// Copyright 2016 The Roughtime Authors.
//
// 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. */

// client is a somewhat featured Roughtime client.
package main

import (
	"crypto/rand"
	"encoding/binary"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net"
	"time"

	"math/big"
	mathrand "math/rand"

	"golang.org/x/crypto/ed25519"
	"roughtime.googlesource.com/go/client/monotime"
	"roughtime.googlesource.com/go/config"
	"roughtime.googlesource.com/go/protocol"
)

const (
	// defaultMaxRadius is the maximum radius that we'll accept from a
	// server.
	defaultMaxRadius = 10 * time.Second

	// defaultMaxDifference is the maximum difference in time between any
	// sample from a server and the quorum-agreed time before we believe
	// that the server might be misbehaving.
	defaultMaxDifference = 60 * time.Second

	// defaultTimeout is the default maximum time that a server has to
	// answer a query.
	defaultTimeout = 2 * time.Second

	// defaultNumQueries is the default number of times we will try to
	// query a server.
	defaultNumQueries = 3
)

// Client represents a Roughtime client and exposes a number of members that
// can be set in order to configure it. The zero value of a Client is always
// ready to use and will set sensible defaults.
type Client struct {
	// Permutation returns a random permutation of [0‥n) that is used to
	// query servers in a random order. If nil, a sensible default is used.
	Permutation func(n int) []int

	// MaxRadiusUs is the maximum interval radius that will be accepted
	// from a server. If zero, a sensible default is used.
	MaxRadius time.Duration

	// MaxDifference is the maximum difference in time between any sample
	// from a server and the quorum-agreed time before that sample is
	// considered suspect. If zero, a sensible default is used.
	MaxDifference time.Duration

	// QueryTimeout is the amount of time a server has to reply to a query.
	// If zero, a sensible default will be used.
	QueryTimeout time.Duration

	// NumQueries is the maximum number of times a query will be sent to a
	// specific server before giving up. If <= zero, a sensible default
	// will be used.
	NumQueries int

	// now returns a monotonic duration from some unspecified epoch. If
	// nil, the system monotonic time will be used.
	nowFunc func() time.Duration
}

func (c *Client) now() time.Duration {
	if c.nowFunc != nil {
		return c.nowFunc()
	}
	return monotime.Now()
}

func (c *Client) permutation(n int) []int {
	if c.Permutation != nil {
		return c.Permutation(n)
	}

	var randBuf [8]byte
	if _, err := io.ReadFull(rand.Reader, randBuf[:]); err != nil {
		panic(err)
	}

	seed := binary.LittleEndian.Uint64(randBuf[:])
	rand := mathrand.New(mathrand.NewSource(int64(seed)))

	return rand.Perm(n)
}

func (c *Client) maxRadius() time.Duration {
	if c.MaxRadius != 0 {
		return c.MaxRadius
	}

	return defaultMaxRadius
}

func (c *Client) maxDifference() time.Duration {
	if c.MaxDifference != 0 {
		return c.MaxDifference
	}

	return defaultMaxDifference
}

func (c *Client) queryTimeout() time.Duration {
	if c.QueryTimeout != 0 {
		return c.QueryTimeout
	}

	return defaultTimeout
}

func (c *Client) numQueries() int {
	if c.NumQueries > 0 {
		return c.NumQueries
	}

	return defaultNumQueries
}

// LoadChain loads a JSON-format chain from the given JSON data.
func LoadChain(jsonData []byte) (chain *config.Chain, err error) {
	chain = new(config.Chain)
	if err := json.Unmarshal(jsonData, chain); err != nil {
		return nil, err
	}

	for i, link := range chain.Links {
		if link.PublicKeyType != "ed25519" {
			return nil, fmt.Errorf("client: link #%d in chain file has unknown public key type %q", i, link.PublicKeyType)
		}

		if l := len(link.PublicKey); l != ed25519.PublicKeySize {
			return nil, fmt.Errorf("client: link #%d in chain file has bad public key of length %d", i, l)
		}

