// Package view derives the operator's rows from the delivery buckets.
package view

import (
	"encoding/json"
	"fmt"
	"slices"
	"sort"
	"strings"
	"sync"
	"time"

	"git.novox.be/novox/mesh-sdk/go/asks"
	"git.novox.be/novox/mesh-sdk/go/delivery"
)

// Entry is one change of a bucket's key, as the watch delivers it.
type Entry struct {
	Bucket  string
	Key     string
	Value   []byte
	At      time.Time
	Deleted bool
	// Revision is the bucket revision of the change, when the watch gives one (the source revision is the gate's source key's).
	Revision uint64
}

// Row is one module on one node.
type Row struct {
	Node     string    `json:"node"`
	Module   string    `json:"module"`
	Status   string    `json:"status"`
	Assigned string    `json:"assigned,omitempty"`
	Have     string    `json:"have,omitempty"`
	Phase    string    `json:"phase,omitempty"`
	Health   string    `json:"health,omitempty"`
	Since    time.Time `json:"since,omitzero"`
	Detail   string    `json:"detail,omitempty"`
	Waits    string    `json:"waits,omitempty"`
	// RequiredBy is what pulled the module onto the node, for an assignment that is pulled in.
	RequiredBy string `json:"required-by,omitempty"`
	Error      string `json:"error,omitempty"`
}

// NodeRow is a node's own facts as counts.
type NodeRow struct {
	Node    string    `json:"node"`
	Windows int       `json:"windows"`
	Busy    int       `json:"busy"`
	Strays  int       `json:"strays"`
	Since   time.Time `json:"since,omitzero"`
	Error   string    `json:"error,omitempty"`
}

// Snapshot is everything the view shows.
type Snapshot struct {
	Rows  []Row     `json:"rows"`
	Nodes []NodeRow `json:"nodes"`
	// Offline is every node and part instance whose presence is offline (the watchman's word).
	Offline []OfflineRow `json:"offline"`
	// Stuck is every key that stopped moving and has not moved again.
	Stuck []StuckRow `json:"stuck"`
	// Unreadable is every watchman value the view could not read, named by its key: never dropped silently.
	Unreadable []UnreadableRow `json:"unreadable"`
	// Groups is every candidate group the indexer holds: the pull requests that share a branch, with the modules each touches.
	Groups []GroupRow `json:"groups"`
	// Fingerprints is every module the indexer fingerprinted at a trunk merge, beside its newest successful trunk build's.
	Fingerprints []FingerprintRow `json:"fingerprints"`
	// Asks is every ask the gate holds that has no answer yet: what waits for the operator's Approve.
	Asks []AskRow `json:"asks"`
	// Wants is every want of the assigner beside the have of its node and where it stands (novox/hq ADR 0539 §8, issue 1906 D7).
	Wants []WantRow `json:"wants"`
	// Source is the state of the mesh's source revision, derived from the wants and the haves (ADR 0429 §1): recorded by no part.
	// Nil when the gate holds no source yet.
	Source *SourceRow `json:"source,omitempty"`
	// Backlog is, for each language and kind of build ask with one waiting or held, the asks waiting for a node, the asks a node
	// holds and the oldest waiting ask's age (issue 1887 A8a).
	Backlog []BacklogRow `json:"backlog"`
}

// The states of a want against the have of its node.
const (
	WantApplied  = "applied"   // the node runs the want's fingerprint, running and healthy
	WantPending  = "pending"   // the node has not yet run the want's fingerprint, or not healthy yet
	WantFailed   = "failed"    // the node runs the want's fingerprint and its health check failed
	WantHeldBack = "held back" // the assigner holds the node back from the newest fingerprint (Rule), or set it back
	WantRemoved  = "removed"   // the node's want is removed: the module is no longer assigned to it
)

// WantRow is a want of the assigner beside the have of its node: the fingerprint wanted, the one the node runs with its phase and
// health, the state of the two, and for a want held back the newest fingerprint, the rule and the assigner's words.
type WantRow struct {
	Node   string `json:"node"`
	Module string `json:"module"`
	Want   string `json:"want,omitempty"`
	Have   string `json:"have,omitempty"`
	Phase  string `json:"phase,omitempty"`
	Health string `json:"health,omitempty"`
	State  string `json:"state"`
	Newest string `json:"newest,omitempty"`
	Rule   int    `json:"rule,omitempty"`
	Words  string `json:"words,omitempty"`
	// Judgement is the controller's judgement of the move to the composition the want names (recreates, moves-data, neither or
	// cannot-judge), and Judged its words; empty when the controller wrote none (ADR 0539 §14).
	Judgement string `json:"judgement,omitempty"`
	Judged    string `json:"judged,omitempty"`
}

// The states of a source revision (ADR 0429 §1).
const (
	SourceDelivered    = "delivered"
	SourceNotDelivered = "not delivered"
)

// SourceRow is the mesh's source revision (the revision of the gate's source key) and whether it is delivered: every want equals its
// module's fingerprint at the indexer and every have equals its want, running and healthy, and every node a module is assigned to
// has a want. Waiting names each module on a node that is not there yet, and why.
type SourceRow struct {
	Revision uint64   `json:"revision"`
	State    string   `json:"state"`
	Waiting  []string `json:"waiting,omitempty"`
	// Wants is how many wants of modules at the indexer were looked at; a revision with none is delivered of nothing.
	Wants int `json:"wants"`
}

// AskRow is an open ask (novox/hq ADR 0437 §10): the gate's ask with no answer under its key in the teller's answers.
type AskRow struct {
	Key      string    `json:"key"`
	Group    string    `json:"group"`
	Headline string    `json:"headline"`
	Words    string    `json:"words"`
	Pulls    []string  `json:"pulls"`
	Expires  time.Time `json:"expires,omitzero"`
	How      string    `json:"how"`
	// Options is what the view offers to answer with, for an ask that needs no proof of who answered (the assigner's hold and
	// restore, ADR 0528 clause 6); Chosen is the option the view wrote that the teller has not filled yet.
	Options []string `json:"options,omitempty"`
	Chosen  string   `json:"chosen,omitempty"`
}

