// Package userlist is the bridge of novox/hq ADR 0547 §8: the node agent of the node holding mesh-broker applies the
// bus's user list from the newest composition of the bus module the controller writes for its node, alone and with no
// want, so a new account is in force on the bus without anybody's hand.
//
// **Why it exists.** The list reached the bus only through a want of the bus module, and ADR 0539 §15 bars the assigner
// from writing one: on 2026-10-25 it stood from 11:23 to 20:50 and every new account was refused until the operator put
// it back by hand (issue 1945). The bus module's entrypoint already reloads the server in place when the file changes;
// what was missing is a writer of the file that does not wait on a want.
//
// **It watches, it never polls.** It follows the node's composition bucket. Each composition is verified as the agent
// verifies it (signed by the controller's or the operator's key) before anything of it is read. Of those that carry the
// bus's user list (a file resource `bus-users`), the newest by revision is the source; its list is written alone
// (apply.WriteAlone) when it differs from the file on the machine, and what was written is reported in the bus
// module's have. It looks again after every apply of the bus module, since an apply of an older composition writes an
// older list, and when asked (Again).
//
// It reads no want and writes none, moves no fingerprint, and touches nothing of the bus module but its user list.
//
// **From take-over it follows the grants part's list instead** (ADR 0547 clauses 7 and 8, issue 1945). On the node whose
// compositions carry the list, it also watches the grants part's one key `user-list.sealed`. While that key says
// FollowGrants, the list is the object the key names, read by its name, opened with this node's sealing key and checked
// against the key's digest, and written to the same file (mode 0600) only when the file's digest differs; otherwise the
// controller's compositions are the source, as before. Never both. The have says which it follows and this node's key,
// which the grants part seals to.
package userlist

import (
	"context"
	"crypto/ed25519"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"regexp"
	"slices"
	"strings"
	"sync"
	"time"

	"git.novox.be/novox/mesh-sdk/go/delivery"
	"git.novox.be/novox/mesh-sdk/go/seal"
	"github.com/novox/mesh-host/internal/agent"
)

// ResourceID is the id the composer gives the bus's user list (mesh-sdk catalogue.BusUsersID); in a composition it may
// carry its module before it, `<module>.bus-users`.
const ResourceID = "bus-users"

// Written is what the follower last did: the source it took the list from, the list's digest there, the file's digest
// on the machine after the write, how many users the list holds, when, and why a write failed.
type Written = delivery.UserListWritten

// Env is everything the follower needs of the node.
type Env struct {
	Node     string
	Composed agent.Watcher
	// Trusted is the keys a composition may be signed by, as the agent trusts them.
	Trusted func() []ed25519.PublicKey
	// Resolve is the list a user-list resource holds: its content, or the object its digest names, fetched and
	// verified (the controller sends a file over 64 KiB by digest, ADR 0486 §1, and the live list is over it).
	Resolve func(ctx context.Context, resource json.RawMessage) ([]byte, error)
	// OnDisk is what the resource's file holds on the machine now; nil when there is none.
	OnDisk func(resource json.RawMessage) ([]byte, error)
	// Write writes the user list's file resource of a module's composition alone, with body as its content, when it
	// differs from the file on the machine, and answers the digest of the file there after it (`sha256:<hex>`) and
	// whether it changed.
	Write func(ctx context.Context, module string, resource json.RawMessage, body []byte) (digest string, changed bool, err error)
	// Report says what was written, in the module's have.
	Report func(ctx context.Context, module string, w Written)
	// Grants watches the grants part's state (its key delivery.GrantsUserListKey); nil follows the controller's alone.
	Grants agent.Watcher
	// Fetch reads an object of the grants part's bucket by its name; Open opens a box sealed to this node's key; Key is
	// this node's sealing key's public half (SPKI PEM), said in the have so the grants part seals to it ("" unknown).
	Fetch func(ctx context.Context, name string) ([]byte, error)
	Open  func(seal.Box) ([]byte, error)
	Key   func() string
	Now   func() time.Time
	Say   func(string)
}

type source struct {
	key      string
	module   string
	rev      uint64
	resource json.RawMessage
	wanted   string
}