		if l := len(link.NonceOrBlind); l != protocol.NonceSize {
			return nil, fmt.Errorf("client: link #%d in chain file has bad nonce/blind of length %d", i, l)
		}

		var nonce [protocol.NonceSize]byte
		if i == 0 {
			copy(nonce[:], link.NonceOrBlind[:])
		} else {
			nonce = protocol.CalculateChainNonce(chain.Links[i-1].Reply, link.NonceOrBlind[:])
		}

		if _, _, err := protocol.VerifyReply(link.Reply, link.PublicKey, nonce); err != nil {
			return nil, fmt.Errorf("client: failed to verify link #%d in chain file", i)
		}
	}

	return chain, nil
}

// timeSample represents a time sample from the network.
type timeSample struct {
	// server references the server that was queried.
	server *config.Server

	// base is a monotonic clock sample that is taken at a time before the
	// network could have answered the query.
	base *big.Int

	// min is the minimum real-time (in Roughtime UTC microseconds) that
	// could correspond to |base| (i.e. midpoint - radius).
	min *big.Int

	// max is the maximum real-time (in Roughtime UTC microseconds) that
	// could correspond to |base| (i.e. midpoint + radius + query time).
	max *big.Int

	// queryDuration contains the amount of time that the server took to
	// answer the query.
	queryDuration time.Duration
}

// midpoint returns the average of the min and max times.
func (s *timeSample) midpoint() *big.Int {
	ret := new(big.Int).Add(s.min, s.max)
	return ret.Rsh(ret, 1)
}

// alignTo updates s so that its base value matches that from reference.
func (s *timeSample) alignTo(reference *timeSample) {
	delta := new(big.Int).Sub(s.base, reference.base)
	delta.Div(delta, big.NewInt(int64(time.Microsecond)))
	s.base.Sub(s.base, delta)
	s.min.Sub(s.min, delta)
	s.max.Sub(s.max, delta)
}

// contains returns true iff p belongs to s
func (s *timeSample) contains(p *big.Int) bool {
	return s.max.Cmp(p) >= 0 && s.min.Cmp(p) <= 0
}

// overlaps returns true iff s and other have any timespan in common.
func (s *timeSample) overlaps(other *timeSample) bool {
	return s.max.Cmp(other.min) >= 0 && other.max.Cmp(s.min) >= 0
}

// query sends a request to s, appends it to chain, and returns the resulting
// timeSample.
func (c *Client) query(server *config.Server, chain *config.Chain) (*timeSample, error) {
	var prevReply []byte
	if len(chain.Links) > 0 {
		prevReply = chain.Links[len(chain.Links)-1].Reply
	}

	var baseTime, replyTime time.Duration
	var reply []byte
	var nonce, blind [protocol.NonceSize]byte

	for attempts := 0; attempts < c.numQueries(); attempts++ {
		var request []byte
		var err error
		if nonce, blind, request, err = protocol.CreateRequest(rand.Reader, prevReply); err != nil {
			return nil, err
		}
		if len(request) < protocol.MinRequestSize {
			panic("internal error: bad request length")
		}

		udpAddr, err := serverUDPAddr(server)
		if err != nil {
			panic(err)
		}

		conn, err := net.DialUDP("udp", nil, udpAddr)
		if err != nil {
			return nil, err
		}

		conn.SetReadDeadline(time.Now().Add(c.queryTimeout()))
		baseTime = c.now()
		conn.Write(request)

		var replyBytes [1024]byte
		n, err := conn.Read(replyBytes[:])
		if err == nil {
			replyTime = c.now()
			reply = replyBytes[:n]
			break
		}

		if netErr, ok := err.(net.Error); ok {
			if !netErr.Timeout() {
				return nil, errors.New("client: error reading from UDP socket: " + err.Error())
			}
		}
	}

	if reply == nil {
		return nil, fmt.Errorf("client: no reply from server %q", server.Name)
	}

	if replyTime < baseTime {
		panic("broken monotonic clock")
	}
	queryDuration := replyTime - baseTime

	midpoint, radius, err := protocol.VerifyReply(reply, server.PublicKey, nonce)
	if err != nil {
		return nil, err
	}

