// Package engine changes Housecarl's configuration. Every change, from the
// API, the CLI or an import, goes through Apply: the change is made on a
// copy of the state, the result is validated as a whole, the ruleset built
// from it is checked by nftables and put in force, and only then are the
// changed objects and a history entry saved together (if saving fails, the
// previous ruleset is put back). Revert undoes a history entry the same
// way.
package engine

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
	"sort"
	"strings"
	"sync"
	"time"

	"housecarl/internal/auth"
	"housecarl/internal/model"
	"housecarl/internal/store"
	"housecarl/internal/validate"
)

// Kinds of errors, for the API's status codes.
const (
	KindBadRequest = "bad_request"
	KindNotFound   = "not_found"
	KindConflict   = "conflict"
	KindValidation = "validation"
	// KindFirewall: nftables refused or failed to apply the ruleset.
	KindFirewall = "firewall"
)

// Firewall puts configurations in force (internal/nft, through
// internal/render).
type Firewall interface {
	// Check builds the ruleset and has nftables check it, changing
	// nothing.
	Check(ctx context.Context, st *model.State) error
	// Apply puts the configuration's ruleset in force, in one
	// transaction.
	Apply(ctx context.Context, st *model.State) error
}

// Error is a refused change.
type Error struct {
	Kind     string             `json:"code"`
	Message  string             `json:"message"`
	Problems []validate.Problem `json:"problems,omitempty"`
}

func (e *Error) Error() string { return e.Message }

// Errorf makes an Error.
func Errorf(kind, format string, args ...any) *Error {
	return &Error{Kind: kind, Message: fmt.Sprintf(format, args...)}
}

// AsError unwraps an Error.
func AsError(err error) (*Error, bool) {
	var e *Error
	ok := errors.As(err, &e)
	return e, ok
}

// Result describes an applied (or previewed) change.
type Result struct {
	Applied   bool               `json:"applied"`
	DryRun    bool               `json:"dry_run,omitempty"`
	HistoryID int64              `json:"history_id,omitempty"`
	Changes   []model.Change     `json:"changes"`
	Warnings  []validate.Problem `json:"warnings,omitempty"`
	// Before and After are the configurations the change goes from and
	// to (After also for a dry run), for diffs and answers.
	Before, After *model.State `json:"-"`
}

// Engine holds the current configuration.
type Engine struct {
	store *store.Store
	log   *slog.Logger
	// Env describes the host's interfaces for validation (nil: skip).
	Env func() *validate.Env
	// Firewall puts each new state in force (nil: none, in tests that
	// only look at the configuration).
	Firewall Firewall
	// OnChange is called with each new state, after it was saved.
	OnChange func(*model.State)

	mu    sync.Mutex
	state *model.State
}

// New loads the configuration.
func New(ctx context.Context, st *store.Store, log *slog.Logger) (*Engine, error) {
	if log == nil {
		log = slog.Default()
	}
	s, err := st.LoadState(ctx)
	if err != nil {
		return nil, err
	}
	Normalize(s)
	return &Engine{store: st, log: log, state: s}, nil
}

// Store is the engine's database.
func (e *Engine) Store() *store.Store { return e.store }

// State returns a copy of the current configuration.
func (e *Engine) State() *model.State {
	e.mu.Lock()
	defer e.mu.Unlock()
	return e.state.Clone()
}

func (e *Engine) env() *validate.Env {
	if e.Env == nil {
		return nil
	}
	return e.Env()
}

// Problems validates the current configuration (for the status page).
func (e *Engine) Problems() []validate.Problem {
	return validate.Check(e.State(), e.env())
}

// Edit changes a configuration in place. NewID gives IDs for new objects.
type Edit func(st *model.State, newID func() int64) error

// Options say how to apply a change.
type Options struct {
	DryRun bool
	Via    string // web, cli, api, import
	// RevertOf is the history entry the change undoes (a changeset with a
	// staged undo), so History links the two.
	RevertOf int64
}

