/*
 * Copyright 2021 Dgraph Labs, Inc. and Contributors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package ristretto

import (
	"bytes"
	"fmt"
	"sync"
	"sync/atomic"

	"github.com/outcaste-io/ristretto/z"
)

type metricType int

const (
	// The following 2 keep track of hits and misses.
	hit = iota
	miss
	// The following 3 keep track of number of keys added, updated and evicted.
	keyAdd
	keyUpdate
	keyEvict
	// The following 2 keep track of cost of keys added and evicted.
	costAdd
	costEvict
	// The following keep track of how many sets were dropped or rejected later.
	dropSets
	rejectSets
	// The following 2 keep track of how many gets were kept and dropped on the
	// floor.
	dropGets
	keepGets
	// This should be the final enum. Other enums should be set before this.
	doNotUse
)

func stringFor(t metricType) string {
	switch t {
	case hit:
		return "hit"
	case miss:
		return "miss"
	case keyAdd:
		return "keys-added"
	case keyUpdate:
		return "keys-updated"
	case keyEvict:
		return "keys-evicted"
	case costAdd:
		return "cost-added"
	case costEvict:
		return "cost-evicted"
	case dropSets:
		return "sets-dropped"
	case rejectSets:
		return "sets-rejected" // by policy.
	case dropGets:
		return "gets-dropped"
	case keepGets:
		return "gets-kept"
	default:
		return "unidentified"
	}
}

// Metrics is a snapshot of performance statistics for the lifetime of a cache instance.
type Metrics struct {
	all [doNotUse][]*uint64

	mu   sync.RWMutex
	life *z.HistogramData // Tracks the life expectancy of a key.
}

// collectMetrics just creates a new *Metrics instance and adds the pointers
// to the cache and policy instances.
func (c *Cache) collectMetrics() {
	c.Metrics = newMetrics()
	c.policy.CollectMetrics(c.Metrics)
}

func newMetrics() *Metrics {
	s := &Metrics{
		life: z.NewHistogramData(z.HistogramBounds(1, 16)),
	}
	for i := 0; i < doNotUse; i++ {
		s.all[i] = make([]*uint64, 256)
		slice := s.all[i]
		for j := range slice {
			slice[j] = new(uint64)
		}
	}
	return s
}

func (p *Metrics) add(t metricType, hash, delta uint64) {
	if p == nil {
		return
	}
	valp := p.all[t]
	// Avoid false sharing by padding at least 64 bytes of space between two
	// atomic counters which would be incremented.
	idx := (hash % 25) * 10
	atomic.AddUint64(valp[idx], delta)
}

func (p *Metrics) get(t metricType) uint64 {
	if p == nil {
		return 0
	}
	valp := p.all[t]
	var total uint64
	for i := range valp {
		total += atomic.LoadUint64(valp[i])
	}
	return total
}

// Hits is the number of Get calls where a value was found for the corresponding key.
func (p *Metrics) Hits() uint64 {
	return p.get(hit)
}

// Misses is the number of Get calls where a value was not found for the corresponding key.
func (p *Metrics) Misses() uint64 {
	return p.get(miss)
}

// KeysAdded is the total number of Set calls where a new key-value item was added.
func (p *Metrics) KeysAdded() uint64 {
	return p.get(keyAdd)
}

// KeysUpdated is the total number of Set calls where the value was updated.
func (p *Metrics) KeysUpdated() uint64 {
	return p.get(keyUpdate)
}

// KeysEvicted is the total number of keys evicted.
func (p *Metrics) KeysEvicted() uint64 {
	return p.get(keyEvict)
}

// CostAdded is the sum of costs that have been added (successful Set calls).
func (p *Metrics) CostAdded() uint64 {
	return p.get(costAdd)
}

// CostEvicted is the sum of all costs that have been evicted.
func (p *Metrics) CostEvicted() uint64 {
	return p.get(costEvict)
}

// SetsDropped is the number of Set calls that don't make it into internal
// buffers (due to contention or some other reason).
func (p *Metrics) SetsDropped() uint64 {
	return p.get(dropSets)
}

// SetsRejected is the number of Set calls rejected by the policy (TinyLFU).
func (p *Metrics) SetsRejected() uint64 {
	return p.get(rejectSets)
}

// GetsDropped is the number of Get counter increments that are dropped
// internally.
func (p *Metrics) GetsDropped() uint64 {
	return p.get(dropGets)
}

// GetsKept is the number of Get counter increments that are kept.
func (p *Metrics) GetsKept() uint64 {
	return p.get(keepGets)
}

// Ratio is the number of Hits over all accesses (Hits + Misses). This is the
// percentage of successful Get calls.
func (p *Metrics) Ratio() float64 {
	if p == nil {
		return 0.0
	}
	hits, misses := p.get(hit), p.get(miss)
	if hits == 0 && misses == 0 {
		return 0.0
	}
	return float64(hits) / float64(hits+misses)
}

func (p *Metrics) trackEviction(numSeconds int64) {
	if p == nil {
		return
	}
	p.mu.Lock()
	defer p.mu.Unlock()
	p.life.Update(numSeconds)
}

func (p *Metrics) LifeExpectancySeconds() *z.HistogramData {
	if p == nil {
		return nil
	}
	p.mu.RLock()
	defer p.mu.RUnlock()
	return p.life.Copy()
}

// Clear resets all the metrics.
func (p *Metrics) Clear() {
	if p == nil {
		return
	}
	for i := 0; i < doNotUse; i++ {
		for j := range p.all[i] {
			atomic.StoreUint64(p.all[i][j], 0)
		}
	}
	p.mu.Lock()
	p.life = z.NewHistogramData(z.HistogramBounds(1, 16))
	p.mu.Unlock()
}

// String returns a string representation of the metrics.
func (p *Metrics) String() string {
	if p == nil {
		return ""
	}
	var buf bytes.Buffer
	for i := 0; i < doNotUse; i++ {
		t := metricType(i)
		fmt.Fprintf(&buf, "%s: %d ", stringFor(t), p.get(t))
	}
	fmt.Fprintf(&buf, "gets-total: %d ", p.get(hit)+p.get(miss))
	fmt.Fprintf(&buf, "hit-ratio: %.2f", p.Ratio())
	return buf.String()
}