package licensing

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"strings"
	"sync"
	"time"

	"golang.org/x/sync/singleflight"
)

// QuotaExceededError is returned by Sign when the License Server explicitly
// rejects the device because the customer's slot quota is full. Unlike
// transient network errors, stale-cache fallback is NOT appropriate — the
// License Server's 403 decision is authoritative, and serving a stale
// signature would silently bypass the operator's cap.
type QuotaExceededError struct {
	Message string // raw error field from the License Server JSON body
}

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

// IsQuotaExceeded reports whether err is, or wraps, a QuotaExceededError.
// Callers (e.g. handleLogin in cmd/main.go) use this to decide whether to
// close the device connection server-side after sending a zeroed signature.
func IsQuotaExceeded(err error) bool {
	var qe *QuotaExceededError
	return errors.As(err, &qe)
}

// RemoteSigner fetches per-login signatures from an operator-hosted License
// Server. ServerURL and Token (a JWT issued offline by the operator) are
// loaded from YAMA_LICENSE_SERVER / YAMA_LICENSE_TOKEN at startup.
//
// Cache strategy: every (startTime, clientID) tuple deterministically
// produces the same HMAC output. So once we've fetched a signature, we
// can serve it from memory for OfflineGrace (default 24h). We do honor the
// grace — the operator may want to revoke license / clear cache during
// outages — returning a stale signature beyond OfflineGrace defeats the
// point of the License Server.
//
// Thundering herd: on cache miss, concurrent Sign() calls for the same
// (startTime, clientID) are coalesced via singleflight so only one HTTPS
// round-trip happens. This matters at customer-side restart when many
// devices reconnect at the same time.
//
// Heartbeat: a background ticker POSTs the cached clientID set to
// /license/heartbeat every heartbeatInterval. This is what lets the
// License Server re-populate its quota view after a restart — without it,
// LS's view stays empty (since cache hits never re-fetch /sign).
//
// On License Server unreachable / non-200: try stale cache; if no cache,
// return ("", err). Caller (sendMasterSetting) ships zeroed signature and
// the device retries on next reconnect.
type RemoteSigner struct {
	serverURL    string
	token        string
	offlineGrace time.Duration
	httpClient   *http.Client
	logger       Logger
	sf           singleflight.Group

	mu    sync.Mutex
	cache map[string]cachedSig

	hbDone chan struct{}
	hbWg   sync.WaitGroup
}

type cachedSig struct {
	sig       string
	fetchedAt time.Time
}

// signRequest mirrors the License Server's POST /license/sign body schema.
type signRequest struct {
	ClientID  string `json:"client_id"`
	StartTime string `json:"start_time"`
}

// signResponse mirrors the License Server's response schema.
type signResponse struct {
	Signature string `json:"signature,omitempty"`
	Error     string `json:"error,omitempty"`
}

// nilLogger drops all log lines; used when callers don't pass a logger so
// the RemoteSigner stays safe to use without panic'ing on nil deref.
type nilLogger struct{}

func (nilLogger) Info(string, ...any)  {}
func (nilLogger) Warn(string, ...any)  {}
func (nilLogger) Error(string, ...any) {}

// heartbeatInterval is the period for /license/heartbeat POSTs. 90s is
// well below the License Server's 5-minute eviction window (quota.go), so
// the customer's devices never get reaped from the LS quota view while
// they're still actively heartbeating to it.
const heartbeatInterval = 90 * time.Second

// ValidateRemoteURL returns nil if u is a safe LICENSE_SERVER URL. We
// require https:// to keep the JWT and signature off the wire in cleartext;
// the only exception is http://localhost / http://127.0.0.1 for testing.
func ValidateRemoteURL(raw string) error {
	u, err := url.Parse(raw)
	if err != nil {
		return fmt.Errorf("not a URL: %w", err)
	}
	switch u.Scheme {
	case "https":
		return nil
	case "http":
		host := u.Hostname()
		if host == "localhost" || host == "127.0.0.1" || host == "::1" {
			return nil
		}
		return fmt.Errorf("http:// scheme exposes JWT in cleartext; use https:// (got %q)", raw)
	default:
		return fmt.Errorf("unsupported scheme %q (need https://)", u.Scheme)
	}
}

func NewRemote(serverURL, token string, offlineGrace time.Duration, lg Logger) *RemoteSigner {
	if lg == nil {
		lg = nilLogger{}
	}
	r := &RemoteSigner{
		serverURL:    strings.TrimRight(serverURL, "/"),
		token:        token,
		offlineGrace: offlineGrace,
		logger:       lg,
		httpClient: &http.Client{
			Timeout: 10 * time.Second,
		},
		cache:  make(map[string]cachedSig),
		hbDone: make(chan struct{}),
	}
	r.hbWg.Add(1)
	go r.heartbeatLoop()
	return r
}

func (r *RemoteSigner) Sign(startTime, clientID string) (string, error) {
	key := startTime + "|" + clientID

	// Fresh cache hit: serve from memory, no network.
	r.mu.Lock()
	if c, ok := r.cache[key]; ok && time.Since(c.fetchedAt) < r.offlineGrace {
		sig := c.sig
		r.mu.Unlock()
		return sig, nil
	}
	r.mu.Unlock()

	// Coalesce concurrent fetches for the same key — protects the License
	// Server from herd reconnects after a network blip.
	v, err, _ := r.sf.Do(key, func() (any, error) {
		return r.fetch(startTime, clientID)
	})
	if err == nil {
		sig := v.(string)
		r.mu.Lock()
		r.cache[key] = cachedSig{sig: sig, fetchedAt: time.Now()}
		r.mu.Unlock()
		return sig, nil
	}