// Follower follows the node's compositions for the bus's user list.
type Follower struct {
	env  Env
	kick chan struct{}
	mu   sync.Mutex
	srcs map[string]source // by composition key
	// cur is the source last taken, and written whether its write went through.
	cur     source
	written bool
	state   Written
	mod     string
	said    map[string]bool
	// grants is the grants part's key as last heard (nil when none), and following the source last taken: FollowGrants
	// or FollowController.
	grants    *delivery.GrantsUserList
	grantsRev uint64
	following string
}

// New is a follower for a node.
func New(env Env) *Follower {
	if env.Now == nil {
		env.Now = time.Now
	}
	if env.Say == nil {
		env.Say = func(string) {}
	}
	return &Follower{env: env, kick: make(chan struct{}, 1), srcs: map[string]source{}, said: map[string]bool{}}
}

// Again asks the follower to look at the file again, though no composition changed: after an apply of the bus module,
// and when a person asks. It never blocks.
func (f *Follower) Again() {
	select {
	case f.kick <- struct{}{}:
	default:
	}
}

// State is the module whose list the follower writes ("" when no composition of this node carries one) and what it
// last did.
func (f *Follower) State() (string, Written) {
	f.mu.Lock()
	defer f.mu.Unlock()
	return f.mod, f.state
}

// Run follows the compositions until ctx ends.
func (f *Follower) Run(ctx context.Context) error {
	updates, err := f.env.Composed.Watch(ctx, f.env.Node+".>")
	if err != nil {
		return fmt.Errorf("the user list cannot be followed: %w", err)
	}
	caught := false
	var grants <-chan agent.Update
	for {
		again := false
		select {
		case <-ctx.Done():
			return nil
		case <-f.kick:
			again = true
		case u, ok := <-updates:
			if !ok {
				return fmt.Errorf("the watch of the compositions ended")
			}
			if u.Caught {
				caught = true
			} else {
				f.take(u)
			}
		case u, ok := <-grants:
			if !ok {
				// Its watch ended (the link, or a grant the bus took away): heard again at the next event.
				grants = nil
				f.sayOnce("the grants part's user list is not followed until its key can be watched again")
				continue
			}
			if u.Caught {
				continue // nothing changed by the watch catching up
			}
			f.takeGrants(u)
		}
		if caught {
			f.settle(ctx, again)
			// The grants part's key is watched on the node whose compositions carry the list: the bus node alone is
			// granted it (mesh-sdk buscompose.GrantsUserListFollows).
			if grants == nil && f.env.Grants != nil && f.busNode() {
				ch, err := f.env.Grants.Watch(ctx, delivery.GrantsUserListKey)
				if err != nil {
					f.sayOnce("the grants part's user list cannot be watched yet, so the controller's is followed: " + err.Error())
				} else {
					grants = ch
				}
			}
		}
	}
}

// busNode says some composition of this node carries the bus's user list.
func (f *Follower) busNode() bool {
	f.mu.Lock()
	defer f.mu.Unlock()
	return f.cur.key != "" || len(f.srcs) > 0
}

// takeGrants keeps the grants part's key as heard; a value it cannot read is said and leaves the controller's list
// followed (no list is followed that the node cannot check).
func (f *Follower) takeGrants(u agent.Update) {
	f.mu.Lock()
	defer f.mu.Unlock()
	f.grantsRev = u.Revision
	if u.Deleted {
		f.grants = nil
		return
	}
	g, err := delivery.DecodeGrantsUserList(u.Value)
	if err != nil {
		f.grants = nil
		f.sayOnceLocked("the grants part's user-list key cannot be read, so the controller's list is followed: " + err.Error())
		return
	}
	f.grants = &g
}

