package cooler

import (
	"context"
	"fmt"
	"log/slog"
	"strconv"
	"time"

	"github.com/maypok86/otter/v2"
)

// forever is how long an entry without a TTL lives in the memory driver:
// otter's documented way to say "no expiration" is an excessively long period.
const forever = 50 * 365 * 24 * time.Hour

// memEntry is a stored value with its absolute expiry; a zero expiresAt
// never expires.
type memEntry struct {
	val       []byte
	expiresAt time.Time
}

func (e memEntry) expired(now time.Time) bool {
	return !e.expiresAt.IsZero() && !now.Before(e.expiresAt)
}

// memoryCache is the in-process Cache on otter v2: size-bounded, with a
// per-entry expiry computed from the absolute expiresAt stored with the value.
type memoryCache struct {
	c      *otter.Cache[string, memEntry]
	prefix string
	warn   *warner
}

// NewMemoryCache returns an in-process Cache holding at most maxEntries
// entries (cache.memory.max_entries, default DefaultMaxEntries); the least
// valuable entries are evicted beyond that. Keys are <prefix>cache:<key>.
// A maxEntries below 1 is an error naming cache.memory.max_entries.
func NewMemoryCache(prefix string, maxEntries int) (Cache, error) {
	if maxEntries < 1 {
		return nil, fmt.Errorf("cooler: %s must be at least 1, got %d", CacheMaxEntriesKey, maxEntries)
	}
	c, err := otter.New(&otter.Options[string, memEntry]{
		MaximumSize: maxEntries,
		ExpiryCalculator: otter.ExpiryWritingFunc(func(e otter.Entry[string, memEntry]) time.Duration {
			if e.Value.expiresAt.IsZero() {
				return forever
			}
			if d := time.Until(e.Value.expiresAt); d > 0 {
				return d
			}
			return time.Nanosecond
		}),
	})
	if err != nil {
		return nil, fmt.Errorf("cooler: memory cache: %w", err)
	}
	return &memoryCache{c: c, prefix: prefix, warn: newWarner(slog.Default())}, nil
}

func (m *memoryCache) key(k string) string { return m.prefix + "cache:" + k }

func (m *memoryCache) setLogger(log *slog.Logger) { m.warn.setLogger(log) }

// lookup returns the live entry for a full key, dropping an expired one.
func (m *memoryCache) lookup(full string) (memEntry, bool) {
	e, ok := m.c.GetIfPresent(full)
	if !ok {
		return memEntry{}, false
	}
	if e.expired(time.Now()) {
		m.c.Invalidate(full)
		return memEntry{}, false
	}
	return e, true
}

func (m *memoryCache) Get(_ context.Context, key string) ([]byte, bool, error) {
	e, ok := m.lookup(m.key(key))
	if !ok {
		return nil, false, nil
	}
	return clone(e.val), true, nil
}

func (m *memoryCache) Set(_ context.Context, key string, value []byte, ttl time.Duration) error {
	e := memEntry{val: clone(value)}
	if ttl > 0 {
		e.expiresAt = time.Now().Add(ttl)
	}
	m.c.Set(m.key(key), e)
	return nil
}

func (m *memoryCache) Has(_ context.Context, key string) (bool, error) {
	_, ok := m.lookup(m.key(key))
	return ok, nil
}

func (m *memoryCache) Forget(_ context.Context, key string) error {
	m.c.Invalidate(m.key(key))
	return nil
}

func (m *memoryCache) Remember(ctx context.Context, key string, ttl time.Duration, fn func(context.Context) ([]byte, error)) ([]byte, error) {
	return remember(ctx, m, m.warn, key, ttl, fn)
}

// Increment adds by to the integer at key atomically (the update runs under
// otter's per-key lock). A missing or expired key starts at 0 and gets no
// TTL; an existing key keeps its expiry, as Redis INCRBY does.
func (m *memoryCache) Increment(_ context.Context, key string, by int64) (int64, error) {
	var (
		n      int64
		valErr error
	)
	now := time.Now()
	m.c.Compute(m.key(key), func(old memEntry, found bool) (memEntry, otter.ComputeOp) {
		cur := int64(0)
		next := memEntry{}
		if found && !old.expired(now) {
			v, err := strconv.ParseInt(string(old.val), 10, 64)
			if err != nil {
				valErr = fmt.Errorf("cooler: cache value at %q is not an integer", key)
				return old, otter.CancelOp
			}
			cur = v
			next.expiresAt = old.expiresAt
		}
		n = cur + by
		next.val = []byte(strconv.FormatInt(n, 10))
		return next, otter.WriteOp
	})
	if valErr != nil {
		return 0, valErr
	}
	return n, nil
}

// Flush removes every entry of this cache.
func (m *memoryCache) Flush(context.Context) error {
	m.c.InvalidateAll()
	return nil
}

func clone(b []byte) []byte {
	if b == nil {
		return nil
	}
	out := make([]byte, len(b))
	copy(out, b)
	return out
}