// Package cooler holds the shared-state services of a SummerCMS application:
// a Laravel-core Cache with a memory (otter) and a Redis driver under a
// per-application key prefix, a Redis-backed fixed-window rate-limit
// store that keeps limits across application instances, and the site-cache
// purge that asks the host site to drop its page caches after admin saves.
// One redis.url key configures Redis for the cache and the store. The
// framework installs cooler at boot (surf's serve command) and FromApp and
// SiteCacheFromApp serve other commands, so applications import nothing.
//
// cooler imports neither surf nor cabana: surf imports cooler, and the
// rate-limit store satisfies surf.Store structurally through Attempter.
package cooler

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"strings"
	"sync"
	"time"

	"git.golem15.com/golem15/summercms/modules/backpack"
	"git.golem15.com/golem15/summercms/modules/compass"
	"github.com/redis/go-redis/v9"
)

// Config keys and defaults cooler reads.
const (
	// RedisURLKey is the shared Redis URL (redis://, rediss:// or unix://),
	// required when a Redis driver is selected. Environment: SUMMER_REDIS__URL.
	RedisURLKey = "redis.url"
	// CacheDriverKey selects the cache driver: memory (default) or redis.
	CacheDriverKey = "cache.driver"
	// CachePrefixKey is the per-application key prefix. The default derives
	// from app.name.
	CachePrefixKey = "cache.prefix"
	// CacheMaxEntriesKey bounds the memory driver's entry count.
	CacheMaxEntriesKey = "cache.memory.max_entries"
	// RateLimitStoreKey selects where rate-limit counters live: memory
	// (default, per instance) or redis (shared).
	RateLimitStoreKey = "http.rate_limit.store"

	// DefaultMaxEntries is the memory driver's size bound when
	// cache.memory.max_entries is unset.
	DefaultMaxEntries = 10000
	// PingTimeout bounds the start-up Ping that Install sends to Redis.
	PingTimeout = 3 * time.Second
	// AttemptTimeout bounds one rate-limit attempt on Redis before the
	// fallback store answers.
	AttemptTimeout = 500 * time.Millisecond
)

// Driver names accepted by cache.driver and http.rate_limit.store.
const (
	driverMemory = "memory"
	driverRedis  = "redis"
)

// defaultPrefix is the key prefix when neither cache.prefix nor app.name is set.
const defaultPrefix = "summer:"

// Attempter is one atomic fixed-window admission. It has exactly the method
// set of surf.Store, so any surf.Store is an Attempter and a RateLimitStore
// is a surf.Store.
type Attempter interface {
	Attempt(key string, max int, decay time.Duration) (allowed bool, attempts int, retryAfter time.Duration)
}

// Cache is a Laravel-core cache: byte values under string keys, each with
// its own TTL, stored under the application's key prefix. A ttl of 0 or less
// keeps the value until it is forgotten, flushed or evicted. There are no tags
// and no locks.
type Cache interface {
	// Get returns the value at key and whether it was found.
	Get(ctx context.Context, key string) ([]byte, bool, error)
	// Set stores value at key for ttl (0 or less: no expiry).
	Set(ctx context.Context, key string, value []byte, ttl time.Duration) error
	// Has reports whether key holds a live value.
	Has(ctx context.Context, key string) (bool, error)
	// Forget removes key.
	Forget(ctx context.Context, key string) error
	// Remember returns the value at key, or calls fn, stores its value for
	// ttl and returns it. It degrades instead of failing: when the read fails
	// it still calls fn and returns its value, a failing write is skipped,
	// and both are logged as a warning at most once per minute. Only an error
	// from fn is returned.
	Remember(ctx context.Context, key string, ttl time.Duration, fn func(context.Context) ([]byte, error)) ([]byte, error)
	// Increment adds by to the integer at key and returns the new value. A
	// missing key starts at 0 and gets no TTL; an existing key keeps its
	// expiry, as Redis INCRBY does.
	Increment(ctx context.Context, key string, by int64) (int64, error)
	// Flush removes every key of this application's cache, and nothing else.
	Flush(ctx context.Context) error
}

