package lrucache

import (
	"encoding/binary"
	"math"
	"strings"
	"sync"
	"time"

	"github.com/bluele/gcache"
	clog "github.com/coredns/coredns/plugin/pkg/log"
	"github.com/miekg/dns"
)

const defaultCacheSize = 1000 // in number of elements

type item struct {
	m    *dns.Msg  // dns message
	when time.Time // time when m was cached
}

type cache struct {
	items        gcache.Cache // cache
	cacheSize    int          // cache size
	sync.RWMutex              // lock
}

func (c *cache) Get(request *dns.Msg) (*dns.Msg, bool) {
	if request == nil {
		return nil, false
	}
	// create key for request
	ok, key := key(request)
	if !ok {
		clog.Debug("key returned !ok")
		return nil, false
	}
	c.Lock()
	if c.items == nil {
		c.Unlock()
		return nil, false
	}
	c.Unlock()
	rawValue, err := c.items.Get(key)
	if err == gcache.KeyNotFoundError {
		// not a real error, just no key found
		return nil, false
	}

	if err != nil {
		// real error
		clog.Errorf("can't get response for %s from cache: %s", request.Question[0].Name, err)
		return nil, false
	}

	cachedValue, ok := rawValue.(item)
	if !ok {
		clog.Errorf("entry with invalid type in cache for %s", request.Question[0].Name)
		return nil, false
	}

	response := cachedValue.fromItem(request)
	return response, true
}

func (c *cache) Set(m *dns.Msg) {
	if m == nil {
		return // no-op
	}
	if !isCacheable(m) {
		return
	}
	ok, key := key(m)
	if !ok {
		return
	}
	i := toItem(m)

	c.Lock()
	// lazy initialization for cache
	if c.items == nil {
		size := defaultCacheSize
		if c.cacheSize > 0 {
			size = c.cacheSize
		}
		c.items = gcache.New(size).LRU().Build()
	}
	c.Unlock()

	// set ttl as expiration time for item
	ttl := time.Duration(findLowestTTL(m)) * time.Second
	err := c.items.SetWithExpire(key, i, ttl)
	if err != nil {
		clog.Warning("Couldn't set cache item")
	}
}

// check if message is cacheable
func isCacheable(m *dns.Msg) bool {
	// truncated messages aren't valid
	if m.Truncated {
		clog.Debug("Refusing to cache truncated message")
		return false
	}

	// if has wrong number of questions, also don't cache
	if len(m.Question) != 1 {
		clog.Debugf("Refusing to cache message with wrong number of questions")
		return false
	}

	qName := m.Question[0].Name
	qType := m.Question[0].Qtype

	ttl := findLowestTTL(m)
	if ttl == 0 {
		return false
	}

	if m.Rcode != dns.RcodeSuccess && m.Rcode != dns.RcodeNameError {
		clog.Debugf("%s: refusing to cache message with response type %s", qName, dns.RcodeToString[m.Rcode])
		return false
	}

	if m.Rcode == dns.RcodeSuccess && (qType == dns.TypeA || qType == dns.TypeAAAA) {
		// Now verify that it contains at least one A or AAAA record
		if len(m.Answer) == 0 {
			clog.Debugf("%s: refusing to cache a NOERROR response with no answers", qName)
			return false
		}

		found := false
		for _, rr := range m.Answer {
			if rr.Header().Rrtype == dns.TypeA || rr.Header().Rrtype == dns.TypeAAAA {
				found = true
				break
			}
		}

		if !found {
			clog.Debugf("%s: refusing to cache a response with no A and AAAA answers", qName)
			return false
		}
	}

	return true
}

func findLowestTTL(m *dns.Msg) uint32 {
	var ttl uint32 = math.MaxUint32

	if m.Answer != nil {
		for _, r := range m.Answer {
			ttl = getTTLIfLower(r.Header(), ttl)
		}
	}

	if m.Ns != nil {
		for _, r := range m.Ns {
			ttl = getTTLIfLower(r.Header(), ttl)
		}
	}

	if m.Extra != nil {
		for _, r := range m.Extra {
			ttl = getTTLIfLower(r.Header(), ttl)
		}
	}

	if ttl == math.MaxUint32 {
		return 0
	}

	return ttl
}

func getTTLIfLower(h *dns.RR_Header, ttl uint32) uint32 {
	if h.Rrtype == dns.TypeOPT {
		return ttl
	}
	if h.Ttl < ttl {
		return h.Ttl
	}
	return ttl
}

// key is binary little endian in sequence:
// uint8(do)
// uint16(qtype)
// uint16(qclass)
// name
func key(m *dns.Msg) (bool, string) {
	if len(m.Question) != 1 {
		clog.Debugf("got msg with len(m.Question) != 1: %d", len(m.Question))
		return false, ""
	}

	q := m.Question[0]
	b := make([]byte, 1+2+2+len(q.Name))

	// put do
	opt := m.IsEdns0()
	do := false
	if opt != nil {
		do = opt.Do()
	}
	if do {
		b[0] = 1
	} else {
		b[0] = 0
	}

	// put qtype, qclass, name
	binary.BigEndian.PutUint16(b[1:], q.Qtype)
	binary.BigEndian.PutUint16(b[3:], q.Qclass)
	name := strings.ToLower(q.Name)
	copy(b[5:], name)
	return true, string(b)
}

func toItem(m *dns.Msg) item {
	return item{
		m:    m,
		when: time.Now(),
	}
}

func (i *item) fromItem(request *dns.Msg) *dns.Msg {
	response := &dns.Msg{}
	response.SetReply(request)

	response.Authoritative = false
	response.AuthenticatedData = i.m.AuthenticatedData
	response.RecursionAvailable = i.m.RecursionAvailable
	response.Rcode = i.m.Rcode

	ttl := findLowestTTL(i.m)
	timeleft := math.Round(float64(ttl) - time.Since(i.when).Seconds())
	var newttl uint32
	if timeleft > 0 {
		newttl = uint32(timeleft)
	}
	for _, r := range i.m.Answer {
		answer := dns.Copy(r)
		answer.Header().Ttl = newttl
		response.Answer = append(response.Answer, answer)
	}
	for _, r := range i.m.Ns {
		ns := dns.Copy(r)
		ns.Header().Ttl = newttl
		response.Ns = append(response.Ns, ns)
	}
	for _, r := range i.m.Extra {
		// don't return OPT records as these are hop-by-hop
		if r.Header().Rrtype == dns.TypeOPT {
			continue
		}
		extra := dns.Copy(r)
		extra.Header().Ttl = newttl
		response.Extra = append(response.Extra, extra)
	}
	return response
}