// Package api serves the admin REST API, the Prometheus metrics endpoint and
// the embedded web console.
//
// The API is the only component that mutates state, so every handler funnels
// through one of three paths: a read from a live component, a config mutation
// (validate -> write file -> swap -> broadcast), or a targeted runtime action
// (flush cache, refresh lists, reload config).
package api

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
	"net"
	"net/http"
	"net/netip"
	"os"
	"strconv"
	"strings"
	"sync"
	"time"

	"git.k8s-git-k6n.net/mats/fancy-dns/internal/config"
)

// Principal is the authenticated identity behind a request.
type Principal struct {
	// Mode is "apikey", "basic", "oidc" or "anonymous".
	Mode string
	// Name is a human label for the key or user.
	Name string
	// ReadOnly is true for a read-only API key.
	ReadOnly bool
	// Loopback is true when the request came from the local machine and
	// trust_loopback is on.
	Loopback bool
}

// CanWrite reports whether the principal may perform mutations.
func (p Principal) CanWrite() bool { return !p.ReadOnly }

// Hooks are the live components the API reads and drives. They are interfaces
// so the api package does not depend on the engine internals.
type Hooks struct {
	// Store is the config store (required).
	Store *config.Store
	// Logger is the structured logger (required).
	Logger *slog.Logger
	// Version is the running build's version.
	Version string
	// StartedAt is the process start time.
	StartedAt time.Time

	// Dashboard returns the aggregated dashboard payload.
	Dashboard func(ctx context.Context, rangeKey string) (any, error)
	// BlocklistState returns the compiled blocklist detail.
	BlocklistState func() any
	// BlocklistStats returns the compiled rule counts.
	BlocklistStats func() any
	// RefreshBlocklists recompiles the blocklist; empty names means all.
	RefreshBlocklists func(ctx context.Context, names []string) error
	// ToggleBlocklist enables or disables one source at runtime.
	ToggleBlocklist func(name string, enabled bool) error
	// DomainCheck explains how a name would be treated.
	DomainCheck func(name string) (any, error)
	// ListDomains pages the compiled rules.
	ListDomains func(q DomainQuery) (any, error)
	// AddDomain adds an inline rule and persists it.
	AddDomain func(ctx context.Context, in DomainInput) (any, error)
	// DeleteDomain removes a rule and persists the change.
	DeleteDomain func(ctx context.Context, name, action, list string) error
	// LocalZones lists, saves and deletes UI-managed DNS master files.
	LocalZones      func(ctx context.Context) ([]ZoneFile, error)
	SaveLocalZone   func(ctx context.Context, name, content string) error
	DeleteLocalZone func(ctx context.Context, name string) error
	// GetACL returns the ACL groups.
	GetACL func() any
	// PutACL validates and persists new ACL groups.
	PutACL func(ctx context.Context, groups []config.Group, def string) (int64, error)
	// GetUpstream returns the upstream pool state.
	GetUpstream func() any
	// PutUpstream validates and persists a new upstream config.
	PutUpstream func(ctx context.Context, up config.Upstream) (int64, error)
	// SearchLog pages the query log.
	SearchLog func(ctx context.Context, q LogQuery) (any, error)
	// LogStats is the live stats event pushed over SSE.
	LogStats func() any
	// SubscribeLog streams new log entries; the cancel func must be called.
	SubscribeLog func() (<-chan LogEntry, func())
	// ClearLog empties the query log.
	ClearLog func() error
	// CacheStats returns the cache snapshot.
	CacheStats func() any
	// CacheTop returns the hottest cached names.
	CacheTop func(limit int) any
	// FlushCache empties the cache.
	FlushCache func() int64
	// Health returns the health payload for /healthz.
	Health func() any
	// Status returns the per-component runtime status.
	Status func() any
	// Flush persists buffered state.
	Flush func()
}