// AnswerOnThePhone is what an open ask says about answering it: the view only shows, and the Approve is answered on the
// operator's phone (ADR 0437 §10: answering in the view is release 5's).
const AnswerOnThePhone = "answer on the phone"

// AnswerInTheView is what an open ask that needs no proof of who answered says (ADR 0528 clause 6): the operator answers it here.
const AnswerInTheView = "answer in the view"

// GroupRow is a candidate group (novox/hq ADR 0437 §5): the branch its pull requests share and the pull requests.
type GroupRow struct {
	Group string      `json:"group"`
	Pulls []GroupPull `json:"pulls"`
}

// GroupPull is one pull request of a group, its head shortened to 12 digits, and the modules it touches.
type GroupPull struct {
	Repository string   `json:"repository"`
	Number     int      `json:"number"`
	Head       string   `json:"head"`
	Modules    []string `json:"modules"`
}

// The states of a module's fingerprint against its newest successful trunk build's.
const (
	FPSame    = "same"
	FPDiffers = "differs"
	FPNone    = "none"
	FPNoBuild = "no build"
	// The states of a fingerprint of the new form against the builder's key of it (I5, issue 1851): built (the key is done),
	// build failed (Reason is its error), building (queued, claimed or building; Reason says what it waits for) and no key.
	FPBuilt       = "built"
	FPBuildFailed = "build failed"
	FPBuilding    = "building"
	FPNoKey       = "no key"
)

// FingerprintRow compares the indexer's fingerprint of a module at a trunk merge with the source fingerprint its newest
// successful trunk build recorded. State is same, differs (both are given), none (the indexer could not fingerprint it:
// Reason says why) or no build (the indexer's alone is given, no successful trunk build recorded one).
type FingerprintRow struct {
	Module     string `json:"module"`
	Repository string `json:"repository,omitempty"`
	Commit     string `json:"commit,omitempty"`
	State      string `json:"state"`
	Indexer    string `json:"indexer,omitempty"`
	Build      string `json:"build,omitempty"`
	Reason     string `json:"reason,omitempty"`
}

// OfflineRow is a node ("node") or a part's instance ("part") that is offline, since the presence entry's time.
type OfflineRow struct {
	Kind     string    `json:"kind"`
	Node     string    `json:"node,omitempty"`
	Part     string    `json:"part,omitempty"`
	Instance string    `json:"instance,omitempty"`
	Since    time.Time `json:"since,omitzero"`
}

// StuckRow is a key that stopped moving: its bucket and key, what it waited for and since when.
type StuckRow struct {
	Bucket string    `json:"bucket"`
	Key    string    `json:"key"`
	Waited string    `json:"waited,omitempty"`
	Since  time.Time `json:"since,omitzero"`
}

// UnreadableRow is a value of the watchman's buckets that did not decode, with the reader's error.
type UnreadableRow struct {
	Bucket string `json:"bucket"`
	Key    string `json:"key"`
	Error  string `json:"error"`
}

// Model holds the rows' inputs in memory and nothing else: a restart fills it again from the buckets' last values.
type Model struct {
	mu             sync.Mutex
	assigns        map[string]assignment
	builds         map[string]buildEntry  // by <module>.build.<id>
	groups         map[string]groupEntry  // the indexer's candidates, by group
	modules        map[string]moduleEntry // the indexer's modules, by module
	keys           map[string]keyEntry    // the builder's build keys, by <module>.<fingerprint>
	asks           map[string]askRecord   // the gate's asks, by key
	wants          map[string]wantEntry   // the assigner's wants, by <node>.<module>
	source         *sourceEntry           // the gate's source key
	deliveredEvent func(DeliveredEvent)
	motionEvent    func(MotionEvent)
	told           map[string]bool                // the motion events said, by their ids
	initial        map[string]bool                // the buckets whose watch handed over what stood at its start
	said           uint64                         // the source revision whose delivery was said
	composed       map[string]composedEntry       // the controller's digests and judgements, by <node>.<module>.<hex>
	aasks          map[string]assignerAskRecord   // the assigner's asks (hold, restore), by key
	chosen         map[string]delivery.ViewAnswer // the answers the view wrote, by ask key
	writer         Writer
	backlog        *Backlog
	answers        map[string]string          // by key: the router id of the asking the teller's answer is for ("" when unreadable)
	answerOf       map[string]delivery.Answer // by key: the teller's answer whole, when readable (holds.go)
	holds          map[string]heldEntry       // the assigner's holds, by held key (holds.go)
	newest         map[string]string          // module -> id
	haves          map[string]haveEntry
	nodes          map[string]nodeEntry
	// the watchman's buckets, by key
	presence map[string]presenceEntry
	stuck    map[string]stuckEntry
}

type presenceEntry struct {
	value delivery.Presence
	err   string
	at    time.Time
}

type stuckEntry struct {
	value delivery.Stuck
	err   string
}

// assignment holds only the fields of the controller's Assignment the view reads.
type assignment struct {
	Node       string    `json:"node"`
	Module     string    `json:"module"`
	Build      string    `json:"build"`
	Commit     string    `json:"commit"`
	Since      time.Time `json:"since"`
	Ended      time.Time `json:"ended"`
	State      string    `json:"state"`
	PulledIn   bool      `json:"pulled-in"`
	RequiredBy string    `json:"required-by"`
}

// open says whether the value is an assignment that stands for the module on its node, as the controller's
// Assignment.Stands does. A module pulled in by another stands too.
func (a assignment) open() bool { return a.Ended.IsZero() && a.State == "" }