// Apply makes a change: edit runs on a copy of the configuration, the
// result is validated, and the changed objects are saved with a history
// entry. A change that touches nothing is not recorded.
func (e *Engine) Apply(ctx context.Context, p *auth.Principal, description string, opts Options, edit Edit) (*Result, error) {
	e.mu.Lock()
	defer e.mu.Unlock()
	next := e.state.Clone()
	var idErr error
	newID := func() int64 {
		ids, err := e.store.AllocateIDs(ctx, 1)
		if err != nil {
			idErr = err
			return 0
		}
		return ids[0]
	}
	if err := edit(next, newID); err != nil {
		return nil, err
	}
	if idErr != nil {
		return nil, idErr
	}
	return e.commit(ctx, p, description, opts, next, opts.RevertOf)
}

// commit validates next and saves it as the new state.
func (e *Engine) commit(ctx context.Context, p *auth.Principal, description string, opts Options, next *model.State, revertOf int64) (*Result, error) {
	Normalize(next)
	next.Sort()
	changes := Diff(e.state, next)
	res := &Result{DryRun: opts.DryRun, Changes: changes, Before: e.state.Clone(), After: next.Clone()}
	if len(changes) == 0 {
		return res, nil
	}
	problems := validate.Check(next, e.env())
	if validate.HasErrors(problems) {
		errs := validate.Errors(problems)
		msg := errs[0].Message
		if errs[0].Object != "" {
			msg = errs[0].Object + ": " + msg
		}
		if len(errs) > 1 {
			msg += fmt.Sprintf(" (and %d more problems)", len(errs)-1)
		}
		return res, &Error{Kind: KindValidation, Message: msg, Problems: errs}
	}
	// Only report warnings the change brings; old ones are on the status
	// page.
	res.Warnings = newProblems(validate.Check(e.state, e.env()), problems)
	res.Warnings = append(res.Warnings, quotaRestarts(e.state, next)...)
	if e.Firewall != nil {
		if err := e.Firewall.Check(ctx, next); err != nil {
			return res, &Error{Kind: KindFirewall, Message: "nftables refused the ruleset: " + err.Error()}
		}
	}
	if opts.DryRun {
		return res, nil
	}
	if e.Firewall != nil {
		if err := e.Firewall.Apply(ctx, next); err != nil {
			return res, &Error{Kind: KindFirewall, Message: "applying the ruleset failed, nothing was changed: " + err.Error()}
		}
	}
	entry := &model.HistoryEntry{Time: time.Now(), Username: p.Name(), Via: opts.Via, Description: description, Changes: changes, RevertOf: revertOf}
	if p != nil && p.User != nil {
		entry.UserID = p.User.ID
	}
	if entry.Description == "" {
		entry.Description = Describe(changes)
	}
	if err := e.store.SaveChanges(ctx, changes, entry); err != nil {
		// The new ruleset is in force but not saved: put the old one back.
		if e.Firewall != nil {
			if rerr := e.Firewall.Apply(ctx, e.state); rerr != nil {
				e.log.Error("saving a change failed, and putting the previous ruleset back failed too", "err", rerr)
			}
		}
		return res, err
	}
	e.state = next
	res.Applied, res.HistoryID = true, entry.ID
	e.Audit(ctx, p, "config.change", fmt.Sprintf("history %d", entry.ID), "success", entry.Description)
	e.log.Info("configuration changed", "history", entry.ID, "user", p.Name(), "via", opts.Via, "description", entry.Description)
	if e.OnChange != nil {
		e.OnChange(next.Clone())
	}
	return res, nil
}

// quotaRestarts warns of the rules whose quota starts again because they
// now count it in a way what they used can't be carried to.
func quotaRestarts(before, after *model.State) []validate.Problem {
	was := map[int64]*model.Rule{}
	for _, r := range before.Rules {
		was[r.ID] = r
	}
	var out []validate.Problem
	for _, r := range after.Rules {
		if carry, why := model.QuotaCarry(was[r.ID], r); carry == model.QuotaRestarts {
			out = append(out, validate.Problem{Severity: validate.Warning, Code: "quota_restarts", Object: r.Label(),
				Message: "its quota starts again: " + why})
		}
	}
	return out
}

func newProblems(before, after []validate.Problem) []validate.Problem {
	seen := map[string]bool{}
	for _, p := range before {
		seen[p.Code+"\x00"+p.Object+"\x00"+p.Message] = true
	}
	var out []validate.Problem
	for _, p := range after {
		if !seen[p.Code+"\x00"+p.Object+"\x00"+p.Message] {
			out = append(out, p)
		}
	}
	return out
}

