// Package engine changes Steward'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, and only then are
// the changed objects and a history entry saved together and the new state
// handed to the DHCP server. Revert undoes a history entry the same way.
package engine

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

	"steward/internal/auth"
	"steward/internal/dhcp4"
	"steward/internal/model"
	"steward/internal/store"
	"steward/internal/validate"
)

// Kinds of errors, for the API's status codes.
const (
	KindBadRequest = "bad_request"
	KindNotFound   = "not_found"
	KindConflict   = "conflict"
	KindValidation = "validation"
)

// 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"`
}

// 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
	// OnChange is called with each new state (the DHCP server's config).
	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
}

// 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, 0)
}

// 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}
	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)
	if opts.DryRun {
		return res, nil
	}
	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 {
		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
}

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
}

// 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()
	cur := objects(e.state)
	next := e.state.Clone()
	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 nil, Errorf(KindConflict, "%s %s has changed since entry %d; undo the later changes first, or change it by hand", c.Kind, what, id)
		}
		if err := put(next, c.Kind, c.ID, c.Before); err != nil {
			return nil, err
		}
	}
	return e.commit(ctx, p, fmt.Sprintf("undo %d: %s", h.ID, h.Description), opts, next, h.ID)
}

// 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 _, s := range st.Subnets {
		m[key{model.KindSubnet, s.ID}] = mustJSON(s)
	}
	for _, r := range st.Reservations {
		m[key{model.KindReservation, r.ID}] = mustJSON(r)
	}
	return m
}

func labels(st *model.State) map[key]string {
	m := map[key]string{{model.KindSettings, 0}: "settings"}
	for _, s := range st.Subnets {
		m[key{model.KindSubnet, s.ID}] = s.Label()
	}
	for _, r := range st.Reservations {
		m[key{model.KindReservation, r.ID}] = r.Label()
	}
	return m
}

// Diff lists what changes from a to b: settings, then subnets, then
// reservations, 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, model.KindSubnet: 1, model.KindReservation: 2}
	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 subnet office (10.0.10.0/24)", 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 {
	switch kind {
	case model.KindSettings:
		if data == nil {
			return fmt.Errorf("settings can't be deleted")
		}
		var s model.Settings
		if err := json.Unmarshal(data, &s); err != nil {
			return err
		}
		st.Settings = s
	case model.KindSubnet:
		var keep []*model.Subnet
		for _, s := range st.Subnets {
			if s.ID != id {
				keep = append(keep, s)
			}
		}
		if data != nil {
			var s model.Subnet
			if err := json.Unmarshal(data, &s); err != nil {
				return err
			}
			keep = append(keep, &s)
		}
		st.Subnets = keep
	case model.KindReservation:
		var keep []*model.Reservation
		for _, r := range st.Reservations {
			if r.ID != id {
				keep = append(keep, r)
			}
		}
		if data != nil {
			var r model.Reservation
			if err := json.Unmarshal(data, &r); err != nil {
				return err
			}
			keep = append(keep, &r)
		}
		st.Reservations = keep
	default:
		return fmt.Errorf("unknown kind %q", kind)
	}
	return nil
}

// Normalize fills in option names and writes option values, pools and
// lists in their canonical form, so equal configurations compare equal.
func Normalize(st *model.State) {
	st.Settings.Options = normOptions(st.Settings.Options)
	if st.Settings.AddressOrder == model.OrderSequential {
		st.Settings.AddressOrder = "" // the default
	}
	if st.Subnets == nil {
		st.Subnets = []*model.Subnet{}
	}
	if st.Reservations == nil {
		st.Reservations = []*model.Reservation{}
	}
	for _, s := range st.Subnets {
		s.Options = normOptions(s.Options)
		if s.Pools == nil {
			s.Pools = []model.Pool{}
		}
		if s.Prefix.IsValid() && s.Prefix.Addr().Is4() {
			s.Prefix = s.Prefix.Masked()
		}
		// The pools' order counts (pool_order listed): it is kept.
		if s.PoolOrder == model.PoolsListed {
			s.PoolOrder = ""
		}
		if s.Access == model.AccessDenyList {
			s.Access = ""
		}
		if len(s.Allow) == 0 {
			s.Allow = nil
		}
		if len(s.Deny) == 0 {
			s.Deny = nil
		}
	}
	for _, r := range st.Reservations {
		r.Options = normOptions(r.Options)
	}
}

func normOptions(opts []model.OptionValue) []model.OptionValue {
	out := []model.OptionValue{}
	for _, o := range opts {
		if o.Code > 0 && o.Code < 255 {
			def := dhcp4.ByCode(byte(o.Code))
			o.Name = def.Name
			typ := def.Type
			if dhcp4.Known(byte(o.Code)) {
				o.Type = ""
			} else if o.Type != "" {
				typ = o.Type
			}
			if v, err := dhcp4.Normalize(typ, o.Value); err == nil {
				o.Value = v
			}
		}
		out = append(out, o)
	}
	sort.SliceStable(out, func(i, j int) bool { return out[i].Code < out[j].Code })
	return out
}