// build holds only the fields of the controller's Build record the view reads. A record with no state and no failure
// is a done build.
type build struct {
	ID     string `json:"id"`
	Module string `json:"module"`
	State  string `json:"state"`
	Commit string `json:"commit"`
	Failed string `json:"failed"`
}

func (b build) done() bool { return b.State == "" && b.Failed == "" }

// buildEntry is a build as the view reads it: its own fields (state, failure) and the SDK's record (the source
// fingerprint, the ref, the time), decoded as the controller writes them.
type buildEntry struct {
	build
	rec delivery.BuildRecord
}

type groupEntry struct {
	value delivery.Candidate
	err   string
}

type keyEntry struct {
	value delivery.BuilderBuild
	err   string
}

type wantEntry struct {
	value delivery.Want
	err   string
	at    time.Time
}

type composedEntry struct {
	value delivery.ComposedDigest
	err   string
}

type sourceEntry struct {
	revision uint64
	group    string
	heads    map[string]string
	err      string
}

type assignerAskRecord struct {
	ask asks.Ask
	err string
}

type moduleEntry struct {
	value delivery.Module
	err   string
}

type askRecord struct {
	value delivery.Ask
	err   string
}

type haveEntry struct {
	have delivery.Have
	err  string
	at   time.Time
}

type nodeEntry struct {
	node delivery.HaveNode
	err  string
	at   time.Time
}

// New is an empty model.
func New() *Model {
	return &Model{assigns: map[string]assignment{}, builds: map[string]buildEntry{}, groups: map[string]groupEntry{}, modules: map[string]moduleEntry{}, keys: map[string]keyEntry{}, asks: map[string]askRecord{}, wants: map[string]wantEntry{}, told: map[string]bool{}, initial: map[string]bool{}, composed: map[string]composedEntry{}, aasks: map[string]assignerAskRecord{}, chosen: map[string]delivery.ViewAnswer{}, answers: map[string]string{}, answerOf: map[string]delivery.Answer{}, holds: map[string]heldEntry{}, newest: map[string]string{},
		haves: map[string]haveEntry{}, nodes: map[string]nodeEntry{},
		presence: map[string]presenceEntry{}, stuck: map[string]stuckEntry{}}
}

// Apply takes one change of a key. A value it cannot read is kept as an error on its row, never guessed. When the change makes
// the source revision delivered, the event of it is said once (SetDeliveredEvent), after the model's lock is released.
func (m *Model) Apply(e Entry) {
	m.mu.Lock()
	m.apply(e)
	event := m.deliveredNow()
	say := m.deliveredEvent
	motion, sayMotion := m.motionNow(e), m.motionEvent
	m.mu.Unlock()
	if event != nil && say != nil {
		say(*event)
	}
	if sayMotion != nil {
		for _, ev := range motion {
			sayMotion(ev)
		}
	}
}

// DeliveredEvent is what the view says when the source revision is delivered (event `delivered` of the view, issue 1906 D9, ADR
// 0539 §17): the revision of the gate's source key, the group that moved it and every repository's merged head at it. A
// consumer takes every earlier revision as covered by it, and takes the same revision twice as once (after a restart of the view).
type DeliveredEvent struct {
	Revision uint64            `json:"revision"`
	Group    string            `json:"group"`
	Heads    map[string]string `json:"heads"`
}

// SetDeliveredEvent sets where the event of a delivered source revision goes; nothing is said until it is set.
func (m *Model) SetDeliveredEvent(say func(DeliveredEvent)) {
	m.mu.Lock()
	defer m.mu.Unlock()
	m.deliveredEvent = say
}

// deliveredNow is the event of the source revision when it is delivered now and has not been said, or nil. A revision that went
// bad after it was delivered and recovered is not said again.
func (m *Model) deliveredNow() *DeliveredEvent {
	if m.deliveredEvent == nil || m.source == nil || m.source.err != "" || m.said == m.source.revision {
		return nil
	}
	row := m.sourceRow()
	if row == nil || row.State != SourceDelivered || row.Wants == 0 {
		return nil
	}
	m.said = m.source.revision
	return &DeliveredEvent{Revision: m.source.revision, Group: m.source.group, Heads: m.source.heads}
}