// DomainQuery pages the compiled rules.
type DomainQuery struct {
	Text    string
	Action  string
	List    string
	Page    int
	PerPage int
}

// DomainInput adds a rule.
type DomainInput struct {
	Name    string `json:"name"`
	Action  string `json:"action"`
	List    string `json:"list"`
	Source  string `json:"source"`
	Content string `json:"content,omitempty"`
}

// ZoneFile is a UI-managed authoritative master file.
type ZoneFile struct {
	Name    string `json:"name"`
	Content string `json:"content"`
}

// LogQuery pages the query log.
type LogQuery struct {
	Limit       int
	Offset      int
	Text        string
	Action      string
	Client      string
	RCode       int
	BlockedOnly bool
	Since       time.Time
}

// LogEntry is one query-log row as the admin API serves it.
//
// These fields are not omitempty, and that is deliberate: omitempty makes the
// key set depend on the row, so a query with no ACL group arrived without
// `client_group` and the console rendered "undefined" where the column should
// be empty. The contract test in internal/app pins the key set, and it can
// only pin one shape if the server emits one.
type LogEntry struct {
	ID       uint64    `json:"id"`
	TS       time.Time `json:"ts"`
	Client   string    `json:"client"`
	Group    string    `json:"client_group"`
	Name     string    `json:"name"`
	QType    uint16    `json:"qtype"`
	QTypeStr string    `json:"qtype_name"`
	RCode    int       `json:"rcode"`
	RCodeStr string    `json:"rcode_name"`
	Action   string    `json:"action"`
	Response string    `json:"response"`
	Duration int64     `json:"duration_ms"`
	Upstream string    `json:"upstream"`
	List     string    `json:"list"`
	Protocol string    `json:"protocol"`
	Blocked  bool      `json:"blocked"`
}

// Server is the admin HTTP server.
type Server struct {
	hooks   Hooks
	log     *slog.Logger
	base    string
	mux     *http.ServeMux
	auth    *authenticator
	trusted []netip.Prefix

	// sessions tracks issued session tokens for the basic-auth flow.
	sessMu   sync.Mutex
	sessions map[string]session

	// limits tracks per-IP rate limits and lockouts.
	limMu  sync.Mutex
	limits map[string]*limitState
	now    func() time.Time
}

type session struct {
	principal Principal
	expires   time.Time
}

type limitState struct {
	window     time.Time
	count      int
	failures   int
	lockedTill time.Time
}

// New builds the API server.
func New(hooks Hooks) (*Server, error) {
	if hooks.Store == nil {
		return nil, errors.New("api: Store is required")
	}
	log := hooks.Logger
	if log == nil {
		log = slog.Default()
	}
	cfg := hooks.Store.Get()
	s := &Server{
		hooks:    hooks,
		log:      log.With("component", "api"),
		base:     cfg.API.BasePath,
		now:      time.Now,
		sessions: make(map[string]session),
		limits:   make(map[string]*limitState),
		auth:     newAuthenticator(cfg.API.Auth),
	}
	for _, c := range cfg.API.TrustedProxies {
		if p, err := netip.ParsePrefix(c); err == nil {
			s.trusted = append(s.trusted, p)
		}
	}
	s.mux = s.routes(cfg)
	return s, nil
}

// Handler returns the HTTP handler, wrapped with logging and panic recovery.
func (s *Server) Handler() http.Handler {
	return s.recoverer(s.cors(s.mux))
}

// routes builds the route table. The stdlib mux handles the path prefix, so
// handlers see paths relative to the base.
func (s *Server) routes(cfg *config.Config) *http.ServeMux {
	mux := http.NewServeMux()
	// base is the prefix every route is built from, so it must end in a
	// slash: "GET " + base + "healthz" has to produce "/api/healthz", not
	// "/apihealthz".
	base := strings.TrimSuffix(s.base, "/") + "/"

