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// Copyright 2018 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package netutil

import (
    "time"

    "github.com/ethereum/go-ethereum/common/mclock"
)

// IPTracker predicts the external endpoint, i.e. IP address and port, of the local host
// based on statements made by other hosts.
type IPTracker struct {
    window          time.Duration
    contactWindow   time.Duration
    minStatements   int
    clock           mclock.Clock
    statements      map[string]ipStatement
    contact         map[string]mclock.AbsTime
    lastStatementGC mclock.AbsTime
    lastContactGC   mclock.AbsTime
}

type ipStatement struct {
    endpoint string
    time     mclock.AbsTime
}

// NewIPTracker creates an IP tracker.
//
// The window parameters configure the amount of past network events which are kept. The
// minStatements parameter enforces a minimum number of statements which must be recorded
// before any prediction is made. Higher values for these parameters decrease 'flapping' of
// predictions as network conditions change. Window duration values should typically be in
// the range of minutes.
func NewIPTracker(window, contactWindow time.Duration, minStatements int) *IPTracker {
    return &IPTracker{
        window:        window,
        contactWindow: contactWindow,
        statements:    make(map[string]ipStatement),
        minStatements: minStatements,
        contact:       make(map[string]mclock.AbsTime),
        clock:         mclock.System{},
    }
}

// PredictFullConeNAT checks whether the local host is behind full cone NAT. It predicts by
// checking whether any statement has been received from a node we didn't contact before
// the statement was made.
func (it *IPTracker) PredictFullConeNAT() bool {
    now := it.clock.Now()
    it.gcContact(now)
    it.gcStatements(now)
    for host, st := range it.statements {
        if c, ok := it.contact[host]; !ok || c > st.time {
            return true
        }
    }
    return false
}

// PredictEndpoint returns the current prediction of the external endpoint.
func (it *IPTracker) PredictEndpoint() string {
    it.gcStatements(it.clock.Now())

    // The current strategy is simple: find the endpoint with most statements.
    counts := make(map[string]int)
    maxcount, max := 0, ""
    for _, s := range it.statements {
        c := counts[s.endpoint] + 1
        counts[s.endpoint] = c
        if c > maxcount && c >= it.minStatements {
            maxcount, max = c, s.endpoint
        }
    }
    return max
}

// AddStatement records that a certain host thinks our external endpoint is the one given.
func (it *IPTracker) AddStatement(host, endpoint string) {
    now := it.clock.Now()
    it.statements[host] = ipStatement{endpoint, now}
    if time.Duration(now-it.lastStatementGC) >= it.window {
        it.gcStatements(now)
    }
}

// AddContact records that a packet containing our endpoint information has been sent to a
// certain host.
func (it *IPTracker) AddContact(host string) {
    now := it.clock.Now()
    it.contact[host] = now
    if time.Duration(now-it.lastContactGC) >= it.contactWindow {
        it.gcContact(now)
    }
}

func (it *IPTracker) gcStatements(now mclock.AbsTime) {
    it.lastStatementGC = now
    cutoff := now.Add(-it.window)
    for host, s := range it.statements {
        if s.time < cutoff {
            delete(it.statements, host)
        }
    }
}

func (it *IPTracker) gcContact(now mclock.AbsTime) {
    it.lastContactGC = now
    cutoff := now.Add(-it.contactWindow)
    for host, ct := range it.contact {
        if ct < cutoff {
            delete(it.contact, host)
        }
    }
}