func (m *Model) apply(e Entry) {
	switch e.Bucket {
	case delivery.HaveBucket:
		node, _, isNode, err := delivery.ParseHaveKey(e.Key)
		if err != nil {
			return
		}
		if isNode {
			if e.Deleted {
				delete(m.nodes, node)
				return
			}
			n, err := delivery.DecodeHaveNode(e.Value)
			m.nodes[node] = nodeEntry{node: n, err: errText(err), at: e.At}
			return
		}
		key := e.Key // the bucket's own key: a module name with a dot is escaped in it (issue 1150)
		if e.Deleted {
			delete(m.haves, key)
			return
		}
		h, err := delivery.DecodeHave(e.Value)
		m.haves[key] = haveEntry{have: h, err: errText(err), at: e.At}
	case delivery.PresenceBucket:
		if e.Deleted {
			delete(m.presence, e.Key)
			return
		}
		p, err := delivery.DecodePresence(e.Value)
		m.presence[e.Key] = presenceEntry{value: p, err: errText(err), at: e.At}
	case delivery.StuckBucket:
		if e.Deleted {
			delete(m.stuck, e.Key)
			return
		}
		st, err := delivery.DecodeStuck(e.Value)
		m.stuck[e.Key] = stuckEntry{value: st, err: errText(err)}
	case AssignmentsBucket:
		if e.Deleted {
			delete(m.assigns, e.Key)
			return
		}
		var a assignment
		if json.Unmarshal(e.Value, &a) != nil || a.Node == "" || a.Module == "" {
			return
		}
		m.assigns[a.Node+"."+a.Module] = a
	case delivery.CandidatesBucket:
		if e.Deleted {
			delete(m.groups, e.Key)
			return
		}
		c, err := delivery.DecodeCandidate(e.Value)
		m.groups[e.Key] = groupEntry{value: c, err: errText(err)}
	case delivery.ModulesBucket:
		if e.Deleted {
			delete(m.modules, e.Key)
			return
		}
		mod, err := delivery.DecodeModule(e.Value)
		m.modules[e.Key] = moduleEntry{value: mod, err: errText(err)}
	case delivery.AsksBucket:
		if e.Deleted {
			delete(m.asks, e.Key)
			return
		}
		a, err := delivery.DecodeAsk(e.Value)
		m.asks[e.Key] = askRecord{value: a, err: errText(err)}
	case delivery.AssignerAsksBucket:
		if e.Deleted {
			delete(m.aasks, e.Key)
			return
		}
		a, err := decodeAssignerAsk(e.Key, e.Value)
		m.aasks[e.Key] = assignerAskRecord{ask: a, err: errText(err)}
	case delivery.ViewAnswersBucket:
		askKey, err := delivery.ParseViewAnswerKey(e.Key)
		if err != nil {
			return
		}
		if e.Deleted {
			delete(m.chosen, askKey)
			return
		}
		if a, err := delivery.DecodeViewAnswer(e.Value); err == nil {
			m.chosen[askKey] = a
		}
	case delivery.AssignerWantsBucket:
		if m.applyHeld(e) {
			return
		}
		node, module, err := delivery.ParseWantKey(e.Key)
		if err != nil {
			return // the node's own key is not a module's want
		}
		key, kerr := delivery.HaveKey(node, module)
		if kerr != nil {
			return
		}
		if e.Deleted {
			delete(m.wants, key)
			return
		}
		w, err := delivery.DecodeWant(e.Value)
		m.wants[key] = wantEntry{value: w, err: errText(err), at: e.At}
	case delivery.ControllerComposedBucket:
		if e.Deleted {
			delete(m.composed, e.Key)
			return
		}
		d, err := delivery.DecodeComposedDigest(e.Value)
		m.composed[e.Key] = composedEntry{value: d, err: errText(err)}
	case delivery.SourceBucket:
		if e.Key != delivery.SourceMainKey {
			return
		}
		if e.Deleted {
			m.source = nil
			return
		}
		src, err := delivery.DecodeSource(e.Value)
		m.source = &sourceEntry{revision: e.Revision, group: src.Group, heads: src.Heads, err: errText(err)}
	case delivery.AnswersBucket:
		if e.Deleted {
			delete(m.answers, e.Key)
			delete(m.answerOf, e.Key)
			return
		}
		// The answer's warrant is `<router id>@<time>`: it ends that asking, and only that one (a later asking of the key is
		// still open). An answer that cannot be read ends none.
		id := ""
		if a, err := delivery.DecodeAnswer(e.Value); err == nil {
			id, _, _ = strings.Cut(a.Warrant, "@")
			m.answerOf[e.Key] = a
		} else {
			delete(m.answerOf, e.Key)
		}
		m.answers[e.Key] = id
	case delivery.BuilderBuildsBucket:
		if e.Deleted {
			delete(m.keys, e.Key)
			return
		}
		b, err := delivery.DecodeBuilderBuild(e.Value)
		if err != nil {
			m.keys[e.Key] = keyEntry{err: err.Error()}
			return
		}
		m.keys[e.Key] = keyEntry{value: b}
	case BuildsBucket:
		module, rest, ok := strings.Cut(e.Key, ".")
		if !ok {
			return
		}
		switch {
		case rest == "newest":
			if e.Deleted {
				delete(m.newest, module)
				return
			}
			m.newest[module] = strings.TrimSpace(string(e.Value))
		case strings.HasPrefix(rest, "build."):
			if e.Deleted {
				delete(m.builds, e.Key)
				return
			}
			var b build
			if json.Unmarshal(e.Value, &b) != nil {
				return
			}
			// The source fingerprint is read with the SDK's record, not with the view's own struct.
			var rec delivery.BuildRecord
			if json.Unmarshal(e.Value, &rec) != nil {
				return
			}
			m.builds[e.Key] = buildEntry{build: b, rec: rec}
		}
	}
}

func errText(err error) string {
	if err == nil {
		return ""
	}
	return err.Error()
}

// Snapshot derives one row per module and node with an open assignment, a row marked left behind for each have with
// none, and a row per node's own key. Rows are ordered by node and module.
func (m *Model) Snapshot() Snapshot {
	s := m.snapshot()
	m.mu.Lock()
	b := m.backlog
	m.mu.Unlock()
	s.Backlog = []BacklogRow{}
	if b != nil {
		s.Backlog = b.Rows() // read from the bus outside the model's lock
	}
	return s
}

// SetBacklog gives the model the seats' workers to read the backlog of asks from.
func (m *Model) SetBacklog(b *Backlog) {
	m.mu.Lock()
	defer m.mu.Unlock()
	m.backlog = b
}