	// Unauthenticated: health and metrics are for probes, not operators.
	mux.Handle("GET "+base+"healthz", s.public(s.handleHealth))
	if cfg.Metrics.Enabled {
		// Bound late so the metrics package's init has run by the time the
		// handler is invoked; a nil handler means "not wired up" (a test, or
		// an embedder that does not import metrics), which must not panic the
		// route table.
		mux.Handle("GET "+base+"metrics", s.protectMetrics(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			if h := metricsHandler(); h != nil {
				h.ServeHTTP(w, r)
				return
			}
			http.Error(w, "metrics not available", http.StatusNotImplemented)
		})))
	}
	mux.Handle("POST "+base+"auth/login", s.limited(s.handleLogin))
	mux.Handle("GET "+base+"auth/config", s.public(s.handleAuthConfig))
	mux.Handle("POST "+base+"auth/logout", s.authed(s.handleLogout, false))

	// Everything below requires a principal.
	mux.Handle("GET "+base+"dashboard", s.authed(s.handleDashboard, false))
	mux.Handle("GET "+base+"status", s.authed(s.handleStatus, false))
	mux.Handle("GET "+base+"blocklists", s.authed(s.handleBlocklists, false))
	mux.Handle("POST "+base+"blocklists/refresh", s.authed(s.handleBlocklistRefresh, true))
	mux.Handle("POST "+base+"blocklists/toggle", s.authed(s.handleBlocklistToggle, true))
	mux.Handle("GET "+base+"domains", s.authed(s.handleDomains, false))
	mux.Handle("POST "+base+"domains", s.authed(s.handleDomainAdd, true))
	mux.Handle("DELETE "+base+"domains/{name...}", s.authed(s.handleDomainDelete, true))
	mux.Handle("GET "+base+"domains/check", s.authed(s.handleDomainCheck, false))
	mux.Handle("GET "+base+"local-zones", s.authed(s.handleLocalZones, false))
	mux.Handle("PUT "+base+"local-zones/{name}", s.authed(s.handleLocalZonePut, true))
	mux.Handle("DELETE "+base+"local-zones/{name}", s.authed(s.handleLocalZoneDelete, true))
	mux.Handle("GET "+base+"clients", s.authed(s.handleClients, false))
	mux.Handle("PUT "+base+"clients", s.authed(s.handleClientsPut, true))
	mux.Handle("GET "+base+"upstreams", s.authed(s.handleUpstreams, false))
	mux.Handle("PUT "+base+"upstreams", s.authed(s.handleUpstreamsPut, true))
	mux.Handle("GET "+base+"logs", s.authed(s.handleLogs, false))
	mux.Handle("GET "+base+"logs/stream", s.authed(s.handleLogStream, false))
	mux.Handle("POST "+base+"logs/clear", s.authed(s.handleLogsClear, true))
	mux.Handle("GET "+base+"cache", s.authed(s.handleCache, false))
	mux.Handle("GET "+base+"cache/top", s.authed(s.handleCacheTop, false))
	mux.Handle("POST "+base+"cache/flush", s.authed(s.handleCacheFlush, true))
	mux.Handle("GET "+base+"settings", s.authed(s.handleSettingsGet, false))
	mux.Handle("PUT "+base+"settings", s.authed(s.handleSettingsPut, true))
	mux.Handle("POST "+base+"settings/validate", s.authed(s.handleSettingsValidate, false))
	mux.Handle("POST "+base+"settings/reload", s.authed(s.handleSettingsReload, true))
	mux.Handle("GET "+base+"whoami", s.authed(s.handleWhoami, false))
	return mux
}

// recoverer turns a panic in a handler into a 500 instead of a dropped
// connection, and logs it.
func (s *Server) recoverer(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		defer func() {
			if rec := recover(); rec != nil {
				s.log.Error("panic in api handler", "path", r.URL.Path, "panic", rec)
				writeError(w, http.StatusInternalServerError, "internal error")
			}
		}()
		next.ServeHTTP(w, r)
	})
}