	// Quota-exceeded is authoritative — skip stale-cache fallback entirely.
	// Serving a cached signature here would bypass the operator's explicit cap
	// decision; the caller (handleLogin) should close the connection instead.
	if IsQuotaExceeded(err) {
		r.logger.Error("RemoteSigner: quota exceeded for clientID=%s (%v); sending zeroed signature",
			clientID, err)
		return "", err
	}

	// Transient failure: fall back to stale cache if any. Better to keep an
	// existing device alive than fail closed during a momentary outage.
	r.mu.Lock()
	c, ok := r.cache[key]
	r.mu.Unlock()
	if ok {
		age := time.Since(c.fetchedAt).Round(time.Second)
		r.logger.Warn("RemoteSigner: License Server unreachable (%v); serving stale cache (age=%s) for clientID=%s",
			err, age, clientID)
		return c.sig, nil
	}
	r.logger.Error("RemoteSigner: License Server unreachable (%v) and no cache for clientID=%s; client will see zeroed signature",
		err, clientID)
	return "", err
}

func (r *RemoteSigner) fetch(startTime, clientID string) (string, error) {
	body, err := json.Marshal(signRequest{ClientID: clientID, StartTime: startTime})
	if err != nil {
		return "", err
	}

	req, err := http.NewRequest("POST", r.serverURL+"/license/sign", bytes.NewReader(body))
	if err != nil {
		return "", err
	}
	if r.token != "" {
		req.Header.Set("Authorization", "Bearer "+r.token)
	}
	req.Header.Set("Content-Type", "application/json")

	resp, err := r.httpClient.Do(req)
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()

	respBody, err := io.ReadAll(io.LimitReader(resp.Body, 8<<10))
	if err != nil {
		return "", err
	}

	if resp.StatusCode != http.StatusOK {
		// 403 with a JSON error body: quota exceeded — this is authoritative,
		// not a transient failure. Return a typed error so Sign() can skip
		// the stale-cache fallback and callers can close the connection.
		if resp.StatusCode == http.StatusForbidden {
			var sr signResponse
			if jsonErr := json.Unmarshal(respBody, &sr); jsonErr == nil && sr.Error != "" {
				return "", &QuotaExceededError{Message: sr.Error}
			}
			return "", &QuotaExceededError{Message: string(respBody)}
		}
		// 401 / 5xx: token rejected or server error — treat as transient.
		return "", fmt.Errorf("License Server returned %d: %s",
			resp.StatusCode, string(respBody))
	}

	var sr signResponse
	if err := json.Unmarshal(respBody, &sr); err != nil {
		return "", fmt.Errorf("malformed License Server response: %w", err)
	}
	if sr.Error != "" {
		return "", errors.New(sr.Error)
	}
	if sr.Signature == "" {
		return "", errors.New("License Server returned empty signature")
	}
	return sr.Signature, nil
}

// heartbeatLoop POSTs the cached clientID set to /license/heartbeat every
// heartbeatInterval. Goal: after a License Server restart, the customer's
// existing devices get re-counted in the LS quota view without each one
// needing to cache-miss /sign first.
func (r *RemoteSigner) heartbeatLoop() {
	defer r.hbWg.Done()
	ticker := time.NewTicker(heartbeatInterval)
	defer ticker.Stop()
	for {
		select {
		case <-r.hbDone:
			return
		case <-ticker.C:
			r.sendHeartbeat()
		}
	}
}

func (r *RemoteSigner) sendHeartbeat() {
	// Snapshot the cache's currently-fresh clientIDs.
	r.mu.Lock()
	cutoff := time.Now().Add(-r.offlineGrace)
	ids := make([]string, 0, len(r.cache))
	for key, c := range r.cache {
		if c.fetchedAt.Before(cutoff) {
			continue
		}
		// key is "startTime|clientID" — extract clientID for the heartbeat.
		if _, cid, ok := strings.Cut(key, "|"); ok {
			ids = append(ids, cid)
		}
	}
	r.mu.Unlock()

	if len(ids) == 0 {
		return // nothing to report yet
	}

	body, err := json.Marshal(heartbeatRequest{
		ActiveDeviceCount: len(ids),
		ActiveDeviceIDs:   ids,
	})
	if err != nil {
		return
	}

	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	req, err := http.NewRequestWithContext(ctx, "POST",
		r.serverURL+"/license/heartbeat", bytes.NewReader(body))
	if err != nil {
		return
	}
	if r.token != "" {
		req.Header.Set("Authorization", "Bearer "+r.token)
	}
	req.Header.Set("Content-Type", "application/json")

	resp, err := r.httpClient.Do(req)
	if err != nil {
		// Transient — don't spam logs every 90s; debug-level if we add one.
		return
	}
	defer resp.Body.Close()
	_, _ = io.Copy(io.Discard, io.LimitReader(resp.Body, 4<<10))
	if resp.StatusCode != http.StatusOK {
		r.logger.Warn("RemoteSigner: heartbeat returned %d", resp.StatusCode)
	}
}

// Mode reports "trial" if this RemoteSigner has no JWT (anonymous
// downstream against the operator's License Server, capped at
// FreeMaxDevices), otherwise "remote" (paid customer with JWT).
func (r *RemoteSigner) Mode() string {
	if r.token == "" {
		return "trial"
	}
	return "remote"
}

func (r *RemoteSigner) Close() error {
	select {
	case <-r.hbDone:
		// already closed
	default:
		close(r.hbDone)
	}
	r.hbWg.Wait()
	r.httpClient.CloseIdleConnections()
	return nil
}