func (m *Model) snapshot() Snapshot {
	m.mu.Lock()
	defer m.mu.Unlock()
	rows := []Row{}
	for key, a := range m.assigns {
		if !a.open() {
			continue
		}
		r := Row{Node: a.Node, Module: a.Module, Status: StatusAssigned, Assigned: a.Build}
		if a.PulledIn {
			r.RequiredBy = a.RequiredBy
		}
		var waits []string
		hk, herr := delivery.HaveKey(a.Node, a.Module)
		if herr != nil {
			hk = key
		}
		h, has := m.haves[hk]
		// The controller no longer writes the build of an assignment (R6): the row is the want and the have of the node.
		w, wanted := m.wants[hk]
		wanted = wanted && w.err == "" && !w.value.Removed && w.value.FP != ""
		if wanted {
			r.Assigned = w.value.FP
		}
		switch {
		case !has:
			waits = append(waits, "no have yet")
		case h.err != "":
			r.Since, r.Error = h.at, h.err
		default:
			r.Have, r.Phase, r.Health = h.have.Build, string(h.have.Phase), string(h.have.Health)
			if r.Have == "" {
				r.Have = h.have.FP
			}
			r.Since, r.Detail = h.at, h.have.Detail
			if wanted {
				if h.have.FP != w.value.FP {
					waits = append(waits, "the node has not applied "+w.value.FP)
				}
			} else if a.Build != "" && h.have.Build != a.Build {
				waits = append(waits, "the node has not applied "+a.Build)
			}
		}
		if id := m.newest[a.Module]; id != "" && a.Build != "" && id != a.Build {
			if b, ok := m.builds[a.Module+".build."+id]; ok && b.done() {
				waits = append(waits, a.Module+" has a newer build "+id+" not yet assigned")
			}
		}
		r.Waits = strings.Join(waits, "; ")
		rows = append(rows, r)
	}
	for key, h := range m.haves {
		node, module, _, perr := delivery.ParseHaveKey(key)
		if perr != nil {
			continue
		}
		if a, ok := m.assigns[node+"."+module]; ok && a.open() {
			continue
		}
		r := Row{Node: node, Module: module, Status: StatusLeftBehind, Since: h.at, Error: h.err}
		if h.err == "" && h.have.Phase == delivery.PhaseRemoved {
			// The node-engine says it took the module away: plainly removed, not left behind.
			r.Status = StatusRemoved
		}
		if h.err == "" {
			r.Have, r.Phase, r.Health, r.Detail = h.have.Build, string(h.have.Phase), string(h.have.Health), h.have.Detail
			if r.Have == "" {
				r.Have = h.have.FP
			}
		}
		rows = append(rows, r)
	}
	sort.Slice(rows, func(i, j int) bool {
		if rows[i].Node != rows[j].Node {
			return rows[i].Node < rows[j].Node
		}
		return rows[i].Module < rows[j].Module
	})
	nodes := []NodeRow{}
	for name, n := range m.nodes {
		nodes = append(nodes, NodeRow{Node: name, Windows: count(n.node.Windows), Busy: count(n.node.Busy),
			Strays: count(n.node.Strays), Since: n.at, Error: n.err})
	}
	sort.Slice(nodes, func(i, j int) bool { return nodes[i].Node < nodes[j].Node })
	offline, stuck, unreadable := []OfflineRow{}, []StuckRow{}, []UnreadableRow{}
	for key, p := range m.presence {
		if p.err != "" {
			unreadable = append(unreadable, UnreadableRow{Bucket: delivery.PresenceBucket, Key: key, Error: p.err})
			continue
		}
		if p.value.Online {
			continue
		}
		node, part, instance, err := delivery.ParsePresenceKey(key)
		if err != nil {
			unreadable = append(unreadable, UnreadableRow{Bucket: delivery.PresenceBucket, Key: key, Error: err.Error()})
			continue
		}
		if part != "" {
			offline = append(offline, OfflineRow{Kind: "part", Part: part, Instance: instance, Since: p.at})
		} else {
			offline = append(offline, OfflineRow{Kind: "node", Node: node, Since: p.at})
		}
	}
	for key, st := range m.stuck {
		if st.err != "" {
			unreadable = append(unreadable, UnreadableRow{Bucket: delivery.StuckBucket, Key: key, Error: st.err})
			continue
		}
		if st.value.Moved {
			continue
		}
		stuck = append(stuck, StuckRow{Bucket: st.value.Bucket, Key: st.value.Key, Waited: st.value.Waited, Since: st.value.Since})
	}
	for key, g := range m.groups {
		if g.err != "" {
			unreadable = append(unreadable, UnreadableRow{Bucket: delivery.CandidatesBucket, Key: key, Error: g.err})
		}
	}
	for key, mod := range m.modules {
		if mod.err != "" {
			unreadable = append(unreadable, UnreadableRow{Bucket: delivery.ModulesBucket, Key: key, Error: mod.err})
		}
	}
	for key, a := range m.asks {
		if a.err != "" {
			unreadable = append(unreadable, UnreadableRow{Bucket: delivery.AsksBucket, Key: key, Error: a.err})
		}
	}
	for key, a := range m.aasks {
		if a.err != "" {
			unreadable = append(unreadable, UnreadableRow{Bucket: delivery.AssignerAsksBucket, Key: key, Error: a.err})
		}
	}
	for key, w := range m.wants {
		if w.err != "" {
			unreadable = append(unreadable, UnreadableRow{Bucket: delivery.AssignerWantsBucket, Key: key, Error: w.err})
		}
	}
	sort.Slice(offline, func(i, j int) bool {
		a, b := offline[i], offline[j]
		return a.Kind+a.Node+a.Part+a.Instance < b.Kind+b.Node+b.Part+b.Instance
	})
	sort.Slice(stuck, func(i, j int) bool {
		return stuck[i].Bucket+"."+stuck[i].Key < stuck[j].Bucket+"."+stuck[j].Key
	})
	sort.Slice(unreadable, func(i, j int) bool {
		return unreadable[i].Bucket+"."+unreadable[i].Key < unreadable[j].Bucket+"."+unreadable[j].Key
	})
	return Snapshot{Rows: rows, Nodes: nodes, Offline: offline, Stuck: stuck, Unreadable: unreadable,
		Groups: m.groupRows(), Fingerprints: m.fingerprintRows(), Asks: m.askRows(), Wants: m.wantRows(), Source: m.sourceRow()}
}

// groupRows are the readable candidate groups, ordered by group, their pull requests by repository and number.
func (m *Model) groupRows() []GroupRow {
	out := []GroupRow{}
	for key, g := range m.groups {
		if g.err != "" {
			continue
		}
		row := GroupRow{Group: key, Pulls: []GroupPull{}}
		for _, p := range g.value.Pulls {
			head := p.Head.Head
			if len(head) > 12 {
				head = head[:12]
			}
			row.Pulls = append(row.Pulls, GroupPull{Repository: p.Repository, Number: p.Number, Head: head, Modules: append([]string{}, p.Modules...)})
		}
		sort.Slice(row.Pulls, func(i, j int) bool {
			a, b := row.Pulls[i], row.Pulls[j]
			return a.Repository < b.Repository || (a.Repository == b.Repository && a.Number < b.Number)
		})
		out = append(out, row)
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Group < out[j].Group })
	return out
}

// fingerprintRows compare each module the indexer fingerprinted with the source fingerprint of the module's newest
// successful trunk build (delivery.BuildRecord.IsTrunkBuild: an outcome, not failed, on main or with no ref) that recorded one. The newest key is any state and ref, so it is not
// used: the records are.
func (m *Model) fingerprintRows() []FingerprintRow {
	newest := map[string]buildEntry{}
	for _, b := range m.builds {
		if b.rec.Module == "" || !b.rec.IsTrunkBuild() || b.rec.SourceFingerprint == "" {
			continue
		}
		if old, ok := newest[b.rec.Module]; !ok || b.rec.At.After(old.rec.At) || (b.rec.At.Equal(old.rec.At) && b.rec.ID > old.rec.ID) {
			newest[b.rec.Module] = b
		}
	}
	out := []FingerprintRow{}
	for name, mod := range m.modules {
		if mod.err != "" {
			continue
		}
		row := FingerprintRow{Module: name, Repository: mod.value.Repository, Commit: mod.value.Commit}
		if mod.value.FP2 != "" || mod.value.None2 != "" {
			// The new form (R4e): the indexer's fp2 against the builder's key of it.
			out = append(out, m.newFormRow(name, row, mod.value))
			continue
		}
		b, hasBuild := newest[name]
		switch {
		case mod.value.None != "":
			row.State, row.Reason = FPNone, mod.value.None
		case !hasBuild:
			row.State, row.Indexer = FPNoBuild, mod.value.FP
		case b.rec.SourceFingerprint == mod.value.FP:
			row.State, row.Indexer, row.Build = FPSame, mod.value.FP, b.rec.SourceFingerprint
		default:
			row.State, row.Indexer, row.Build = FPDiffers, mod.value.FP, b.rec.SourceFingerprint
		}
		out = append(out, row)
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Module < out[j].Module })
	return out
}