// cors adds the configured origins. A wildcard is allowed but echoed per
// request rather than as "*", so credentials keep working.
func (s *Server) cors(next http.Handler) http.Handler {
	allowed := map[string]bool{}
	wildcard := false
	for _, o := range s.hooks.Store.Get().API.CORSOrigins {
		if o == "*" {
			wildcard = true
		}
		allowed[o] = true
	}
	if len(allowed) == 0 && !wildcard {
		return next
	}
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		origin := r.Header.Get("Origin")
		if origin != "" && (wildcard || allowed[origin]) {
			w.Header().Set("Access-Control-Allow-Origin", origin)
			w.Header().Add("Vary", "Origin")
			w.Header().Set("Access-Control-Allow-Credentials", "true")
			w.Header().Set("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS")
			w.Header().Set("Access-Control-Allow-Headers", "Content-Type, X-API-Key, Authorization")
		}
		if r.Method == http.MethodOptions {
			w.WriteHeader(http.StatusNoContent)
			return
		}
		next.ServeHTTP(w, r)
	})
}

// public marks a handler as reachable without a principal.
func (s *Server) public(h http.HandlerFunc) http.Handler { return h }

// limited applies the per-IP rate limit to a handler that is reachable without
// a principal. The login endpoint is the reason the limit exists: guessing
// API keys is bounded by nothing but the attacker's patience, so the lockout
// the login handler records has to be able to stop the *next* attempt. It did
// not, because the limiter ran behind authed() and login is public, so N bad
// keys gave N 401s and the lockout only locked the caller out of the rest of
// the API.
func (s *Server) limited(h http.HandlerFunc) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if !s.allowRequest(w, r) {
			return
		}
		h(w, r)
	})
}

// cfg is a small accessor so handlers read the current config once.

// authed wraps a handler with authentication and, for mutations, a write
// permission check.
func (s *Server) authed(h http.HandlerFunc, write bool) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		p, err := s.principal(r)
		if err != nil {
			if s.hooks.Store.Get().API.Auth.Mode != "none" && !s.cfgTrustLoopback(r) {
				writeError(w, http.StatusUnauthorized, "authentication required")
				return
			}
			// auth mode none, or a trusted loopback client: run as a local
			// principal.
			p = Principal{Mode: "loopback", Name: "loopback", Loopback: true}
		}
		if write && !p.CanWrite() {
			writeError(w, http.StatusForbidden, "this key is read-only")
			return
		}
		if !s.allowRequest(w, r) {
			return
		}
		ctx := context.WithValue(r.Context(), principalKey, p)
		h(w, r.WithContext(ctx))
	})
}

type ctxKey string

const principalKey ctxKey = "fancydns.principal"

// PrincipalFrom returns the authenticated principal, if any.
func PrincipalFrom(ctx context.Context) (Principal, bool) {
	p, ok := ctx.Value(principalKey).(Principal)
	return p, ok
}

// principal authenticates a request.
func (s *Server) principal(r *http.Request) (Principal, error) {
	// API key in the header wins: it is the machine path.
	if key := r.Header.Get("X-API-Key"); key != "" {
		return s.auth.apiKey(key)
	}
	if h := r.Header.Get("Authorization"); h != "" {
		scheme, value, ok := strings.Cut(h, " ")
		if !ok {
			return Principal{}, errors.New("malformed Authorization header")
		}
		switch strings.ToLower(scheme) {
		case "bearer":
			// Local sessions minted by /auth/login are 64-character hex
			// strings. OIDC access tokens are compact JWTs (three segments).
			// Keep key-based login sessions usable alongside OIDC for scripted
			// clients, while sending JWT-shaped credentials to the verifier.
			if s.auth.mode == "oidc" && strings.Count(value, ".") == 2 {
				return s.auth.oidcPrincipal(value)
			}
			return s.auth.session(value)
		case "basic":
			user, pass, ok := r.BasicAuth()
			if !ok {
				return Principal{}, errors.New("malformed basic credentials")
			}
			return s.auth.basic(user, pass)
		}
		return Principal{}, fmt.Errorf("unsupported auth scheme %q", scheme)
	}
	if s.cfgTrustLoopback(r) {
		return Principal{Mode: "loopback", Name: "loopback", Loopback: true}, nil
	}
	return Principal{}, errors.New("no credentials")
}

