer {
		out[k] = v
	}
	for k, v := range kinds {
		if k == "prefs" {
			merged := map[string]any{}
			if was, ok := out["prefs"].(map[string]any); ok {
				for n, x := range was {
					merged[n] = x
				}
			}
			for n, x := range v.(map[string]any) {
				merged[n] = x
			}
			out[k] = merged
			continue
		}
		out[k] = v
	}
	return out
}

func contains(list []string, s string) bool {
	for _, x := range list {
		if x == s {
			return true
		}
	}
	return false
}

// readLayer is the module's layer as the controller holds it: the mesh-wide one for node "", else that node's.
func readLayer(node string) (map[string]any, error) {
	args := map[string]any{"module": "betterbird"}
	if node != "" {
		args["node"] = node
	}
	raw, err := asker("seat:mesh-controller.settings", args)
	if err != nil {
		return nil, fmt.Errorf("the controller's settings verb did not answer: %v", err)
	}
	said, ok := verbSaid(raw)
	if !ok {
		return nil, fmt.Errorf("the controller did not answer the layer: %s", said)
	}
	// A missing layer is said in exactly these words; anything else must read as the layer's JSON. A proposal
	// replaces the layer whole, so an answer read wrongly as empty would drop what the layer holds: refused, loudly.
	text := strings.TrimSpace(said)
	where := "the whole mesh"
	if node != "" {
		where = node
	}
	if strings.HasPrefix(text, "betterbird on "+where+": no layer") {
		return map[string]any{}, nil
	}
	out := map[string]any{}
	if err := json.NewDecoder(strings.NewReader(text)).Decode(&out); err != nil || !strings.HasPrefix(text, "{") {
		first, _, _ := strings.Cut(text, "\n")
		return nil, fmt.Errorf("the controller's answer for the layer of betterbird on %s cannot be read as the layer "+
			"(it begins %q), so nothing is proposed: a layer is replaced whole and would lose what it holds", where, first)
	}
	return out, nil
}

// proposeLayer proposes a layer whole for the operator's Approve (ADR 0277); replace says a key the layer had
// and the proposal does not hold is meant to go, which a re-derive needs when an entry left every profile.
func proposeLayer(node string, values map[string]any, replace bool) (string, error) {
	body, err := json.Marshal(values)
	if err != nil {
		return "", err
	}
	args := map[string]any{"module": "betterbird", "values": string(body), "propose": "true"}
	if replace {
		args["replace"] = "true"
	}
	if node != "" {
		args["node"] = node
	}
	raw, err := asker("seat:mesh-controller.settings", args)
	if err != nil {
		return "", fmt.Errorf("the controller's settings verb did not answer: %v", err)
	}
	said, ok := verbSaid(raw)
	if !ok {
		return said, fmt.Errorf("the controller did not take the proposal: %s", said)
	}
	return said, nil
}

var (
	proposalHead = regexp.MustCompile(`^(\S+)\s+(?:set|clear) betterbird on (.+?)\s+\(`)
	waitingLine  = regexp.MustCompile(`^\s+waiting for the operator until`)
)

// openProposals are the module's proposals that wait for the operator, by id and layer.
func openProposals() ([]string, error) {
	raw, err := asker("seat:mesh-controller.settings", map[string]any{"proposals": "true"})
	if err != nil {
		return nil, err
	}
	said, ok := verbSaid(raw)
	if !ok {
		return nil, errors.New(said)
	}
	var out []string
	current := ""
	for _, line := range strings.Split(said, "\n") {
		if mm := proposalHead.FindStringSubmatch(line); mm != nil {
			current = mm[1] + " (" + mm[2] + ")"
			continue
		}
		if !strings.HasPrefix(line, " ") {
			current = ""
		}
		if current != "" && waitingLine.MatchString(line) {
			out = append(out, current)
			current = ""
		}
	}
	return out, nil
}

// holdersOfBetterbird are the nodes the controller has assigned the module to: the desktop nodes.
func holdersOfBetterbird() ([]string, error) {
	raw, err := asker("seat:mesh-controller.nodes", map[string]any{})
	if err != nil {
		return nil, fmt.Errorf("the controller's nodes verb did not answer: %v", err)
	}
	said, _ := verbSaid(raw)
	var nodes []struct {
		Name string `json:"name"`
	}
	if err := json.Unmarshal([]byte(said), &nodes); err != nil {
		return nil, fmt.Errorf("the controller's nodes cannot be read: %v", err)
	}
	var out []string
	for _, n := range nodes {
		raw, err := asker("seat:mesh-controller.node", map[string]any{"node": n.Name})
		if err != nil {
			return nil, fmt.Errorf("the controller's node verb did not answer for %s: %v", n.Name, err)
		}
		said, _ := verbSaid(raw)
		for _, line := range strings.Split(said, "\n") {
			if rest, ok := strings.CutPrefix(strings.TrimSpace(line), "assigned:"); ok {
				for _, mod := range strings.Split(rest, ",") {
					if strings.TrimSpace(mod) == "betterbird" {
						out = append(out, n.Name)
					}
				}
			}
		}
	}
	sort.Strings(out)
	if len(out) == 0 {
		return nil, errors.New("the controller names no node the module is assigned to")
	}
	return out, nil
}

// deriveOn asks another node's betterbird for what its profile holds.
func deriveOn(node string, kinds []string) (nodeDerivation, error) {
	raw, err := asker("betterbird.betterbird_derive@"+node, map[string]any{"here": true, "kinds": kinds})
	if err != nil {
		return nodeDerivation{}, err
	}
	said, ok := verbSaid(raw)
	if !ok {
		return nodeDerivation{}, errors.New(said)
	}
	var a DeriveAnswer
	if err := json.Unmarshal([]byte(said), &a); err != nil || a.Here == nil {
		return nodeDerivation{}, fmt.Errorf("its answer cannot be read")
	}
	a.Here.Node = node
	return *a.Here, nil
}