package kubenat

import (
	"bufio"
	"bytes"
	"context"
	"fmt"
	"io/ioutil"
	"net"
	"strconv"
	"strings"

	"github.com/golang/glog"
)

// translationReq is a request passed to the translationWorker.
type translationReq struct {
	t   *Tuple4
	res chan *translationResp
}

// translationResp is a response from the translationWorker, sent over the res
// channel in a translationReq.
type translationResp struct {
	localIP   net.IP
	localPort uint16
}

// reply sends a reply to the given translationReq based on a conntrackEntry,
// sending nil if the entry is nil.
func (r *translationReq) reply(e *conntrackEntry) {
	if e == nil {
		r.res <- nil
		return
	}
	localPort, err := strconv.ParseUint(e.request["sport"], 10, 16)
	if err != nil {
		r.res <- nil
		return
	}
	r.res <- &translationResp{
		localIP:   net.ParseIP(e.request["src"]),
		localPort: uint16(localPort),
	}
}

// translate performs a translationReq/translationResp exchange under a context
// that can be used to time out the query.
func (r *Resolver) translate(ctx context.Context, t *Tuple4) (*translationResp, error) {
	resC := make(chan *translationResp, 1)
	r.translationC <- &translationReq{
		t:   t,
		res: resC,
	}

	select {
	case <-ctx.Done():
		return nil, ctx.Err()
	case res := <-resC:
		return res, nil
	}
}

// conntrackEntry is an entry parsed from /proc/net/nf_conntrack. The format is
// not well documented, and the best resource I could find is:
//   https://stackoverflow.com/questions/16034698/details-of-proc-net-ip-conntrack-and-proc-net-nf-conntrack
type conntrackEntry struct {
	// networkProtocol is currently always "ipv4".
	networkProtocol string
	// transmissionProtocol is currently "tcp" or "udp".
	transmissionProtocol string
	invalidateTimeout    int64

	state string

	// request key-value pairs. For NAT, these are entries relating to the
	// connection as seen as the 'inside' of the NAT, eg. the pod-originated
	// connection.
	request map[string]string
	// response key-value parirs. For NAT, these are entries relating to the
	// connection as seen by the 'outside' of the NAT, eg. the internet.
	response map[string]string
	tags     map[string]bool
}

// conntrackParseEntry parses a line from /proc/net/nf_conntrack into a conntrackEntry.
func conntrackParseEntry(line string) (*conntrackEntry, error) {
	entry := conntrackEntry{
		request:  make(map[string]string),
		response: make(map[string]string),
		tags:     make(map[string]bool),
	}

	fields := strings.Fields(line)
	if len(fields) < 5 {
		// This should never happen unless the file format drastically
		// changed. Don't bother to parse the rest, error early, and let
		// someone debug this.
		return nil, fmt.Errorf("invalid field count: %v", fields)
	}
	switch fields[0] {
	case "ipv4":
		if fields[1] != "2" {
			return nil, fmt.Errorf("ipv4 with proto number %q, wanted 2", fields[1])
		}
		// TODO(q3k): support IPv6 when we get it on prod.
	default:
		return nil, nil
	}
	entry.networkProtocol = fields[0]

	rest := fields[5:]
	switch fields[2] {
	case "tcp":
		if fields[3] != "6" {
			return nil, fmt.Errorf("tcp with proto number %q, wanted 6", fields[3])
		}
		if len(fields) < 6 {
			return nil, fmt.Errorf("tcp with missing state field")
		}
		entry.state = fields[5]
		rest = fields[6:]
	case "udp":
		if fields[3] != "17" {
			return nil, fmt.Errorf("udp with proto number %q, wanted 17", fields[3])
		}
	default:
		return nil, nil
	}
	entry.transmissionProtocol = fields[2]

	invalidateTimeout, err := strconv.ParseInt(fields[4], 10, 64)
	if err != nil {
		return nil, fmt.Errorf("unparseable timeout %q", fields[4])
	}
	entry.invalidateTimeout = invalidateTimeout

	for _, el := range rest {
		parts := strings.Split(el, "=")
		switch len(parts) {
		case 1:
			// This is a tag.
			tag := parts[0]
			// Ensure the tag starts and ends with [] (eg. [ASSURED].
			if !strings.HasPrefix(tag, "[") || !strings.HasSuffix(tag, "]") {
				continue
			}
			// Strip [ and ].
			tag = tag[1:]
			tag = tag[:len(tag)-1]
			if _, ok := entry.tags[tag]; ok {
				return nil, fmt.Errorf("repeated tag %q", tag)
			}
			entry.tags[tag] = true
		case 2:
			// This is a k/v field.
			k := parts[0]
			v := parts[1]
			if _, ok := entry.request[k]; ok {
				if _, ok := entry.response[k]; ok {
					return nil, fmt.Errorf("field %q encountered more than twice", k)
				} else {
					entry.response[k] = v
				}
			} else {
				entry.request[k] = v
			}
		default:
			return nil, fmt.Errorf("unparseable column %q", el)
		}
	}
	return &entry, nil