// Following is which list the follower takes now: the grants part's while its key says so, the controller's otherwise.
func (f *Follower) Following() string {
	f.mu.Lock()
	defer f.mu.Unlock()
	return f.followingLocked()
}

func (f *Follower) followingLocked() string {
	if f.grants != nil && f.grants.Follow == delivery.FollowGrants {
		return delivery.FollowGrants
	}
	return delivery.FollowController
}

func (f *Follower) key() string {
	if f.env.Key == nil {
		return ""
	}
	return f.env.Key()
}

// take keeps a composition key's change when it carries the bus's user list, and forgets a key deleted.
func (f *Follower) take(u agent.Update) {
	node, module, _, isNode, err := delivery.ParseComposedKey(u.Key)
	if err != nil || node != f.env.Node || isNode {
		return
	}
	f.mu.Lock()
	defer f.mu.Unlock()
	if u.Deleted {
		delete(f.srcs, u.Key)
		return
	}
	c, err := delivery.DecodeComposed(u.Value)
	if err != nil {
		delete(f.srcs, u.Key)
		return
	}
	var trusted []ed25519.PublicKey
	if f.env.Trusted != nil {
		trusted = f.env.Trusted()
	}
	if _, err := delivery.VerifyComposed(c, trusted); err != nil {
		// The agent says it is refused; a refused composition is never a source of the list.
		delete(f.srcs, u.Key)
		return
	}
	if c.Node != f.env.Node || c.Module != module {
		// Signed, and for another node or module than its key says: a re-put of another composition is no source.
		delete(f.srcs, u.Key)
		f.sayOnceLocked(fmt.Sprintf("the composition under %s is %s's of %s, and is no source of the bus's user list",
			u.Key, c.Node, c.Module))
		return
	}
	raw, content, digest, ok := userList(c.Resources)
	if !ok {
		delete(f.srcs, u.Key)
		return
	}
	wanted := digest
	if wanted == "" {
		wanted = Digest([]byte(content))
	}
	f.srcs[u.Key] = source{key: u.Key, module: module, rev: u.Revision, resource: raw, wanted: wanted}
}

// settle writes the newest source's list when it is newer than the one written, or the one written again when asked.
// A source never goes back: a key deleted, or a newer one refused, leaves the list written as it is, since an older
// list would take away accounts a newer one gave.
func (f *Follower) settle(ctx context.Context, again bool) {
	f.mu.Lock()
	follow := f.followingLocked()
	if follow != f.following {
		// The switch turned: the other source is written, though neither changed.
		again = true
	}
	f.mu.Unlock()
	if follow == delivery.FollowGrants {
		f.settleGrants(ctx, again)
		return
	}
	f.mu.Lock()
	newest := f.cur
	for _, s := range f.srcs {
		if s.rev > newest.rev {
			newest = s
		}
	}
	if newest.key == "" || (newest.rev == f.cur.rev && f.written && !again) {
		f.mu.Unlock()
		return
	}
	f.cur = newest
	at := fmt.Sprintf("%s@%d", newest.key, newest.rev)
	f.mu.Unlock()

	w := Written{Source: newest.key, Revision: newest.rev, Wanted: newest.wanted, At: f.env.Now().UTC(),
		Follows: delivery.FollowController, Key: f.key()}
	var changed bool
	body, err := f.env.Resolve(ctx, newest.resource)
	if err == nil {
		w.Users = Users(body)
		err = f.guard(newest.resource, body)
		if err != nil {
			// Not written: what is on the machine stays, and the have says why, loudly.
			if disk, derr := f.env.OnDisk(newest.resource); derr == nil && disk != nil {
				w.Digest = Digest(disk)
			}
		}
	}
	if err == nil {
		w.Digest, changed, err = f.env.Write(ctx, newest.module, newest.resource, body)
	}
	if err != nil {
		w.Error = err.Error()
	}
	f.mu.Lock()
	prev := f.state
	f.mod, f.state = newest.module, w
	f.written = err == nil
	f.following = delivery.FollowController
	f.mu.Unlock()
	if err != nil {
		f.sayOnce(fmt.Sprintf("the bus's user list of %s could not be written: %v", at, err))
	} else if changed {
		f.env.Say(fmt.Sprintf("the bus's user list from %s is written alone (%d users, %s); the bus reloads it itself",
			at, w.Users, short(w.Digest)))
	}
	// Reported when what it says changed: a look again that found the file as it was says nothing new.
	if f.env.Report != nil && (changed || reportable(prev, w)) {
		f.env.Report(ctx, newest.module, w)
	}
}