// ApplyCurrent puts the current configuration in force again: at start,
// and when a flowtable device appears. It holds the engine's lock, so a
// change can't be applied in between.
func (e *Engine) ApplyCurrent(ctx context.Context) error {
	if e.Firewall == nil {
		return nil
	}
	e.mu.Lock()
	defer e.mu.Unlock()
	return e.Firewall.Apply(ctx, e.state.Clone())
}

// Revert undoes a history entry, if the objects it changed are still as
// it left them.
func (e *Engine) Revert(ctx context.Context, p *auth.Principal, id int64, opts Options) (*Result, error) {
	h, err := e.store.GetHistory(ctx, id)
	if errors.Is(err, store.ErrNotFound) {
		return nil, Errorf(KindNotFound, "history entry %d not found", id)
	}
	if err != nil {
		return nil, err
	}
	if h.RevertedBy != 0 {
		return nil, Errorf(KindConflict, "history entry %d was already undone by entry %d", id, h.RevertedBy)
	}
	e.mu.Lock()
	defer e.mu.Unlock()
	next := e.state.Clone()
	if err := Undo(next, h); err != nil {
		return nil, err
	}
	return e.commit(ctx, p, UndoDescription(h), opts, next, h.ID)
}

// UndoDescription is an undo's description: "undo 12: create rule …".
func UndoDescription(h *model.HistoryEntry) string {
	return fmt.Sprintf("undo %d: %s", h.ID, h.Description)
}

// Undo applies a history entry backwards to st, if every object it
// changed is still as it left it (a change made since would be lost).
func Undo(st *model.State, h *model.HistoryEntry) error {
	cur := objects(st)
	for i := len(h.Changes) - 1; i >= 0; i-- {
		c := h.Changes[i]
		now := cur[key{c.Kind, c.ID}]
		if !sameJSON(now, c.After) {
			what := c.Label
			if what == "" {
				what = fmt.Sprintf("%s %d", c.Kind, c.ID)
			}
			return Errorf(KindConflict, "%s %s has changed since entry %d; undo the later changes first, or change it by hand", c.Kind, what, h.ID)
		}
		if err := put(st, c.Kind, c.ID, c.Before); err != nil {
			return err
		}
	}
	return nil
}

// Rewind applies a history entry backwards to st without Undo's check:
// stepping back through the entries newest first brings back the
// configuration as it was before each.
func Rewind(st *model.State, h *model.HistoryEntry) error {
	for i := len(h.Changes) - 1; i >= 0; i-- {
		c := h.Changes[i]
		if c.Kind == model.KindSettings && c.Before == nil {
			continue
		}
		if err := put(st, c.Kind, c.ID, c.Before); err != nil {
			return err
		}
	}
	// put appends what it brings back; its position puts it in place.
	st.Sort()
	return nil
}

// Replace makes the configuration next as a whole (imports).
func (e *Engine) Replace(ctx context.Context, p *auth.Principal, description string, opts Options, next *model.State) (*Result, error) {
	e.mu.Lock()
	defer e.mu.Unlock()
	return e.commit(ctx, p, description, opts, next.Clone(), 0)
}

// Audit records an action in the audit log.
func (e *Engine) Audit(ctx context.Context, p *auth.Principal, action, object, result, detail string) {
	ev := &model.AuditEvent{Action: action, Object: object, Result: result, Detail: detail}
	if p != nil {
		ev.Username, ev.Auth, ev.IP = p.Name(), p.Method, p.IP
		if p.User != nil {
			ev.UserID = p.User.ID
		}
		if p.Token != nil {
			ev.TokenID = p.Token.ID
		}
	}
	if err := e.store.AddAudit(ctx, ev); err != nil {
		e.log.Warn("writing the audit log failed", "err", err)
	}
}