}

// conntrackParse parses the contents of a /proc/net/nf_conntrack file into
// multiple entries. If the majority of the entries could not be parsed, an
// error is returned.
func conntrackParse(data []byte) ([]conntrackEntry, error) {
	buf := bytes.NewBuffer(data)
	scanner := bufio.NewScanner(buf)
	var res []conntrackEntry
	var errors []error
	for scanner.Scan() {
		line := strings.TrimSpace(scanner.Text())
		if line == "" {
			continue
		}

		entry, err := conntrackParseEntry(line)
		if err != nil {
			glog.Errorf("Error while parsing %q: %v", line, err)
			errors = append(errors, err)
		} else if entry != nil {
			res = append(res, *entry)
		}
	}

	if len(errors) == 0 || len(errors) < len(res) {
		return res, nil
	} else {
		return nil, fmt.Errorf("encountered too many errors during conntrack parse, check logs; first error: %w", errors[0])
	}
}

// contrackIndex is an index into a list of conntrackEntries. It allows lookup
// by request/response k/v pairs.
type conntrackIndex struct {
	entries []conntrackEntry
	// byRequest is a map from key to value to list of indixes into entries.
	byRequest map[string]map[string][]int
	// byResponse is a map from key to value to list of indixes into entries.
	byResponse map[string]map[string][]int
}

// buildIndex builds a conntrackIndex from a list of conntrackEntries.
func buildIndex(entries []conntrackEntry) *conntrackIndex {
	ix := conntrackIndex{
		entries:    entries,
		byRequest:  make(map[string]map[string][]int),
		byResponse: make(map[string]map[string][]int),
	}
	for i, entry := range ix.entries {
		for k, v := range entry.request {
			if _, ok := ix.byRequest[k]; !ok {
				ix.byRequest[k] = make(map[string][]int)
			}
			ix.byRequest[k][v] = append(ix.byRequest[k][v], i)
		}
		for k, v := range entry.response {
			if _, ok := ix.byResponse[k]; !ok {
				ix.byResponse[k] = make(map[string][]int)
			}
			ix.byResponse[k][v] = append(ix.byResponse[k][v], i)
		}
	}
	return &ix
}

// getByRequest returns conntrackEntries that match a given k/v pair in their
// request fields.
func (c *conntrackIndex) getByRequest(k, v string) []*conntrackEntry {
	m, ok := c.byRequest[k]
	if !ok {
		return nil
	}
	ixes, ok := m[v]
	if !ok {
		return nil
	}
	res := make([]*conntrackEntry, len(ixes))
	for i, ix := range ixes {
		res[i] = &c.entries[ix]
	}
	return res
}

// getByResponse returns conntrackEntries that match a given k/v pair in their
// response fields.
func (c *conntrackIndex) getByResponse(k, v string) []*conntrackEntry {
	m, ok := c.byResponse[k]
	if !ok {
		return nil
	}
	ixes, ok := m[v]
	if !ok {
		return nil
	}
	res := make([]*conntrackEntry, len(ixes))
	for i, ix := range ixes {
		res[i] = &c.entries[ix]
	}
	return res
}

// find returns a conntrackEntry corresponding to a TCP connection defined on
// the 'outside' of the NAT by a 4-tuple, or nil if no such connection is
// found.
func (c *conntrackIndex) find(t *Tuple4) *conntrackEntry {
	// TODO(q3k): support IPv6
	if t.RemoteIP.To4() == nil || t.LocalIP.To4() == nil {
		return nil
	}
	entries := c.getByResponse("src", t.RemoteIP.String())
	for _, entry := range entries {
		if entry.transmissionProtocol != "tcp" {
			continue
		}
		if entry.response["sport"] != fmt.Sprintf("%d", t.RemotePort) {
			continue
		}
		if entry.response["dst"] != t.LocalIP.String() {
			continue
		}
		if entry.response["dport"] != fmt.Sprintf("%d", t.LocalPort) {
			continue
		}
		return entry
	}
	return nil
}

// runTranslationWorker runs the conntrack 'translation worker'. It responds to
// requests over translationC until ctx is canceled.
func (r *Resolver) runTranslationWorker(ctx context.Context) {
	var ix *conntrackIndex
	readConntrack := func() {
		var entries []conntrackEntry
		data, err := ioutil.ReadFile(r.conntrackPath)
		if err != nil {
			glog.Errorf("Failed to read conntrack file: %v", err)
		} else {
			entries, err = conntrackParse(data)
			if err != nil {
				glog.Errorf("failed to parse conntrack entries: %v", err)
			}
		}
		ix = buildIndex(entries)
	}
	readConntrack()

	for {
		select {
		case req := <-r.translationC:
			entry := ix.find(req.t)
			if entry != nil {
				req.reply(entry)
			} else {
				readConntrack()
				entry = ix.find(req.t)
				if entry != nil {
					req.reply(entry)
				} else {
					req.reply(nil)
				}
			}
		case <-ctx.Done():
			return
		}
	}
}