// Package cache gives anchorage a small layer of bounded in-memory caches
// on top of ristretto. Every cache has a byte budget and a default TTL,
// so no matter how hot traffic gets the RSS cost is known in advance.
//
// Values are cached per-node. Cross-node consistency is handled by the
// invalidation bus (see invalidate.go) which publishes a NATS message
// whenever a writer mutates the underlying record; subscribers on other
// nodes drop their cached entry and the next read re-fetches from Postgres.
package cache

import (
	"errors"
	"sync"
	"time"

	"github.com/dgraph-io/ristretto/v2"
)

// Options configures a single cache.
type Options struct {
	// Name is used in log messages and metric labels; should be short and
	// lowercase (e.g., "org/byid", "pin/byid").
	Name string
	// MaxCost is the byte budget. Once the internal cost estimate exceeds
	// this, ristretto evicts entries using admission-controlled TinyLFU.
	MaxCost int64
	// NumCounters is ristretto's frequency-estimator size. ~10x the expected
	// entry count is a good default.
	NumCounters int64
	// DefaultTTL is applied when callers set a zero TTL. Zero here means
	// "no default TTL" — entries live until evicted.
	DefaultTTL time.Duration
}

// Cache is a typed, bounded in-memory cache.
//
// Keys must satisfy ristretto's Key constraint (string, int, byte slice,
// etc.). anchorage conventionally uses strings for keys — typeid strings,
// compound keys like "<orgID>:<pinID>".
type Cache[K ristretto.Key, V any] struct {
	opts Options
	r    *ristretto.Cache[K, V]
}

// New returns a Cache backed by ristretto.
//
// MaxCost must be > 0; NumCounters defaults to 10×expected-entries if zero
// (falls back to a safe minimum otherwise).
func New[K ristretto.Key, V any](opts Options) (*Cache[K, V], error) {
	if opts.MaxCost <= 0 {
		return nil, errors.New("cache: MaxCost must be > 0")
	}
	if opts.NumCounters == 0 {
		// ~10× a rough estimate of entry count. Ristretto docs recommend
		// 10× expected-items; we derive a safe minimum from MaxCost.
		opts.NumCounters = opts.MaxCost / 64
		if opts.NumCounters < 1000 {
			opts.NumCounters = 1000
		}
	}

	r, err := ristretto.NewCache(&ristretto.Config[K, V]{
		MaxCost:     opts.MaxCost,
		NumCounters: opts.NumCounters,
		BufferItems: 64,
		// Metrics has a small per-op cost; worth paying for the
		// /v1/admin/cache-stats endpoint + any future /metrics wiring.
		Metrics: true,
	})
	if err != nil {
		return nil, err
	}
	c := &Cache[K, V]{opts: opts, r: r}
	Register(c)
	return c, nil
}

// Stats is a point-in-time snapshot of a cache's counters. Emitted by
// /v1/admin/cache-stats. Fields are monotonic counters or instantaneous
// gauges — differences between successive snapshots give rates.
type Stats struct {
	Name        string `json:"name"`
	Hits        uint64 `json:"hits"`
	Misses      uint64 `json:"misses"`
	KeysAdded   uint64 `json:"keys_added"`
	KeysEvicted uint64 `json:"keys_evicted"`
	CostAdded   uint64 `json:"cost_added"`
	CostEvicted uint64 `json:"cost_evicted"`
}

// Stats returns a snapshot of this cache's current counters.
func (c *Cache[K, V]) Stats() Stats {
	m := c.r.Metrics
	if m == nil {
		return Stats{Name: c.opts.Name}
	}
	return Stats{
		Name:        c.opts.Name,
		Hits:        m.Hits(),
		Misses:      m.Misses(),
		KeysAdded:   m.KeysAdded(),
		KeysEvicted: m.KeysEvicted(),
		CostAdded:   m.CostAdded(),
		CostEvicted: m.CostEvicted(),
	}
}

// StatsProvider is implemented by anything that can expose a Stats
// snapshot — both *Cache[K,V] and package-external caches (e.g.,
// pin.CachedLiveNodes) which keep their own counters.
type StatsProvider interface {
	Stats() Stats
}

// Global stats registry. Every cache.New() call registers itself;
// external providers (pin.CachedLiveNodes) register explicitly via
// cache.Register(provider). The admin cache-stats endpoint reads
// AllStats() to produce its response.
var registry struct {
	mu        sync.RWMutex
	providers []StatsProvider
}

// Register adds a StatsProvider to the global registry. Safe to call
// from multiple goroutines; idempotent-ish (dedup is caller's concern —
// most caches are constructed once during app startup).
func Register(p StatsProvider) {
	registry.mu.Lock()
	registry.providers = append(registry.providers, p)
	registry.mu.Unlock()
}

// AllStats returns snapshots for every registered cache. Order is
// registration order; callers that care about stable ordering should
// sort by Stats.Name.
func AllStats() []Stats {
	registry.mu.RLock()
	defer registry.mu.RUnlock()
	out := make([]Stats, 0, len(registry.providers))
	for _, p := range registry.providers {
		out = append(out, p.Stats())
	}
	return out
}

// ResetRegistry clears the global registry. Tests call this between
// runs; not for production use.
func ResetRegistry() {
	registry.mu.Lock()
	registry.providers = nil
	registry.mu.Unlock()
}

// Get returns the cached value and whether it was present.
func (c *Cache[K, V]) Get(k K) (V, bool) {
	return c.r.Get(k)
}

// Set stores v with a cost estimate and TTL. ttl == 0 falls back to
// opts.DefaultTTL; cost == 0 uses the length of the key's string form
// as a (very rough) estimate.
//
// Ristretto's Set is asynchronous: the value may not be queryable
// immediately. Call Wait() in tests that need strict read-your-writes.
func (c *Cache[K, V]) Set(k K, v V, cost int64, ttl time.Duration) bool {
	if ttl == 0 {
		ttl = c.opts.DefaultTTL
	}
	if cost <= 0 {
		cost = 1
	}
	return c.r.SetWithTTL(k, v, cost, ttl)
}

// Delete removes k from the cache if present.
func (c *Cache[K, V]) Delete(k K) {
	c.r.Del(k)
}

// Wait blocks until pending async Sets are visible to Get. Intended for
// tests and for the writer-inline update pattern (see invalidate.go).
func (c *Cache[K, V]) Wait() {
	c.r.Wait()
}

// Close releases ristretto's background goroutines. Only called at
// process shutdown.
func (c *Cache[K, V]) Close() {
	c.r.Close()
}

// Name returns the cache's configured name. Used by the invalidation
// bus to form subject names.
func (c *Cache[K, V]) Name() string { return c.opts.Name }