// Handle is what Install set up. Close releases the Redis client Install
// opened, if any.
type Handle struct {
	client    *redis.Client
	prefix    string
	rateRedis bool
	cache     Cache
	log       *slog.Logger
}

// Close closes the Redis client Install opened. It is safe on a nil Handle
// and when no Redis driver was selected. A Redis-backed Cache that Install
// built on that client stops working; serve closes the handle on shutdown.
func (h *Handle) Close() error {
	if h == nil || h.client == nil {
		return nil
	}
	err := h.client.Close()
	h.client = nil
	return err
}

// Cache returns the Cache Install published (or the one FromApp had
// published before it).
func (h *Handle) Cache() Cache {
	if h == nil {
		return nil
	}
	return h.cache
}

// RateLimitRedis reports whether http.rate_limit.store selects the Redis store.
func (h *Handle) RateLimitRedis() bool {
	return h != nil && h.rateRedis && h.client != nil
}

// RateLimitStore returns the Redis rate-limit store, falling back to fallback
// while Redis does not answer. When the Redis store is not selected it
// returns fallback itself.
func (h *Handle) RateLimitStore(fallback Attempter) Attempter {
	if !h.RateLimitRedis() {
		return fallback
	}
	s := NewRateLimitStore(h.client, h.prefix, fallback)
	s.warn.setLogger(h.log)
	return s
}

// Install reads cooler's configuration, opens what it selects and publishes
// the Cache on app (reusing one FromApp already published).
//
// cache.driver selects the cache (memory, the default, or redis) and
// http.rate_limit.store the rate-limit store (memory or redis). When either
// selects Redis, Install builds one client from redis.url, shared by both,
// and pings it within PingTimeout. An unreachable Redis is logged as a
// warning (the endpoint only, never the URL) and start-up continues: the rate
// limiter falls back to per-instance memory and Remember degrades until Redis
// answers. A missing or invalid redis.url, an unknown driver name or a
// non-positive cache.memory.max_entries is a boot error naming the key.
//
// Install also publishes the site-cache purger (SiteCachePurger) built from
// site_cache.purge_url and site_cache.purge_token, unless a plugin already
// published one; an invalid site_cache configuration is a boot error naming
// the key, and an enabled purger is logged with its Endpoint.
//
// Only the serve command calls Install; route:list and tests never dial Redis.
func Install(ctx context.Context, app *backpack.App) (*Handle, error) {
	if app == nil {
		return nil, fmt.Errorf("cooler: app is nil")
	}
	if ctx == nil {
		ctx = context.Background()
	}
	cfg := app.Config
	h := &Handle{prefix: keyPrefix(cfg), log: loggerFromApp(app)}
	if err := installSiteCache(app, h.log); err != nil {
		return nil, err
	}
	store, err := driverName(cfg, RateLimitStoreKey)
	if err != nil {
		return nil, err
	}
	cacheDriver, err := driverName(cfg, CacheDriverKey)
	if err != nil {
		return nil, err
	}
	h.rateRedis = store == driverRedis
	existing, published := app.Lookup[Cache]()
	published = published && existing != nil
	needRedis := h.rateRedis || (!published && cacheDriver == driverRedis)
	if needRedis {
		selectedBy := RateLimitStoreKey
		if !h.rateRedis {
			selectedBy = CacheDriverKey
		}
		client, err := openFromConfig(ctx, cfg, selectedBy)
		if err != nil {
			return nil, err
		}
		h.client = client
	}
	if published {
		h.cache = existing
	} else {
		c, err := buildCache(cfg, cacheDriver, h.prefix, h.client)
		if err != nil {
			_ = h.Close()
			return nil, err
		}
		setLogger(c, h.log)
		if err := app.Publish[Cache](c); err != nil {
			_ = h.Close()
			return nil, fmt.Errorf("cooler: %w", err)
		}
		h.cache = c
	}
	if h.client != nil {
		ping(ctx, h.client, h.log)
	}
	return h, nil
}

