// Package starcache provides a short-TTL in-memory cache for pb.Star metadata
// looked up via UserSocialService.GetFanIdentities.
//
// Why this exists:
// Gateway controllers used to call ctrl.userServiceClient.GetFanIdentities
// after every Register/Login/GetCurrentUser/GetMyProfile/AddIdentity/SwitchIdentity
// just to fetch a single Star by ID. The full identity list is small (~few dozen
// stars) and effectively static, so we cache it for 60s with singleflight to
// collapse concurrent refreshes.
//
// Security: the cache only holds pb.Star structs (public star metadata). It
// does NOT hold any user-specific fields (mobile, token, password, fan_profiles).
// user_id is the cache key, but its only use is to look up the corresponding
// star_id - it is not stored in any cache entry. Star ID is treated as public
// data (it's what the registration UI itself displays).
package starcache

import (
	"context"
	"sync"
	"time"

	"github.com/topfans/backend/pkg/logger"
	pb "github.com/topfans/backend/pkg/proto/user"
	"go.uber.org/zap"
)

// CacheTTL is the per-star expiration window. Short enough that star metadata
// changes (e.g. avatar update) propagate within a minute, long enough that the
// chained GetFanIdentities calls in Register/Login/GetCurrentUser/etc. reuse
// one RPC round-trip per minute of traffic.
const CacheTTL = 60 * time.Second

// Cache wraps a TTL map + singleflight for GetFanIdentities.
//
// Concurrency model:
//   - mu guards stars/expiry (RWMutex: many readers, one writer).
//   - sfMu guards the inflight map (separate lock so the slow path doesn't
//     block the hot read path).
type Cache struct {
	cli pb.UserSocialService

	mu     sync.RWMutex
	stars  map[int64]*pb.Star
	expiry map[int64]time.Time

	sfMu     sync.Mutex
	inflight map[int64]chan struct{}
}

// New constructs a Cache backed by the given UserSocialService client.
func New(cli pb.UserSocialService) *Cache {
	return &Cache{
		cli:      cli,
		stars:    make(map[int64]*pb.Star),
		expiry:   make(map[int64]time.Time),
		inflight: make(map[int64]chan struct{}),
	}
}

// GetStar returns the cached pb.Star for starID, refreshing from the upstream
// RPC on miss or expiry. Returns (nil, err) on upstream error; callers should
// treat nil as "star info unavailable" and continue with the rest of the
// response.
//
// starID == 0 is a no-op (returns nil, nil) so that callers don't need to
// pre-check empty identities.
func (c *Cache) GetStar(ctx context.Context, starID int64) (*pb.Star, error) {
	if starID == 0 {
		return nil, nil
	}

	// Hot path: read lock.
	c.mu.RLock()
	if s, ok := c.stars[starID]; ok {
		if exp, ok := c.expiry[starID]; ok && time.Now().Before(exp) {
			c.mu.RUnlock()
			return s, nil
		}
	}
	c.mu.RUnlock()

	// Singleflight: ensure only one refresh per starID at a time.
	c.sfMu.Lock()
	if ch, ok := c.inflight[starID]; ok {
		c.sfMu.Unlock()
		<-ch
		// Re-read cache after the leader populated it.
		c.mu.RLock()
		defer c.mu.RUnlock()
		return c.stars[starID], nil
	}
	ch := make(chan struct{})
	c.inflight[starID] = ch
	c.sfMu.Unlock()

	defer func() {
		close(ch)
		c.sfMu.Lock()
		delete(c.inflight, starID)
		c.sfMu.Unlock()
	}()

	resp, err := c.cli.GetFanIdentities(ctx, &pb.GetFanIdentitiesRequest{})
	if err != nil {
		logger.Logger.Warn("starcache: GetFanIdentities refresh failed",
			zap.Int64("star_id", starID),
			zap.Error(err),
		)
		return nil, err
	}

	c.mu.Lock()
	now := time.Now()
	for _, s := range resp.Stars {
		c.stars[s.StarId] = s
		c.expiry[s.StarId] = now.Add(CacheTTL)
	}
	c.mu.Unlock()

	c.mu.RLock()
	defer c.mu.RUnlock()
	return c.stars[starID], nil
}

// Invalidate removes the cached entry for starID. Call this when star metadata
// changes (e.g. admin updates a star's avatar) so the next GetStar refreshes.
//
// Safe to call with starID == 0 (no-op).
func (c *Cache) Invalidate(starID int64) {
	if starID == 0 {
		return
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	delete(c.stars, starID)
	delete(c.expiry, starID)
}

// Len returns the number of cached stars. Useful for tests and metrics.
func (c *Cache) Len() int {
	c.mu.RLock()
	defer c.mu.RUnlock()
	return len(c.stars)
}