// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package portlist

import (
	"fmt"
	"sort"
	"strings"

	"tailscale.com/envknob"
)

// Port is a listening port on the machine.
type Port struct {
	Proto   string // "tcp" or "udp"
	Port    uint16 // port number
	Process string // optional process name, if found

	inode string // OS-specific; "socket:[165614651]" on Linux
}

// List is a list of Ports.
type List []Port

func (a *Port) lessThan(b *Port) bool {
	if a.Port < b.Port {
		return true
	} else if a.Port > b.Port {
		return false
	}

	if a.Proto < b.Proto {
		return true
	} else if a.Proto > b.Proto {
		return false
	}

	if a.inode < b.inode {
		return true
	} else if a.inode > b.inode {
		return false
	}

	if a.Process < b.Process {
		return true
	} else if a.Process > b.Process {
		return false
	}
	return false
}

func (a List) sameInodes(b List) bool {
	if a == nil || b == nil || len(a) != len(b) {
		return false
	}
	for i := range a {
		if a[i].Proto != b[i].Proto ||
			a[i].Port != b[i].Port ||
			a[i].inode != b[i].inode {
			return false
		}
	}
	return true
}

func (pl List) String() string {
	var sb strings.Builder
	for _, v := range pl {
		fmt.Fprintf(&sb, "%-3s %5d %-17s %#v\n",
			v.Proto, v.Port, v.inode, v.Process)
	}
	return strings.TrimRight(sb.String(), "\n")
}

var debugDisablePortlist = envknob.RegisterBool("TS_DEBUG_DISABLE_PORTLIST")

func (p *Poller) getList() (List, error) {
	if debugDisablePortlist() {
		return nil, nil
	}
	var err error
	p.scratch, err = appendListeningPorts(p.scratch[:0])
	if err != nil {
		return nil, fmt.Errorf("listPorts: %s", err)
	}
	pl := sortAndDedup(p.scratch)
	if pl.sameInodes(p.prev) {
		// Nothing changed, skip inode lookup
		return p.prev, nil
	}
	pl, err = addProcesses(pl)
	if err != nil {
		return nil, fmt.Errorf("addProcesses: %s", err)
	}
	return pl, nil
}

// sortAndDedup sorts ps in place (by Port.lessThan) and then returns
// a subset of it with duplicate (Proto, Port) removed.
func sortAndDedup(ps List) List {
	sort.Slice(ps, func(i, j int) bool {
		return (&ps[i]).lessThan(&ps[j])
	})
	out := ps[:0]
	var last Port
	for _, p := range ps {
		protoPort := Port{Proto: p.Proto, Port: p.Port}
		if last == protoPort {
			continue
		}
		out = append(out, p)
		last = protoPort
	}
	return out
}