// FromApp returns the Cache published on app, or builds the configured one
// and publishes it. It never pings: go-redis dials on the first command, so a
// job worker, a console command or a plugin's Boot can call it without a
// running Redis. When another caller published first, FromApp returns that
// Cache. A Redis client FromApp opens lives as long as the process.
func FromApp(app *backpack.App) (Cache, error) {
	if app == nil {
		return nil, fmt.Errorf("cooler: app is nil")
	}
	if c, ok := app.Lookup[Cache](); ok && c != nil {
		return c, nil
	}
	cfg := app.Config
	driver, err := driverName(cfg, CacheDriverKey)
	if err != nil {
		return nil, err
	}
	prefix := keyPrefix(cfg)
	var client *redis.Client
	if driver == driverRedis {
		client, err = openFromConfig(context.Background(), cfg, CacheDriverKey)
		if err != nil {
			return nil, err
		}
	}
	c, err := buildCache(cfg, driver, prefix, client)
	if err != nil {
		if client != nil {
			_ = client.Close()
		}
		return nil, err
	}
	setLogger(c, loggerFromApp(app))
	if err := app.Publish[Cache](c); err != nil {
		if client != nil {
			_ = client.Close()
		}
		if winner, ok := app.Lookup[Cache](); ok && winner != nil {
			return winner, nil
		}
		return nil, fmt.Errorf("cooler: %w", err)
	}
	return c, nil
}

// buildCache builds the selected driver. client is used by the Redis driver.
func buildCache(cfg *compass.Config, driver, prefix string, client *redis.Client) (Cache, error) {
	if driver == driverRedis {
		if client == nil {
			return nil, errNilClient
		}
		return NewRedisCache(client, prefix), nil
	}
	n, err := maxEntries(cfg)
	if err != nil {
		return nil, err
	}
	return NewMemoryCache(prefix, n)
}

// maxEntries reads cache.memory.max_entries: unset means DefaultMaxEntries,
// anything below 1 (or not a number) is an error naming the key.
func maxEntries(cfg *compass.Config) (int, error) {
	if cfg == nil || !cfg.Has(CacheMaxEntriesKey) {
		return DefaultMaxEntries, nil
	}
	n := cfg.Int(CacheMaxEntriesKey)
	if n < 1 {
		return 0, fmt.Errorf("cooler: %s must be a whole number of at least 1", CacheMaxEntriesKey)
	}
	return n, nil
}

// openFromConfig builds the Redis client from redis.url, naming the key that
// selected Redis when the URL is missing.
func openFromConfig(ctx context.Context, cfg *compass.Config, selectedBy string) (*redis.Client, error) {
	raw := ""
	if cfg != nil {
		raw = strings.TrimSpace(cfg.String(RedisURLKey))
	}
	if raw == "" {
		return nil, fmt.Errorf("cooler: %s is required when %s is %s (set it in config/redis.yaml or SUMMER_REDIS__URL)", RedisURLKey, selectedBy, driverRedis)
	}
	return OpenRedis(ctx, raw)
}

// ping checks Redis once within PingTimeout and logs a warning on failure.
func ping(ctx context.Context, client *redis.Client, log *slog.Logger) {
	pctx, cancel := context.WithTimeout(ctx, PingTimeout)
	defer cancel()
	if err := client.Ping(pctx).Err(); err != nil {
		log.Warn("cooler: redis did not answer at start-up; rate limits count per instance and the cache degrades until it does",
			"endpoint", redisEndpoint(client), "error", err)
	}
}