	if time.Duration(radius)*time.Microsecond > c.maxRadius() {
		return nil, fmt.Errorf("client: radius (%d) too large", radius)
	}

	nonceOrBlind := blind[:]
	if len(prevReply) == 0 {
		nonceOrBlind = nonce[:]
	}

	chain.Links = append(chain.Links, config.Link{
		PublicKeyType: "ed25519",
		PublicKey:     server.PublicKey,
		NonceOrBlind:  nonceOrBlind,
		Reply:         reply,
	})

	queryDurationBig := new(big.Int).SetInt64(int64(queryDuration / time.Microsecond))
	bigRadius := new(big.Int).SetUint64(uint64(radius))
	min := new(big.Int).SetUint64(midpoint)
	min.Sub(min, bigRadius)
	min.Sub(min, queryDurationBig)

	max := new(big.Int).SetUint64(midpoint)
	max.Add(max, bigRadius)

	return &timeSample{
		server:        server,
		base:          new(big.Int).SetInt64(int64(baseTime)),
		min:           min,
		max:           max,
		queryDuration: queryDuration,
	}, nil
}

func serverUDPAddr(server *config.Server) (*net.UDPAddr, error) {
	for _, addr := range server.Addresses {
		if addr.Protocol != "udp" {
			continue
		}

		return net.ResolveUDPAddr("udp", addr.Address)
	}

	return nil, nil
}

// LoadServers loads information about known servers from the given JSON data.
// It only extracts information about servers with Ed25519 public keys and UDP
// address. The number of servers skipped because of unsupported requirements
// is returned in numSkipped.
func LoadServers(jsonData []byte) (servers []config.Server, numSkipped int, err error) {
	var serversJSON config.ServersJSON
	if err := json.Unmarshal(jsonData, &serversJSON); err != nil {
		return nil, 0, err
	}

	seenNames := make(map[string]struct{})

	for _, candidate := range serversJSON.Servers {
		if _, ok := seenNames[candidate.Name]; ok {
			return nil, 0, fmt.Errorf("client: duplicate server name %q", candidate.Name)
		}
		seenNames[candidate.Name] = struct{}{}

		if candidate.PublicKeyType != "ed25519" {
			numSkipped++
			continue
		}

		udpAddr, err := serverUDPAddr(&candidate)

		if err != nil {
			return nil, 0, fmt.Errorf("client: server %q lists invalid UDP address: %s", candidate.Name, err)
		}

		if udpAddr == nil {
			numSkipped++
			continue
		}

		servers = append(servers, candidate)
	}

	if len(servers) == 0 {
		return nil, 0, errors.New("client: no usable servers found")
	}

	return servers, 0, nil
}

// trimChain drops elements from the beginning of chain, as needed, so that its
// length is <= n.
func trimChain(chain *config.Chain, n int) {
	if n <= 0 {
		chain.Links = nil
		return
	}

	if len(chain.Links) <= n {
		return
	}

	numToTrim := len(chain.Links) - n
	for i := 0; i < numToTrim; i++ {
		// The NonceOrBlind of the first element is special because
		// it's an nonce. All the others are blinds and are combined
		// with the previous reply to make the nonce. That's not
		// possible for the first element because there is no previous
		// reply. Therefore, when removing the first element the blind
		// of the next element needs to be converted to an nonce.
		nonce := protocol.CalculateChainNonce(chain.Links[0].Reply, chain.Links[1].NonceOrBlind[:])
		chain.Links[1].NonceOrBlind = nonce[:]
		chain.Links = chain.Links[1:]
	}
}

