package dns

import (
	"context"
	"fmt"
	"net"
	"sync"
	"time"

	"github.com/miekg/dns"
)

// Resolver is the interface for sending DNS queries.
// Implementations may cache, mock, or delegate to the real network.
type Resolver interface {
	Query(ctx context.Context, server net.IP, name string, qtype uint16, cfg *QueryConfig) (*dns.Msg, error)
}

// BasicResolver implements Resolver by issuing real UDP/TCP DNS queries.
type BasicResolver struct{}

// NewBasicResolver returns a BasicResolver ready for use.
func NewBasicResolver() *BasicResolver {
	return &BasicResolver{}
}

func (br *BasicResolver) Query(ctx context.Context, server net.IP, name string, qtype uint16, cfg *QueryConfig) (*dns.Msg, error) {
	return Query(ctx, server, name, qtype, cfg)
}

type cacheKey struct {
	server string
	name   string
	qtype  uint16
	qclass uint16
}

type cacheEntry struct {
	msg      *dns.Msg
	expireAt time.Time
}

func (e *cacheEntry) expired() bool {
	return time.Now().After(e.expireAt)
}

// CachingResolver wraps another Resolver and caches successful responses.
// Responses are evicted based on the minimum DNS TTL in the message, with a
// configurable fallback default TTL for zero-TTL responses.
// All operations are safe for concurrent use.
type CachingResolver struct {
	inner      Resolver
	mu         sync.RWMutex
	cache      map[cacheKey]*cacheEntry
	defaultTTL time.Duration
}

// NewCachingResolver creates a CachingResolver wrapping inner.
// When inner is nil, a BasicResolver is used.
// Optional opts may be supplied to customise the default TTL (see WithDefaultTTL).
func NewCachingResolver(inner Resolver, opts ...CachingResolverOption) *CachingResolver {
	if inner == nil {
		inner = NewBasicResolver()
	}

	cfg := &cachingResolverConfig{
		defaultTTL: 5 * time.Second,
	}
	for _, opt := range opts {
		opt(cfg)
	}

	return &CachingResolver{
		inner:      inner,
		cache:      make(map[cacheKey]*cacheEntry),
		defaultTTL: cfg.defaultTTL,
	}
}

func (cr *CachingResolver) Query(ctx context.Context, server net.IP, name string, qtype uint16, cfg *QueryConfig) (*dns.Msg, error) {
	if cfg == nil {
		cfg = DefaultQueryConfig()
	}

	fqdn := dns.Fqdn(name)
	key := cacheKey{
		server: server.String(),
		name:   fqdn,
		qtype:  qtype,
		qclass: dns.ClassINET,
	}

	if resp, ok := cr.lookup(key); ok {
		return resp, nil
	}

	resp, err := cr.inner.Query(ctx, server, name, qtype, cfg)
	if err != nil {
		return nil, fmt.Errorf("caching resolver query: %w", err)
	}

	cr.store(key, resp)
	return resp, nil
}

func (cr *CachingResolver) lookup(key cacheKey) (*dns.Msg, bool) {
	cr.mu.RLock()
	entry, ok := cr.cache[key]
	cr.mu.RUnlock()
	if !ok {
		return nil, false
	}
	if entry.expired() {
		return nil, false
	}
	return entry.msg.Copy(), true
}

func (cr *CachingResolver) store(key cacheKey, msg *dns.Msg) {
	ttl := minTTLFromMsg(msg)
	if ttl <= 0 {
		ttl = cr.defaultTTL
	}

	cr.mu.Lock()
	cr.cache[key] = &cacheEntry{
		msg:      msg.Copy(),
		expireAt: time.Now().Add(ttl),
	}
	cr.mu.Unlock()
}

// Len returns the current number of entries in the cache, including expired ones.
func (cr *CachingResolver) Len() int {
	cr.mu.RLock()
	n := len(cr.cache)
	cr.mu.RUnlock()
	return n
}

// Clear removes all entries from the cache.
func (cr *CachingResolver) Clear() {
	cr.mu.Lock()
	cr.cache = make(map[cacheKey]*cacheEntry)
	cr.mu.Unlock()
}

// PurgeExpired removes all expired entries from the cache and returns the count removed.
func (cr *CachingResolver) PurgeExpired() int {
	cr.mu.Lock()
	count := 0
	for k, e := range cr.cache {
		if e.expired() {
			delete(cr.cache, k)
			count++
		}
	}
	cr.mu.Unlock()
	return count
}

// CachingResolverOption is a functional option for NewCachingResolver.
type CachingResolverOption func(*cachingResolverConfig)

type cachingResolverConfig struct {
	defaultTTL time.Duration
}

// WithDefaultTTL sets the TTL used for cache entries when the DNS response
// contains no TTL information (or when TTL is zero).
func WithDefaultTTL(d time.Duration) CachingResolverOption {
	return func(c *cachingResolverConfig) {
		c.defaultTTL = d
	}
}

func minTTLFromMsg(msg *dns.Msg) time.Duration {
	if msg == nil {
		return 0
	}

	var min uint32
	found := false

	for _, rr := range msg.Answer {
		ttl := rr.Header().Ttl
		if !found || ttl < min {
			min = ttl
			found = true
		}
	}
	for _, rr := range msg.Ns {
		ttl := rr.Header().Ttl
		if !found || ttl < min {
			min = ttl
			found = true
		}
	}
	for _, rr := range msg.Extra {
		if _, ok := rr.(*dns.OPT); ok {
			continue
		}
		ttl := rr.Header().Ttl
		if !found || ttl < min {
			min = ttl
			found = true
		}
	}

	if !found {
		return 0
	}

	return time.Duration(min) * time.Second
}