// ---- diffs

type key struct {
	kind string
	id   int64
}

func objects(st *model.State) map[key]json.RawMessage {
	m := map[key]json.RawMessage{{model.KindSettings, 0}: mustJSON(st.Settings)}
	for _, k := range model.Kinds {
		for _, o := range st.Objects(k) {
			m[key{k, o.GetID()}] = mustJSON(o)
		}
	}
	return m
}

func labels(st *model.State) map[key]string {
	m := map[key]string{{model.KindSettings, 0}: "settings"}
	for _, k := range model.Kinds {
		for _, o := range st.Objects(k) {
			m[key{k, o.GetID()}] = o.Label()
		}
	}
	return m
}

// Diff lists what changes from a to b: settings, then each kind of
// object (zones, sets, services, rules, NAT), each by ID.
func Diff(a, b *model.State) []model.Change {
	oa, ob := objects(a), objects(b)
	la, lb := labels(a), labels(b)
	var keys []key
	for k := range oa {
		keys = append(keys, k)
	}
	for k := range ob {
		if _, ok := oa[k]; !ok {
			keys = append(keys, k)
		}
	}
	order := map[string]int{model.KindSettings: 0}
	for i, k := range model.Kinds {
		order[k] = i + 1
	}
	sort.Slice(keys, func(i, j int) bool {
		if keys[i].kind != keys[j].kind {
			return order[keys[i].kind] < order[keys[j].kind]
		}
		return keys[i].id < keys[j].id
	})
	var out []model.Change
	for _, k := range keys {
		before, after := oa[k], ob[k]
		if sameJSON(before, after) {
			continue
		}
		label := lb[k]
		if label == "" {
			label = la[k]
		}
		out = append(out, model.Change{Kind: k.kind, ID: k.id, Label: label, Before: before, After: after})
	}
	return out
}

// Describe sums up changes: "create zone lan (eth1)", or
// "3 changes" for many.
func Describe(cs []model.Change) string {
	if len(cs) == 0 {
		return "no changes"
	}
	parts := []string{}
	for _, c := range cs {
		if c.Kind == model.KindSettings {
			parts = append(parts, "update settings")
			continue
		}
		parts = append(parts, c.Action()+" "+c.Kind+" "+c.Label)
	}
	if len(parts) > 3 {
		return fmt.Sprintf("%s and %d more changes", strings.Join(parts[:2], ", "), len(parts)-2)
	}
	return strings.Join(parts, ", ")
}

func mustJSON(v any) json.RawMessage {
	b, err := json.Marshal(v)
	if err != nil {
		panic(err)
	}
	return b
}

func sameJSON(a, b json.RawMessage) bool {
	if a == nil || b == nil {
		return a == nil && b == nil
	}
	return bytes.Equal(a, b)
}

// put sets (or with nil data deletes) one object in st.
func put(st *model.State, kind string, id int64, data json.RawMessage) error {
	if kind == model.KindSettings {
		if data == nil {
			return fmt.Errorf("settings can't be deleted")
		}
		var set model.Settings
		if err := json.Unmarshal(data, &set); err != nil {
			return err
		}
		st.Settings = set
		return nil
	}
	return st.Put(kind, id, data)
}