// newFormRow is a module's row of the new form: none with its reason, or its fp2 against the state of the builder's key
// of it (I5, issue 1851; the build records name no fingerprint of the new form).
func (m *Model) newFormRow(name string, row FingerprintRow, v delivery.Module) FingerprintRow {
	if v.FP2 == "" {
		row.State, row.Reason = FPNone, v.None2
		return row
	}
	row.Indexer = v.FP2
	key, err := delivery.BuilderBuildKey(name, v.FP2)
	if err != nil {
		row.State, row.Reason = FPNoKey, err.Error()
		return row
	}
	k, ok := m.keys[key]
	switch {
	case !ok:
		row.State = FPNoKey
	case k.err != "":
		row.State, row.Reason = FPBuildFailed, "the key cannot be read: "+k.err
	case k.value.State == delivery.BuildDone:
		row.State, row.Build = FPBuilt, v.FP2
	case k.value.State == delivery.BuildFailed:
		row.State, row.Reason = FPBuildFailed, k.value.Error
	default:
		row.State, row.Reason = FPBuilding, strings.TrimSpace(k.value.State+" "+k.value.Waits)
	}
	return row
}

// count is the number of items of a carried JSON array or object, and 0 for anything else.
func count(raw json.RawMessage) int {
	var list []json.RawMessage
	if json.Unmarshal(raw, &list) == nil {
		return len(list)
	}
	var obj map[string]json.RawMessage
	if json.Unmarshal(raw, &obj) == nil {
		return len(obj)
	}
	return 0
}

// Counted is the number of rows that are modules on the nodes, not left behind.
func (s Snapshot) Counted() int {
	n := 0
	for _, r := range s.Rows {
		if r.Status == StatusAssigned {
			n++
		}
	}
	return n
}

// The buckets the view reads besides the have bucket (which mesh-sdk's delivery package names).
const (
	AssignmentsBucket = "mesh-controller_assignments"
	BuildsBucket      = "mesh-controller_builds"
)

// The statuses of a row.
const (
	StatusAssigned   = "assigned"
	StatusLeftBehind = "left behind"
	StatusRemoved    = "removed"
)

// askRows are the readable asks with no answer under their key, ordered by key: each with its headline, its words, the
// pull requests it covers (repository, number, head shortened to 12 digits) and how to answer.
func (m *Model) askRows() []AskRow {
	out := []AskRow{}
	for key, a := range m.asks {
		if a.err != "" || m.answers[key] == a.value.Ask.ID {
			continue
		}
		row := AskRow{Key: key, Group: a.value.Group, Headline: a.value.Ask.Headline, Words: a.value.Ask.Explanation,
			Expires: a.value.Ask.Expires, How: AnswerOnThePhone, Pulls: []string{}}
		for _, h := range a.value.Heads {
			head := h.Head
			if len(head) > 12 {
				head = head[:12]
			}
			row.Pulls = append(row.Pulls, fmt.Sprintf("%s#%d @%s", h.Repository, h.Number, head))
		}
		out = append(out, row)
	}
	for key, a := range m.aasks {
		if a.err != "" || m.answers[key] == a.ask.ID {
			continue
		}
		row := AskRow{Key: key, Headline: a.ask.Headline, Words: a.ask.Explanation, Expires: a.ask.Expires, How: AnswerInTheView, Pulls: []string{},
			Chosen: m.chosenOf(key, a.ask)}
		for _, o := range a.ask.Options {
			row.Options = append(row.Options, o.ID)
		}
		out = append(out, row)
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Key < out[j].Key })
	return out
}

// decodeAssignerAsk reads a key of the assigner's asks (mesh-sdk go/delivery DecodeAssignerAsk): the router's ask under its
// asking's id, of a kind that needs no proof, offering an option.
func decodeAssignerAsk(key string, value []byte) (asks.Ask, error) {
	a, err := delivery.DecodeAssignerAsk(key, value)
	if err != nil {
		return asks.Ask{}, fmt.Errorf("the assigner's ask %s is not readable: %w", key, err)
	}
	return a.Ask, nil
}

// chosenOf is the option the view wrote for this asking of the ask: an answer to another asking (a stale one) is none.
func (m *Model) chosenOf(key string, ask asks.Ask) string {
	if a, ok := m.chosen[key]; ok && a.Asking == ask.ID {
		return a.Option
	}
	return ""
}

// Writer puts a key of the view's bucket answers, the only state the view writes.
type Writer interface {
	Put(key string, value []byte) error
}

// SetWriter gives the model the bucket its answers are written to.
func (m *Model) SetWriter(w Writer) {
	m.mu.Lock()
	defer m.mu.Unlock()
	m.writer = w
}