// reportable says a have's user list says something new: a look again that found the file as it was says nothing.
func reportable(prev, w Written) bool {
	return prev.Source != w.Source || prev.Revision != w.Revision || prev.Digest != w.Digest || prev.Error != w.Error ||
		prev.Follows != w.Follows || prev.Key != w.Key
}

// settleGrants writes the grants part's list, the object its key names, into the file the controller's composition of
// the bus module declares, made 0600 (ADR 0547 clause 7): when the key moved, when the switch turned, or when asked.
func (f *Follower) settleGrants(ctx context.Context, again bool) {
	f.mu.Lock()
	g, rev := *f.grants, f.grantsRev
	file := f.cur
	for _, s := range f.srcs {
		if s.rev > file.rev {
			file = s
		}
	}
	if !again && f.following == delivery.FollowGrants && f.state.Revision == rev && f.written {
		f.mu.Unlock()
		return
	}
	f.mu.Unlock()

	w := Written{Source: delivery.GrantsUserListSource, Revision: rev, Wanted: g.Digest, At: f.env.Now().UTC(),
		Follows: delivery.FollowGrants, Key: f.key()}
	var err error
	var changed bool
	resource, rerr := mode0600(file.resource)
	switch {
	case file.key == "" || rerr != nil:
		err = fmt.Errorf("no composition of this node declares the bus's user-list file, so the grants part's list has nowhere to be written")
	case g.Digest == "":
		err = fmt.Errorf("the grants part's key names no list yet; the list on the machine is kept")
	case f.env.Fetch == nil || f.env.Open == nil:
		err = fmt.Errorf("this engine cannot read or open the grants part's list")
	case g.SealedTo != "" && w.Key != "" && g.SealedTo != delivery.SealingKeyID(w.Key):
		err = fmt.Errorf("the grants part's list is sealed to another key than this node's (%s); the list on the machine is kept until it seals to this one", g.SealedTo)
	}
	var body []byte
	if err == nil {
		var object []byte
		if object, err = f.env.Fetch(ctx, g.Object); err == nil {
			body, err = delivery.OpenUserList(object, g.Digest, f.env.Open)
		}
		if err != nil {
			err = fmt.Errorf("the grants part's list %s could not be had: %w", short(g.Digest), err)
		}
	}
	if err == nil {
		w.Users = Users(body)
		if err = f.guard(resource, body); err != nil {
			if disk, derr := f.env.OnDisk(resource); derr == nil && disk != nil {
				w.Digest = Digest(disk)
			}
		}
	}
	if err == nil {
		w.Digest, changed, err = f.env.Write(ctx, file.module, resource, body)
	} else if w.Digest == "" && file.key != "" && f.env.OnDisk != nil {
		if disk, derr := f.env.OnDisk(resource); derr == nil && disk != nil {
			w.Digest = Digest(disk)
		}
	}
	if err != nil {
		w.Error = err.Error()
	}
	f.mu.Lock()
	prev := f.state
	f.mod, f.state = file.module, w
	f.written = err == nil
	f.following = delivery.FollowGrants
	f.mu.Unlock()
	at := fmt.Sprintf("%s@%d", delivery.GrantsUserListSource, rev)
	if err != nil {
		f.sayOnce(fmt.Sprintf("the bus's user list of %s could not be written: %v", at, err))
	} else if changed {
		f.env.Say(fmt.Sprintf("the bus's user list from the grants part (%s, revision %d) is written alone (%d users, %s); the bus reloads it itself",
			at, g.Revision, w.Users, short(w.Digest)))
	}
	if f.env.Report != nil && file.module != "" && (changed || reportable(prev, w)) {
		f.env.Report(ctx, file.module, w)
	}
}