func (s *Server) cfgTrustLoopback(r *http.Request) bool {
	cfg := s.hooks.Store.Get()
	if !cfg.API.Auth.TrustLoopback {
		return false
	}
	ip := s.clientIP(r)
	a, err := netip.ParseAddr(ip)
	if err != nil {
		return false
	}
	return a.IsLoopback()
}

// clientIP resolves the client address, honouring X-Forwarded-For only from a
// trusted proxy. Trusting it from anyone would let a client spoof its identity
// past the ACL and the rate limiter.
func (s *Server) clientIP(r *http.Request) string {
	host, _, err := net.SplitHostPort(r.RemoteAddr)
	if err != nil {
		host = r.RemoteAddr
	}
	addr, err := netip.ParseAddr(host)
	if err != nil {
		return host
	}
	if !s.isTrusted(addr) {
		return addr.String()
	}
	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
		// Left-most entry is the original client.
		first := strings.TrimSpace(strings.Split(xff, ",")[0])
		if a, err := netip.ParseAddr(first); err == nil {
			return a.String()
		}
	}
	if xr := r.Header.Get("X-Real-IP"); xr != "" {
		if a, err := netip.ParseAddr(strings.TrimSpace(xr)); err == nil {
			return a.String()
		}
	}
	return addr.String()
}

func (s *Server) isTrusted(a netip.Addr) bool {
	for _, p := range s.trusted {
		if p.Contains(a) {
			return true
		}
	}
	return false
}

// allowRequest applies the API rate limit per client IP.
func (s *Server) allowRequest(w http.ResponseWriter, r *http.Request) bool {
	cfg := s.hooks.Store.Get()
	if cfg.API.RateLimit.PerMinute <= 0 {
		return true
	}
	ip := s.clientIP(r)
	now := s.now()
	s.limMu.Lock()
	defer s.limMu.Unlock()
	st, ok := s.limits[ip]
	if !ok {
		st = &limitState{window: now}
		s.limits[ip] = st
	}
	if now.Before(st.lockedTill) {
		w.Header().Set("Retry-After", strconv.Itoa(int(st.lockedTill.Sub(now).Seconds())+1))
		writeError(w, http.StatusTooManyRequests, "rate limit exceeded")
		return false
	}
	if now.Sub(st.window) >= time.Minute {
		st.window = now
		st.count = 0
	}
	st.count++
	if st.count > cfg.API.RateLimit.PerMinute {
		w.Header().Set("Retry-After", "60")
		writeError(w, http.StatusTooManyRequests, "rate limit exceeded")
		return false
	}
	// Opportunistic cleanup keeps the map bounded without a sweeper.
	if len(s.limits) > 4096 {
		for k, v := range s.limits {
			if now.Sub(v.window) > 10*time.Minute {
				delete(s.limits, k)
			}
		}
	}
	return true
}