// intersection returns the timespan common to all the elements in samples,
// which must be aligned to the same base. The caller must ensure that such a
// timespan exists.
func intersection(samples []*timeSample) *timeSample {
	ret := &timeSample{
		base: samples[0].base,
		min:  new(big.Int).Set(samples[0].min),
		max:  new(big.Int).Set(samples[0].max),
	}

	for _, sample := range samples[1:] {
		if ret.min.Cmp(sample.min) < 0 {
			ret.min.Set(sample.min)
		}
		if ret.max.Cmp(sample.max) > 0 {
			ret.max.Set(sample.max)
		}
	}

	return ret
}

// findNOverlapping finds an n-element subset of samples where all the
// members overlap. It returns the intersection if such a subset exists.
func findNOverlapping(samples []*timeSample, n int) (sampleIntersection *timeSample, ok bool) {
	switch {
	case n <= 0:
		return nil, false
	case n == 1:
		return samples[0], true
	}

	overlapping := make([]*timeSample, 0, n)

	for _, initial := range samples {
		// An intersection of any subset of intervals will be an interval that contains
		// the starting point of one of the intervals (possibly as its own starting point).
		point := initial.min

		for _, candidate := range samples {
			if candidate.contains(point) {
				overlapping = append(overlapping, candidate)
			}

			if len(overlapping) == n {
				return intersection(overlapping), true
			}
		}

		overlapping = overlapping[:0]
	}

	return nil, false
}

// TimeResult is the result of trying to establish the current time by querying
// a number of servers.
type TimeResult struct {
	// MonoUTCDelta may be nil, in which case a time could not be
	// established. Otherwise it contains the difference between the
	// Roughtime epoch and the monotonic clock.
	MonoUTCDelta *time.Duration

	// ServerErrors maps from server name to query error.
	ServerErrors map[string]error

	// ServerInfo contains information about each server that was queried.
	ServerInfo map[string]ServerInfo

	// OutOfRangeAnswer is true if one or more of the queries contained a
	// significantly incorrect time, as defined by MaxDifference. In this
	// case, the reply will have been recorded in the chain.
	OutOfRangeAnswer bool
}

// ServerInfo contains information from a specific server.
type ServerInfo struct {
	// QueryDuration is the amount of time that the server took to answer.
	QueryDuration time.Duration

	// Min and Max specify the time window given by the server. These
	// values have been adjusted so that they are comparible across
	// servers, even though they are queried at different times.
	Min, Max *big.Int
}

// EstablishTime queries a number of servers until it has a quorum of
// overlapping results, or it runs out of servers. Results from the querying
// the servers are appended to chain.
func (c *Client) EstablishTime(chain *config.Chain, quorum int, servers []config.Server) (TimeResult, error) {
	perm := c.permutation(len(servers))
	var samples []*timeSample
	var intersection *timeSample
	var result TimeResult

	for len(perm) > 0 {
		server := &servers[perm[0]]
		perm = perm[1:]

		sample, err := c.query(server, chain)
		if err != nil {
			if result.ServerErrors == nil {
				result.ServerErrors = make(map[string]error)
			}
			result.ServerErrors[server.Name] = err
			continue
		}

		if len(samples) > 0 {
			sample.alignTo(samples[0])
		}
		samples = append(samples, sample)

		if result.ServerInfo == nil {
			result.ServerInfo = make(map[string]ServerInfo)
		}
		result.ServerInfo[server.Name] = ServerInfo{
			QueryDuration: sample.queryDuration,
			Min:           sample.min,
			Max:           sample.max,
		}

		var ok bool
		if intersection, ok = findNOverlapping(samples, quorum); ok {
			break
		}
		intersection = nil
	}

	if intersection == nil {
		return result, nil
	}
	midpoint := intersection.midpoint()

	maxDifference := new(big.Int).SetUint64(uint64(c.maxDifference() / time.Microsecond))
	for _, sample := range samples {
		delta := new(big.Int).Sub(midpoint, sample.midpoint())
		delta.Abs(delta)

		if delta.Cmp(maxDifference) > 0 {
			result.OutOfRangeAnswer = true
			break
		}
	}

	midpoint.Mul(midpoint, big.NewInt(1000))
	delta := new(big.Int).Sub(midpoint, intersection.base)
	if delta.BitLen() > 63 {
		return result, errors.New("client: cannot represent difference between monotonic and UTC time")
	}
	monoUTCDelta := time.Duration(delta.Int64())
	result.MonoUTCDelta = &monoUTCDelta

	return result, nil
}