// Normalize writes names, addresses, ports and lists in their canonical
// form, so equal configurations compare equal, and spaces rule positions
// out again when insertions have crowded them.
func Normalize(st *model.State) {
	st.Fill()
	set := &st.Settings
	set.AntispoofV4 = normAddrs(set.AntispoofV4)
	set.AntispoofV6 = normAddrs(set.AntispoofV6)
	set.OwnPrefixes = normAddrs(set.OwnPrefixes)
	set.FlowtableDevices = normList(set.FlowtableDevices)
	for _, z := range st.Zones {
		z.Name = strings.TrimSpace(z.Name)
		z.Interfaces = normList(z.Interfaces)
	}
	for _, x := range st.Sets {
		x.Name = strings.TrimSpace(x.Name)
		x.Entries = normAddrs(x.Entries)
		if j := x.Jarl; j != nil {
			for i, n := range j.Names {
				j.Names[i] = model.ParseJarlName(n.Name)
				if n.View != "" {
					j.Names[i].View = strings.TrimSpace(n.View)
				}
			}
			j.Within = normAddrs(j.Within) // zone names stay as they are
		}
	}
	for _, x := range st.Services {
		x.Name = strings.TrimSpace(x.Name)
		x.Entries = normServices(x.Entries)
	}
	for _, r := range st.Rules {
		normMatch(&r.Match)
		r.CtState = normList(lower(r.CtState))
		r.CtStatus = normList(lower(r.CtStatus))
		r.Raw = strings.TrimSpace(r.Raw)
	}
	for _, f := range st.Forwards {
		f.Protocols = normList(lower(f.Protocols))
		f.PublicAddresses = normAddrs(f.PublicAddresses)
		f.Sources = normAddrs(f.Sources)
		f.HairpinSources = normAddrs(f.HairpinSources)
		f.Ports = normPorts(f.Ports)
		f.ToPort = normPorts(f.ToPort)
		f.To = normAddr(f.To)
		f.To6 = normAddr(f.To6)
	}
	for _, x := range st.SNATs {
		normMatch(&x.Match)
		x.To = normAddr(x.To)
	}
	for _, x := range st.Redirects {
		normMatch(&x.Match)
		x.ToPort = normPorts(x.ToPort)
	}
	for _, b := range st.BanLists {
		b.Name = strings.TrimSpace(b.Name)
		b.Scope = strings.ToLower(strings.TrimSpace(b.Scope))
		b.Hosts = normAddrs(b.Hosts)
		b.Never = normAddrs(b.Never)
	}
	// Positions are fractions after insertions between rules; renumber
	// only when two have come too close to put another between.
	st.Sort()
	crowded := func(pos []float64) bool {
		for i := 1; i < len(pos); i++ {
			if pos[i]-pos[i-1] < 1e-6 {
				return true
			}
		}
		return false
	}
	byChain := map[string][]float64{}
	for _, r := range st.Rules {
		byChain[r.Chain] = append(byChain[r.Chain], r.Position)
	}
	re := false
	for _, pos := range byChain {
		re = re || crowded(pos)
	}
	var fp, sp, rp []float64
	for _, x := range st.Forwards {
		fp = append(fp, x.Position)
	}
	for _, x := range st.SNATs {
		sp = append(sp, x.Position)
	}
	for _, x := range st.Redirects {
		rp = append(rp, x.Position)
	}
	if re || crowded(fp) || crowded(sp) || crowded(rp) {
		st.Renumber()
	}
}

func normMatch(m *model.Match) {
	m.InZones = normList(m.InZones)
	m.OutZones = normList(m.OutZones)
	m.Sources = normAddrs(m.Sources)
	m.Destinations = normAddrs(m.Destinations)
	m.Services = normServices(m.Services)
	m.Family = strings.ToLower(strings.TrimSpace(m.Family))
}

// normList trims, drops empty entries and duplicates, keeping the order.
func normList(in []string) []string {
	var out []string
	seen := map[string]bool{}
	for _, s := range in {
		s = strings.TrimSpace(s)
		if s != "" && !seen[s] {
			seen[s] = true
			out = append(out, s)
		}
	}
	return out
}

func lower(in []string) []string {
	out := make([]string, len(in))
	for i, s := range in {
		out[i] = strings.ToLower(s)
	}
	return out
}

// normAddrs writes addresses canonically; set names stay as they are.
func normAddrs(in []string) []string {
	out := normList(in)
	for i, s := range out {
		out[i] = normAddr(s)
	}
	return normList(out)
}

func normAddr(s string) string {
	s = strings.TrimSpace(s)
	if n, err := model.NormalizeAddress(s); err == nil {
		return n
	}
	return s
}

func normServices(in []string) []string {
	out := normList(in)
	for i, s := range out {
		if e, err := model.ParseServiceEntry(s); err == nil {
			out[i] = e.String()
		}
	}
	return normList(out)
}

func normPorts(s string) string {
	if n, err := model.NormalizePorts(s); err == nil {
		return n
	}
	return strings.TrimSpace(s)
}