// Package cache implements the hot answer cache: a sharded LRU over packed
// wire-format DNS messages, backed by a bbolt snapshot so a restart does not
// start cold.
//
// Design notes that matter for performance:
//
//   - Entries are stored as packed wire bytes, not as *dns.Msg. A hit costs
//     one Unpack and yields a private copy, so serving never mutates shared
//     state and TTL ageing is free. This is the same trade dnsmasq makes.
//   - Keys are Go strings built from the already-allocated question name, so
//     the lookup path does not allocate on a hit.
//   - The LRU is sharded by name hash: a hot domain only contends with other
//     names in its own shard.
//   - Expired-but-useful entries are kept for a serve-stale window so a dead
//     upstream degrades latency instead of availability.
package cache

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

	"github.com/miekg/dns"
)

// Key identifies a cache entry. Scope lets several logically distinct
// resolvers share one cache (for example per-upstream-group ACL overrides)
// without colliding.
type Key struct {
	Name  string
	Type  uint16
	Class uint16
	// DO mirrors the request's DNSSEC OK bit. Since we pass AD/DO through
	// without validating, a DO query legitimately gets a different answer.
	DO bool
	// Scope is a small caller-defined namespace, usually a group/ACL key.
	Scope uint8
	// CD (checking disabled) is not part of the key: RFC 4035 says a CD
	// query is answered from a non-CD cached entry, which is what we do.
}

// QueryKey builds a Key from a DNS question.
func QueryKey(name string, qtype, qclass uint16, do bool, scope uint8) Key {
	return Key{
		Name:  canonical(name),
		Type:  qtype,
		Class: qclass,
		DO:    do,
		Scope: scope,
	}
}

// FromMsg builds a Key from the first question of a message.
func FromMsg(m *dns.Msg, scope uint8) (Key, bool) {
	if len(m.Question) == 0 {
		return Key{}, false
	}
	q := m.Question[0]
	return QueryKey(q.Name, q.Qtype, q.Qclass, Do(m), scope), true
}

// Do reports whether the message carries the DNSSEC OK bit.
func Do(m *dns.Msg) bool {
	if o := m.IsEdns0(); o != nil {
		return o.Do()
	}
	return false
}

// canonical lowercases and strips the trailing dot of a name so that
// "Example.COM." and "example.com" share one entry.
func canonical(name string) string {
	if n := len(name); n > 0 && name[n-1] == '.' {
		name = name[:n-1]
	}
	// A fast path for the overwhelmingly common all-lowercase case keeps this
	// allocation-free.
	for i := 0; i < len(name); i++ {
		c := name[i]
		if c >= 'A' && c <= 'Z' {
			return strings.ToLower(name)
		}
	}
	return name
}

// packKey serialises a key for the bbolt snapshot. The wire format is
// length-prefixed so it is unambiguous and compact:
//
//	varint len | wire name | uint16 type | uint16 class | uint8 flags
func packKey(buf []byte, k Key) ([]byte, error) {
	name := k.Name
	if n := len(name); n > 255*4 {
		return nil, errKeyTooLong
	}
	var tmp [256]byte
	wireLen, err := dns.PackDomainName(name, tmp[:], 0, nil, false)
	if err != nil {
		return nil, err
	}
	buf = binary.AppendUvarint(buf, uint64(wireLen))
	buf = append(buf, tmp[:wireLen]...)
	buf = binary.LittleEndian.AppendUint16(buf, k.Type)
	buf = binary.LittleEndian.AppendUint16(buf, k.Class)
	var flags uint8
	if k.DO {
		flags |= 1
	}
	flags |= k.Scope << 1
	return append(buf, flags), nil
}

func unpackKey(buf []byte) (Key, []byte, error) {
	l, n := binary.Uvarint(buf)
	if n <= 0 || l > uint64(len(buf)-n) {
		return Key{}, nil, errBadKey
	}
	buf = buf[n:]
	wire := buf[:int(l)]
	buf = buf[l:]
	if len(buf) < 5 {
		return Key{}, nil, errBadKey
	}
	name, _, err := dns.UnpackDomainName(wire, 0)
	if err != nil {
		return Key{}, nil, err
	}
	var k Key
	k.Name = canonical(name)
	k.Type = binary.LittleEndian.Uint16(buf[0:])
	k.Class = binary.LittleEndian.Uint16(buf[2:])
	flags := buf[4]
	k.DO = flags&1 != 0
	k.Scope = flags >> 1
	return k, buf[5:], nil
}

// Entry is one cached response plus its timing metadata.
//
// Wire is the packed response exactly as it will be served (minus ID and TTL
// ageing, both applied on unpack). Origin records where the answer came from
// for the UI.
type Entry struct {
	Wire []byte
	// TTL is the value the entry was stored with, used to age the answer on
	// every serve so clients see a monotonically decreasing TTL.
	TTL uint32
	// Age is the logical age at store time (0 for a fresh answer).
	Age time.Duration
	// Origin is a small enum: "upstream", "prewarm", "local", "blocked".
	Origin string
	// Upstream is the address the answer came from, shown in the UI.
	Upstream string
	// CreatedAt is the wall clock when the entry was stored.
	CreatedAt time.Time
	// ExpiresAt is when the entry stops being fresh.
	ExpiresAt time.Time
	// StaleUntil is when the entry is finally evicted. It is ExpiresAt plus
	// the serve-stale window.
	StaleUntil time.Time
	// Bytes is the accounted size, used by the LRU and the size gauges.
	Bytes int
	// Hits counts serves since it was stored.
	Hits uint32
	// Neg marks a cached NXDOMAIN or NODATA answer.
	Neg bool
	// Truncated records that the upstream answer did not fit in 512 bytes,
	// so a cache hit must re-truncate for a small client buffer.
	Truncated bool
}

// AgeTTL returns the TTL an entry should be served with, given the elapsed
// time since it was stored, clamped at zero.
func (e *Entry) AgeTTL(now time.Time) uint32 {
	if e.CreatedAt.IsZero() {
		return e.TTL
	}
	elapsed := uint32(now.Sub(e.CreatedAt).Seconds())
	if elapsed >= e.TTL {
		return 0
	}
	return e.TTL - elapsed
}

// Remaining returns how long the entry stays fresh.
func (e *Entry) Remaining(now time.Time) time.Duration {
	if !e.ExpiresAt.After(now) {
		return 0
	}
	return e.ExpiresAt.Sub(now)
}

// StaleRemaining returns how long the entry remains serveable as stale data.
func (e *Entry) StaleRemaining(now time.Time) time.Duration {
	if !e.StaleUntil.After(now) {
		return 0
	}
	return e.StaleUntil.Sub(now)
}

// Expired reports whether the entry is past its freshness window.
func (e *Entry) Expired(now time.Time) bool { return !e.ExpiresAt.After(now) }

// BlockResponse is a synthesised answer (blocked or locally defined) held in
// the cache so repeated blocked queries never touch the matcher or the wire.
type BlockResponse struct {
	Wire []byte
	TTL  uint32
}