// Answer writes the view's answer to the open ask at askKey: option, one the ask offers, and the operator's words. It writes
// `<ask key>.view` and nothing else; an ask that needs proof of who answered (the merge's Approve, issue 953) is refused with
// its reason, as is an ask the view does not hold open, an option the ask does not offer, and one the view answered already.
func (m *Model) Answer(askKey, option, words string) error {
	key, err := delivery.ViewAnswerKey(askKey)
	if err != nil {
		return err
	}
	option = strings.TrimSpace(option)
	if option == "" {
		return fmt.Errorf("an answer names no option")
	}
	m.mu.Lock()
	a, held := m.aasks[askKey]
	open := held && a.err == "" && m.answers[askKey] != a.ask.ID
	chosen, w := m.chosenOf(askKey, a.ask), m.writer
	m.mu.Unlock()
	if !open {
		return fmt.Errorf("the view holds no open ask %s", askKey)
	}
	if !slices.ContainsFunc(a.ask.Options, func(o asks.Option) bool { return o.ID == option }) {
		ids := []string{}
		for _, o := range a.ask.Options {
			ids = append(ids, o.ID)
		}
		return fmt.Errorf("the ask %s offers no option %q (it offers %s)", askKey, option, strings.Join(ids, ", "))
	}
	if chosen != "" {
		return fmt.Errorf("the view answered %s with %q already: the teller fills the ask from it", askKey, chosen)
	}
	if w == nil {
		return fmt.Errorf("the view cannot write its answers here")
	}
	body, err := delivery.EncodeViewAnswer(delivery.ViewAnswer{Asking: a.ask.ID, Option: option, Words: strings.TrimSpace(words)})
	if err != nil {
		return err
	}
	return w.Put(key, body)
}

// wantRows are the readable wants beside the haves of their nodes, ordered by node and module.
func (m *Model) wantRows() []WantRow {
	out := []WantRow{}
	for key, w := range m.wants {
		if w.err != "" {
			continue
		}
		node, module := splitKey(key)
		v := w.value
		row := WantRow{Node: node, Module: module, Want: v.FP, Newest: v.Newest, Rule: v.Rule, Words: v.Words}
		h, has := m.haves[key]
		if has && h.err == "" {
			row.Have, row.Phase, row.Health = h.have.FP, string(h.have.Phase), string(h.have.Health)
		}
		if ckey, err := delivery.ComposedKey(node, module, v.FP); err == nil && !v.Removed {
			if c, ok := m.composed[ckey]; ok && c.err == "" {
				row.Judgement, row.Judged = string(c.value.Judgement), c.value.Words
			}
		}
		switch {
		case v.Removed:
			row.State = WantRemoved
		case v.Rule != 0 || v.Newest != "":
			row.State = WantHeldBack
		case row.Have == v.FP && h.have.Health == delivery.HealthFailed:
			row.State = WantFailed
		case row.Have == v.FP && h.have.Phase == delivery.PhaseRunning && h.have.Health == delivery.HealthOK:
			row.State = WantApplied
		default:
			row.State = WantPending
		}
		out = append(out, row)
	}
	sort.Slice(out, func(i, j int) bool {
		if out[i].Node != out[j].Node {
			return out[i].Node < out[j].Node
		}
		return out[i].Module < out[j].Module
	})
	return out
}

// sourceRow derives the state of the source revision the gate holds (ADR 0429 §1; recorded by no part): delivered when every want
// equals its module's fingerprint at the indexer and every have equals its want, running and healthy, and every node a module is
// assigned to has a want. It is nil when the gate holds no readable source.
func (m *Model) sourceRow() *SourceRow {
	if m.source == nil || m.source.err != "" {
		return nil
	}
	waiting := []string{}
	short := func(fp string) string {
		fp = strings.TrimPrefix(fp, delivery.SourcePrefix2)
		if len(fp) > 12 {
			fp = fp[:12]
		}
		return fp
	}
	looked := 0
	for key, w := range m.wants {
		node, module := splitKey(key)
		mod, ok := m.modules[module]
		if w.err != "" || w.value.Removed || !ok || mod.err != "" || mod.value.FP2 == "" {
			continue
		}
		looked++
		h := m.haves[key]
		switch {
		case w.value.FP != mod.value.FP2:
			waiting = append(waiting, fmt.Sprintf("%s on %s wants %s, not the module's %s", module, node, short(w.value.FP), short(mod.value.FP2)))
		case h.err != "" || h.have.FP != w.value.FP:
			waiting = append(waiting, fmt.Sprintf("%s on %s does not yet run %s", module, node, short(w.value.FP)))
		case h.have.Health == delivery.HealthFailed:
			waiting = append(waiting, fmt.Sprintf("%s on %s runs %s and its health check failed", module, node, short(w.value.FP)))
		case h.have.Phase != delivery.PhaseRunning || h.have.Health != delivery.HealthOK:
			waiting = append(waiting, fmt.Sprintf("%s on %s runs %s, not yet running and healthy", module, node, short(w.value.FP)))
		}
	}
	for key, a := range m.assigns {
		if !a.open() {
			continue
		}
		wk, werr := delivery.HaveKey(a.Node, a.Module)
		if werr != nil {
			wk = key
		}
		if _, ok := m.wants[wk]; ok {
			continue
		}
		if mod, ok := m.modules[a.Module]; ok && mod.err == "" && mod.value.FP2 != "" {
			waiting = append(waiting, fmt.Sprintf("%s on %s has no want yet", a.Module, a.Node))
		}
	}
	sort.Strings(waiting)
	row := &SourceRow{Revision: m.source.revision, State: SourceDelivered, Wants: looked}
	if len(waiting) > 0 {
		row.State, row.Waiting = SourceNotDelivered, waiting
	}
	return row
}

// splitKey is the node and the module (unescaped, issue 1150) of a model key, which is the have bucket's key form.
func splitKey(key string) (node, module string) {
	node, module, isNode, err := delivery.ParseHaveKey(key)
	if err != nil || isNode {
		node, module, _ = strings.Cut(key, ".")
	}
	return node, module
}

// The events of a module in motion (issue 1906 D10, for the messenger, ADR 0308): with no plan to say it, the view says that a
// module's want on a node differs from its have (moving), that the have reached the want running and healthy (settled), or that it
// is held or failed or stopped moving past the watchman's bound (stuck). Each is said once for a node, a module and a composition.
const (
	MotionMoving  = "moving"
	MotionSettled = "settled"
	MotionStuck   = "stuck"
	// MotionBoundSeconds is how long a node may take to apply a want before the watchman says it stuck (watch.StepBound).
	MotionBoundSeconds = 1800
)

