cache.go

package util

import (
	"errors"
	"math"
	"sync"
	"time"

	"github.com/asaskevich/EventBus"
	"github.com/benbjohnson/clock"
	"github.com/evcc-io/evcc/api"
)

var bus = EventBus.New()

const (
	reset           = "reset"
	backoffDuration = 5 * time.Second
)

func ResetCached() {
	bus.Publish(reset)
}

// cached wraps a getter with a cache
type cached[T any] struct {
	mux            sync.Mutex
	clock          clock.Clock
	updated        time.Time
	retried        time.Time
	cache          time.Duration
	backoffCounter int
	g              func() (T, error)
	val            T
	err            error
}

// Cached wraps a getter with a cache
func Cached[T any](g func() (T, error), cache time.Duration) func() (T, error) {
	c := ResettableCached(g, cache)
	return c.Get
}

// Cacheable is the interface for a resettable cache
type Cacheable[T any] interface {
	Get() (T, error)
	Reset()
}

var _ Cacheable[int64] = (*cached[int64])(nil)

// ResettableCached wraps a getter with a cache. It returns a `Cacheable`.
// Instead of the cached getter, the `Get()` and `Reset()` methods are exposed.
func ResettableCached[T any](g func() (T, error), cache time.Duration) *cached[T] {
	clock := clock.New()
	c := &cached[T]{
		clock: clock,
		cache: cache,
		g:     g,
	}
	_ = bus.Subscribe(reset, c.Reset)
	return c
}

func (c *cached[T]) Get() (T, error) {
	c.mux.Lock()
	defer c.mux.Unlock()

	if c.mustUpdate() {
		c.val, c.err = c.g()
		c.updated = c.clock.Now()
		c.retried = c.clock.Now()

		if c.err == nil {
			c.backoffCounter = 0
		}
	}

	return c.val, c.err
}

func (c *cached[T]) Reset() {
	c.mux.Lock()
	c.updated = time.Time{}
	c.retried = time.Time{}
	c.mux.Unlock()
}

func (c *cached[T]) mustUpdate() bool {
	return c.clock.Since(c.updated) > c.cache ||
		errors.Is(c.err, api.ErrMustRetry) ||
		c.err != nil && c.shouldRetryWithBackoff()
}

// shouldRetryWithBackoff returns true when exponential back-off duration has elapsed since last retry
func (c *cached[T]) shouldRetryWithBackoff() bool {
	if c.clock.Since(c.retried) > backoffDuration*time.Duration(math.Pow(2, float64(c.backoffCounter))) {
		c.backoffCounter++
		return true
	}

	return false
}

// Value is a cacheable value that can expire
type Value[T any] struct {
	mux     sync.RWMutex
	clock   clock.Clock
	updated time.Time
	cache   time.Duration
	val     T
}

func NewValue[T any](cache time.Duration) *Value[T] {
	return &Value[T]{
		clock: clock.New(),
		cache: cache,
	}
}

func (v *Value[T]) Get() (T, error) {
	v.mux.RLock()
	defer v.mux.RUnlock()

	if v.clock.Since(v.updated) > v.cache {
		var zero T
		return zero, api.ErrTimeout
	}

	return v.val, nil
}

func (v *Value[T]) Set(val T) {
	v.mux.Lock()
	defer v.mux.Unlock()

	v.val = val
	v.updated = v.clock.Now()
}