// mode0600 is a user-list resource made mode 0600, whatever its composition said (ADR 0547 clause 7).
func mode0600(raw json.RawMessage) (json.RawMessage, error) {
	if len(raw) == 0 {
		return nil, fmt.Errorf("no resource")
	}
	var r map[string]any
	if err := json.Unmarshal(raw, &r); err != nil {
		return nil, err
	}
	r["mode"] = "0600"
	return json.Marshal(r)
}

// ControllerUser is the controller's own user in the list: a list without it would lock the controller out of the bus.
const ControllerUser = "controller"

// guard refuses a list that would lock the mesh out of its own bus: one without the controller's own user, or one that
// drops more than half the users the file on the machine holds. A list that shrinks that far is a composer that read
// half the mesh, not half the mesh gone; it waits for a person, and the bus keeps the list it has.
func (f *Follower) guard(resource json.RawMessage, body []byte) error {
	names := Names(body)
	if !slices.Contains(names, ControllerUser) {
		return fmt.Errorf("refused: the list holds no user %q, and written it would lock the controller out of the "+
			"bus; the list on the machine is kept", ControllerUser)
	}
	if f.env.OnDisk == nil {
		return nil
	}
	disk, err := f.env.OnDisk(resource)
	if err != nil || disk == nil {
		return nil
	}
	if had := Users(disk); had > 0 && len(names)*2 < had {
		return fmt.Errorf("refused: the list holds %d users and the one on the machine %d; a list that drops more "+
			"than half is not written, and the list on the machine is kept", len(names), had)
	}
	return nil
}

func (f *Follower) sayOnceLocked(line string) {
	if !f.said[line] {
		f.said[line] = true
		f.env.Say(line)
	}
}

func (f *Follower) sayOnce(line string) {
	f.mu.Lock()
	said := f.said[line]
	f.said[line] = true
	f.mu.Unlock()
	if !said {
		f.env.Say(line)
	}
}

// userList is the bus's user list among a composition's resources: the resource, its content, and its digest when it
// is given by digest rather than content.
func userList(resources []json.RawMessage) (json.RawMessage, string, string, bool) {
	for _, raw := range resources {
		var head struct {
			ID      string `json:"id"`
			Type    string `json:"type"`
			Content string `json:"content"`
			Digest  string `json:"digest"`
		}
		if json.Unmarshal(raw, &head) != nil || head.Type != "file" {
			continue
		}
		if head.ID == ResourceID || strings.HasSuffix(head.ID, "."+ResourceID) {
			return raw, head.Content, head.Digest, true
		}
	}
	return nil, "", "", false
}

// Digest is content's digest as the have says it, `sha256:<hex>`.
func Digest(content []byte) string {
	sum := sha256.Sum256(content)
	return "sha256:" + hex.EncodeToString(sum[:])
}

// userEntry is one user of the list as the composer writes it (and the bus module's own tools read it).
var userEntry = regexp.MustCompile(`\{\s*user:\s*"([^"]+)"`)

// Users is how many users a list holds.
func Users(content []byte) int { return len(userEntry.FindAllIndex(content, -1)) }

// Names is the users a list holds, as it names them.
func Names(content []byte) []string {
	var out []string
	for _, m := range userEntry.FindAllSubmatch(content, -1) {
		out = append(out, string(m[1]))
	}
	return out
}

func short(d string) string {
	d = strings.TrimPrefix(d, "sha256:")
	if len(d) > 12 {
		d = d[:12]
	}
	return d
}