// MotionBody is the body of the three events. Outcome and Words are those of settled and stuck: settled is "running"; stuck is
// "failed" (a health check failed) or "stopped" (the module is held, or the watchman says the want stopped moving past its bound),
// with the have's own words or the watchman's in Words.
type MotionBody struct {
	Node         string    `json:"node"`
	Module       string    `json:"module"`
	FromFP       string    `json:"from_fp"`
	ToFP         string    `json:"to_fp"`
	Composed     string    `json:"composed"`
	Since        time.Time `json:"since"`
	BoundSeconds int       `json:"bound_seconds"`
	// At is when the have or the want last changed, the time the event is about.
	At      time.Time `json:"at"`
	Outcome string    `json:"outcome,omitempty"`
	Words   string    `json:"words,omitempty"`
}

// MotionEvent is an event to say: its type (moving, settled or stuck), its id (the bus's dedup id and the event id header, one for
// a type, a node, a module and a composition) and its body.
type MotionEvent struct {
	Type string
	ID   string
	Body MotionBody
}

// SetMotionEvent sets where the events of modules in motion go; nothing is said until it is set.
func (m *Model) SetMotionEvent(say func(MotionEvent)) {
	m.mu.Lock()
	defer m.mu.Unlock()
	m.motionEvent = say
}

// motionNow is the events to say now about the node and module the change touched: a want, a have, or the watchman's stuck key of a
// want.
func (m *Model) motionNow(e Entry) []MotionEvent {
	if m.motionEvent == nil || !m.motionReady() {
		return nil
	}
	var hk string
	switch e.Bucket {
	case delivery.AssignerWantsBucket:
		node, module, err := delivery.ParseWantKey(e.Key)
		if err != nil {
			return nil
		}
		hk, _ = delivery.HaveKey(node, module)
	case delivery.HaveBucket:
		if _, _, isNode, err := delivery.ParseHaveKey(e.Key); err != nil || isNode {
			return nil
		}
		hk = e.Key
	case delivery.StuckBucket:
		st, ok := m.stuck[e.Key]
		if !ok || st.err != "" || st.value.Bucket != delivery.AssignerWantsBucket {
			return nil
		}
		node, module, err := delivery.ParseWantKey(st.value.Key)
		if err != nil {
			return nil
		}
		hk, _ = delivery.HaveKey(node, module)
	default:
		return nil
	}
	return m.motionOf(hk)
}

// motionReady says whether the view has read what stands in the buckets motion is derived from (the wants, the haves and the
// watchman's stuck keys): before, a want read ahead of its have would be said moving though the node runs it.
func (m *Model) motionReady() bool {
	for _, b := range []string{delivery.AssignerWantsBucket, delivery.HaveBucket, delivery.StuckBucket} {
		if !m.initial[b] {
			return false
		}
	}
	return true
}

// Initial tells the model that the watch of a bucket has handed over every key that stood at its start. When the buckets motion is
// derived from have all done so, the events of the modules in motion at that moment are said (the ids deduplicate what a restart
// of the view says again).
func (m *Model) Initial(bucket string) {
	m.mu.Lock()
	was := m.motionReady()
	m.initial[bucket] = true
	var out []MotionEvent
	if !was && m.motionReady() && m.motionEvent != nil {
		keys := make([]string, 0, len(m.wants))
		for k := range m.wants {
			keys = append(keys, k)
		}
		sort.Strings(keys)
		for _, k := range keys {
			out = append(out, m.motionOf(k)...)
		}
	}
	say := m.motionEvent
	m.mu.Unlock()
	for _, ev := range out {
		say(ev)
	}
}

// motionOf is the events to say now about the node and module of a model key, from their want, have and stuck key.
func (m *Model) motionOf(hk string) []MotionEvent {
	w, ok := m.wants[hk]
	if !ok || w.err != "" || w.value.Removed || w.value.FP == "" {
		return nil
	}
	node, module := splitKey(hk)
	h, has := m.haves[hk]
	has = has && h.err == ""
	body := MotionBody{Node: node, Module: module, ToFP: w.value.FP, Composed: w.value.Composed, Since: w.at, BoundSeconds: MotionBoundSeconds}
	body.At = w.at
	if has {
		body.FromFP = h.have.FP
		if h.at.After(body.At) {
			body.At = h.at
		}
	}
	id := func(kind string) string {
		return kind + "." + node + "." + module + "." + strings.TrimPrefix(w.value.Composed, "sha256:")
	}
	applied := has && h.have.FP == w.value.FP && (h.have.Composed == "" || h.have.Composed == w.value.Composed)
	var out []MotionEvent
	say := func(kind string, b MotionBody) {
		if m.told[id(kind)] {
			return
		}
		m.told[id(kind)] = true
		out = append(out, MotionEvent{Type: kind, ID: id(kind), Body: b})
	}
	switch {
	case applied && h.have.Phase == delivery.PhaseRunning && h.have.Health == delivery.HealthOK:
		if m.told[id(MotionMoving)] {
			b := body
			b.Outcome = "running"
			say(MotionSettled, b)
		}
		return out
	case !m.told[id(MotionMoving)]:
		say(MotionMoving, body)
	}
	stop := body
	switch {
	case has && h.have.Phase == delivery.PhaseHeld:
		stop.Outcome, stop.Words = "stopped", h.have.Detail
	case has && (h.have.Health == delivery.HealthFailed || h.have.Phase == delivery.PhaseFailed) && h.have.FP == w.value.FP:
		stop.Outcome, stop.Words = "failed", h.have.Detail
	default:
		if s, ok := m.stuckWant(w.value, node, module); ok {
			stop.Outcome, stop.Words = "stopped", s
		} else {
			return out
		}
	}
	say(MotionStuck, stop)
	return out
}

// stuckWant is the watchman's words for the want of module on node, when it says the want stopped moving and not yet moved.
func (m *Model) stuckWant(_ delivery.Want, node, module string) (string, bool) {
	wk, err := delivery.WantKey(node, module)
	if err != nil {
		return "", false
	}
	sk, err := delivery.StuckKey(delivery.AssignerWantsBucket, wk)
	if err != nil {
		return "", false
	}
	if st, ok := m.stuck[sk]; ok && st.err == "" && !st.value.Moved {
		return st.value.Waited, true
	}
	return "", false
}