// driverName reads a memory|redis selector. Empty means memory.
func driverName(cfg *compass.Config, key string) (string, error) {
	if cfg == nil {
		return driverMemory, nil
	}
	v := strings.ToLower(strings.TrimSpace(cfg.String(key)))
	switch v {
	case "", driverMemory:
		return driverMemory, nil
	case driverRedis:
		return driverRedis, nil
	}
	return "", fmt.Errorf("cooler: %s must be %s or %s, got %q", key, driverMemory, driverRedis, v)
}

// keyPrefix returns the key prefix every cooler key starts with: cache.prefix
// when set, otherwise app.name lower-cased with every run of characters
// outside [a-z0-9] replaced by "_" (trimmed at both ends) and a trailing ":",
// or "summer:" when app.name is empty too. Cache keys are <prefix>cache:<key>,
// rate-limit keys <prefix>rl:<key>.
func keyPrefix(cfg *compass.Config) string {
	if cfg == nil {
		return defaultPrefix
	}
	if p := strings.TrimSpace(cfg.String(CachePrefixKey)); p != "" {
		return p
	}
	name := slug(cfg.String("app.name"))
	if name == "" {
		return defaultPrefix
	}
	return name + ":"
}

func slug(s string) string {
	var b strings.Builder
	under := false
	for _, r := range strings.ToLower(s) {
		if (r >= 'a' && r <= 'z') || (r >= '0' && r <= '9') {
			b.WriteRune(r)
			under = false
			continue
		}
		if !under {
			b.WriteByte('_')
			under = true
		}
	}
	return strings.Trim(b.String(), "_")
}

func loggerFromApp(app *backpack.App) *slog.Logger {
	if app != nil {
		if log, ok := app.Lookup[*slog.Logger](); ok && log != nil {
			return log
		}
	}
	return slog.Default()
}

// errNilClient is returned when a Redis driver is built without a client.
var errNilClient = errors.New("cooler: redis client is nil")

// warnEvery bounds how often a degraded call is logged.
const warnEvery = time.Minute

// warner logs a message at most once per warnEvery.
type warner struct {
	mu   sync.Mutex
	last time.Time
	log  *slog.Logger
}

func newWarner(log *slog.Logger) *warner { return &warner{log: log} }

func (w *warner) setLogger(log *slog.Logger) {
	if w == nil || log == nil {
		return
	}
	w.mu.Lock()
	w.log = log
	w.mu.Unlock()
}

func (w *warner) warn(msg string, args ...any) {
	if w == nil {
		return
	}
	w.mu.Lock()
	now := time.Now()
	if !w.last.IsZero() && now.Sub(w.last) < warnEvery {
		w.mu.Unlock()
		return
	}
	w.last = now
	log := w.log
	w.mu.Unlock()
	if log == nil {
		log = slog.Default()
	}
	log.Warn(msg, args...)
}

// setLogger hands the app's logger to a driver built by this package.
func setLogger(c Cache, log *slog.Logger) {
	if s, ok := c.(interface{ setLogger(*slog.Logger) }); ok {
		s.setLogger(log)
	}
}

// remember is Cache.Remember for every driver: read, else compute and store,
// degrading on cache errors instead of failing the caller.
func remember(ctx context.Context, c Cache, w *warner, key string, ttl time.Duration, fn func(context.Context) ([]byte, error)) ([]byte, error) {
	if fn == nil {
		return nil, fmt.Errorf("cooler: Remember %q: callback is nil", key)
	}
	v, ok, err := c.Get(ctx, key)
	if err == nil && ok {
		return v, nil
	}
	if err != nil {
		w.warn("cooler: cache read failed; computing the value without the cache", "error", err)
	}
	v, ferr := fn(ctx)
	if ferr != nil {
		return nil, ferr
	}
	if err == nil {
		if serr := c.Set(ctx, key, v, ttl); serr != nil {
			w.warn("cooler: cache write failed; the value was not stored", "error", serr)
		}
	}
	return v, nil
}