/*
MIT License
Copyright © 2016 <dev@jpillora.com>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

package server

import (
	"log"
	"net"
	"net/http"
	"os"
	"sync"

	"github.com/tomasen/realip"
)

// IPFilterOptions for ipFilter. Allowed takes precedence over Blocked.
// IPs can be IPv4 or IPv6 and can optionally contain subnet
// masks (/24). Note however, determining if a given IP is
// included in a subnet requires a linear scan so is less performant
// than looking up single IPs.
//
// This could be improved with some algorithmic magic.
type IPFilterOptions struct {
	//explicity allowed IPs
	AllowedIPs []string
	//explicity blocked IPs
	BlockedIPs []string
	//block by default (defaults to allow)
	BlockByDefault bool
	// TrustProxy enable check request IP from proxy
	TrustProxy bool

	Logger interface {
		Printf(format string, v ...interface{})
	}
}

// ipFilter
type ipFilter struct {
	//mut protects the below
	//rw since writes are rare
	mut            sync.RWMutex
	defaultAllowed bool
	ips            map[string]bool
	subnets        []*subnet
}

type subnet struct {
	str     string
	ipnet   *net.IPNet
	allowed bool
}

func newIPFilter(opts *IPFilterOptions) *ipFilter {
	if opts.Logger == nil {
		flags := log.LstdFlags
		opts.Logger = log.New(os.Stdout, "", flags)
	}
	f := &ipFilter{
		ips:            map[string]bool{},
		defaultAllowed: !opts.BlockByDefault,
	}
	for _, ip := range opts.BlockedIPs {
		f.BlockIP(ip)
	}
	for _, ip := range opts.AllowedIPs {
		f.AllowIP(ip)
	}
	return f
}

func (f *ipFilter) AllowIP(ip string) bool {
	return f.ToggleIP(ip, true)
}

func (f *ipFilter) BlockIP(ip string) bool {
	return f.ToggleIP(ip, false)
}

func (f *ipFilter) ToggleIP(str string, allowed bool) bool {
	//check if provided string describes a subnet
	if ip, network, err := net.ParseCIDR(str); err == nil {
		// containing only one ip?
		if n, total := network.Mask.Size(); n == total {
			f.mut.Lock()
			f.ips[ip.String()] = allowed
			f.mut.Unlock()
			return true
		}
		//check for existing
		f.mut.Lock()
		found := false
		for _, subnet := range f.subnets {
			if subnet.str == str {
				found = true
				subnet.allowed = allowed
				break
			}
		}
		if !found {
			f.subnets = append(f.subnets, &subnet{
				str:     str,
				ipnet:   network,
				allowed: allowed,
			})
		}
		f.mut.Unlock()
		return true
	}
	//check if plain ip
	if ip := net.ParseIP(str); ip != nil {
		f.mut.Lock()
		f.ips[ip.String()] = allowed
		f.mut.Unlock()
		return true
	}
	return false
}

// ToggleDefault alters the default setting
func (f *ipFilter) ToggleDefault(allowed bool) {
	f.mut.Lock()
	f.defaultAllowed = allowed
	f.mut.Unlock()
}

// Allowed returns if a given IP can pass through the filter
func (f *ipFilter) Allowed(ipstr string) bool {
	return f.NetAllowed(net.ParseIP(ipstr))
}

// NetAllowed returns if a given net.IP can pass through the filter
func (f *ipFilter) NetAllowed(ip net.IP) bool {
	//invalid ip
	if ip == nil {
		return false
	}
	//read lock entire function
	//except for db access
	f.mut.RLock()
	defer f.mut.RUnlock()
	//check single ips
	allowed, ok := f.ips[ip.String()]
	if ok {
		return allowed
	}
	//scan subnets for any allow/block
	blocked := false
	for _, subnet := range f.subnets {
		if subnet.ipnet.Contains(ip) {
			if subnet.allowed {
				return true
			}
			blocked = true
		}
	}
	if blocked {
		return false
	}

	//use default setting
	return f.defaultAllowed
}

// Blocked returns if a given IP can NOT pass through the filter
func (f *ipFilter) Blocked(ip string) bool {
	return !f.Allowed(ip)
}

// NetBlocked returns if a given net.IP can NOT pass through the filter
func (f *ipFilter) NetBlocked(ip net.IP) bool {
	return !f.NetAllowed(ip)
}

// Wrap the provided handler with simple IP blocking middleware
// using this IP filter and its configuration
func (f *ipFilter) Wrap(next http.Handler) http.Handler {
	return &ipFilterMiddleware{ipFilter: f, next: next}
}

// WrapIPFilter is equivalent to newIPFilter(opts) then Wrap(next)
func WrapIPFilter(next http.Handler, opts *IPFilterOptions) http.Handler {
	return newIPFilter(opts).Wrap(next)
}

type ipFilterMiddleware struct {
	*ipFilter
	next http.Handler
}

func (m *ipFilterMiddleware) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	remoteIP := realip.FromRequest(r)

	if !m.ipFilter.Allowed(remoteIP) {
		//show simple forbidden text
		http.Error(w, http.StatusText(http.StatusForbidden), http.StatusForbidden)
		return
	}

	//success!
	m.next.ServeHTTP(w, r)
}