// recordLoginFailure locks an IP out after repeated bad logins.
func (s *Server) recordLoginFailure(ip string) {
	cfg := s.hooks.Store.Get()
	if cfg.API.RateLimit.Failures <= 0 {
		return
	}
	s.limMu.Lock()
	defer s.limMu.Unlock()
	st, ok := s.limits[ip]
	if !ok {
		st = &limitState{window: s.now()}
		s.limits[ip] = st
	}
	st.failures++
	if st.failures >= cfg.API.RateLimit.Failures {
		st.lockedTill = s.now().Add(cfg.API.RateLimit.Lockout.D())
		st.failures = 0
	}
	s.log.Warn("failed admin login", "client", ip)
}

func (s *Server) clearLoginFailures(ip string) {
	s.limMu.Lock()
	defer s.limMu.Unlock()
	st, ok := s.limits[ip]
	if !ok {
		return
	}
	// Only the auth-failure state is reset. Deleting the whole entry would
	// also reset the per-minute request window, so a single successful login
	// in the middle of a burst would hand the client a fresh budget — which
	// is exactly what an attacker holding one valid key wants.
	st.failures = 0
	st.lockedTill = time.Time{}
}

// protectMetrics gates /metrics behind an API key when configured.
func (s *Server) protectMetrics(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		cfg := s.hooks.Store.Get()
		if !cfg.Metrics.Protect {
			next.ServeHTTP(w, r)
			return
		}
		if _, err := s.principal(r); err != nil {
			writeError(w, http.StatusUnauthorized, "metrics are protected")
			return
		}
		next.ServeHTTP(w, r)
	})
}

// ---- response helpers -------------------------------------------------------

func writeJSON(w http.ResponseWriter, code int, v any) {
	w.Header().Set("Content-Type", "application/json; charset=utf-8")
	w.Header().Set("Cache-Control", "no-store")
	w.WriteHeader(code)
	enc := json.NewEncoder(w)
	enc.SetEscapeHTML(false)
	if err := enc.Encode(v); err != nil {
		// The status line is already sent, so the only useful thing left is a
		// truncated-object fallback the client can detect.
		_, _ = w.Write([]byte(`{"error":"response encoding failed"}`))
	}
}

type apiError struct {
	Error  string   `json:"error"`
	Fields []string `json:"fields,omitempty"`
}

func writeError(w http.ResponseWriter, code int, msg string) {
	writeJSON(w, code, apiError{Error: msg})
}

func writeErrorFields(w http.ResponseWriter, code int, msg string, fields []string) {
	writeJSON(w, code, apiError{Error: msg, Fields: fields})
}

// decodeJSON reads a JSON body with a size cap and strict unknown-field
// rejection, so a typo in a UI payload is an error rather than a silent no-op.
func decodeJSON(r *http.Request, v any) error {
	defer func() { _ = r.Body.Close() }()
	dec := json.NewDecoder(http.MaxBytesReader(nil, r.Body, 4<<20))
	dec.DisallowUnknownFields()
	if err := dec.Decode(v); err != nil {
		return err
	}
	return nil
}

func queryInt(r *http.Request, key string, def int) int {
	v := r.URL.Query().Get(key)
	if v == "" {
		return def
	}
	n, err := strconv.Atoi(v)
	if err != nil {
		return def
	}
	return n
}

func queryBool(r *http.Request, key string) bool {
	v := strings.ToLower(r.URL.Query().Get(key))
	return v == "1" || v == "true" || v == "yes"
}

func queryTime(r *http.Request, key string) time.Time {
	v := r.URL.Query().Get(key)
	if v == "" {
		return time.Time{}
	}
	if n, err := strconv.ParseInt(v, 10, 64); err == nil {
		if n > 1e12 {
			return time.UnixMilli(n)
		}
		return time.Unix(n, 0)
	}
	if t, err := time.Parse(time.RFC3339, v); err == nil {
		return t
	}
	return time.Time{}
}

func isFileWritable(path string) bool {
	if path == "" {
		return false
	}
	f, err := os.OpenFile(path, os.O_WRONLY, 0)
	if err != nil {
		// A missing file in a writable directory is still writable.
		dir := path
		if i := strings.LastIndexByte(dir, '/'); i > 0 {
			dir = dir[:i]
		}
		d, derr := os.Stat(dir)
		if derr != nil {
			return false
		}
		return d.IsDir()
	}
	_ = f.Close()
	return true
}