package portallocator

import (
	"bufio"
	"errors"
	"fmt"
	"net"
	"os"
	"sync"
)

const (
	// DefaultPortRangeStart indicates the first port in port range
	DefaultPortRangeStart = 49153
	// DefaultPortRangeEnd indicates the last port in port range
	DefaultPortRangeEnd = 65535
)

type ipMapping map[string]protoMap

var (
	// ErrAllPortsAllocated is returned when no more ports are available
	ErrAllPortsAllocated = errors.New("all ports are allocated")
	// ErrUnknownProtocol is returned when an unknown protocol was specified
	ErrUnknownProtocol = errors.New("unknown protocol")
	defaultIP          = net.ParseIP("0.0.0.0")
	once               sync.Once
	instance           *PortAllocator
	createInstance     = func() { instance = newInstance() }
)

// ErrPortAlreadyAllocated is the returned error information when a requested port is already being used
type ErrPortAlreadyAllocated struct {
	ip   string
	port int
}

func newErrPortAlreadyAllocated(ip string, port int) ErrPortAlreadyAllocated {
	return ErrPortAlreadyAllocated{
		ip:   ip,
		port: port,
	}
}

// IP returns the address to which the used port is associated
func (e ErrPortAlreadyAllocated) IP() string {
	return e.ip
}

// Port returns the value of the already used port
func (e ErrPortAlreadyAllocated) Port() int {
	return e.port
}

// IPPort returns the address and the port in the form ip:port
func (e ErrPortAlreadyAllocated) IPPort() string {
	return fmt.Sprintf("%s:%d", e.ip, e.port)
}

// Error is the implementation of error.Error interface
func (e ErrPortAlreadyAllocated) Error() string {
	return fmt.Sprintf("Bind for %s:%d failed: port is already allocated", e.ip, e.port)
}

type (
	// PortAllocator manages the transport ports database
	PortAllocator struct {
		mutex sync.Mutex
		ipMap ipMapping
		Begin int
		End   int
	}
	portMap struct {
		p          map[int]struct{}
		begin, end int
		last       int
	}
	protoMap map[string]*portMap
)

// Get returns the default instance of PortAllocator
func Get() *PortAllocator {
	// Port Allocator is a singleton
	// Note: Long term solution will be each PortAllocator will have access to
	// the OS so that it can have up to date view of the OS port allocation.
	// When this happens singleton behavior will be removed. Clients do not
	// need to worry about this, they will not see a change in behavior.
	once.Do(createInstance)
	return instance
}

func newInstance() *PortAllocator {
	start, end, err := getDynamicPortRange()
	if err != nil {
		start, end = DefaultPortRangeStart, DefaultPortRangeEnd
	}
	return &PortAllocator{
		ipMap: ipMapping{},
		Begin: start,
		End:   end,
	}
}

func getDynamicPortRange() (start int, end int, err error) {
	const portRangeKernelParam = "/proc/sys/net/ipv4/ip_local_port_range"
	portRangeFallback := fmt.Sprintf("using fallback port range %d-%d", DefaultPortRangeStart, DefaultPortRangeEnd)
	file, err := os.Open(portRangeKernelParam)
	if err != nil {
		return 0, 0, fmt.Errorf("port allocator - %s due to error: %v", portRangeFallback, err)
	}
	n, err := fmt.Fscanf(bufio.NewReader(file), "%d\t%d", &start, &end)
	if n != 2 || err != nil {
		if err == nil {
			err = fmt.Errorf("unexpected count of parsed numbers (%d)", n)
		}
		return 0, 0, fmt.Errorf("port allocator - failed to parse system ephemeral port range from %s - %s: %v", portRangeKernelParam, portRangeFallback, err)
	}
	return start, end, nil
}

// RequestPort requests new port from global ports pool for specified ip and proto.
// If port is 0 it returns first free port. Otherwise it checks port availability
// in pool and return that port or error if port is already busy.
func (p *PortAllocator) RequestPort(ip net.IP, proto string, port int) (int, error) {
	p.mutex.Lock()
	defer p.mutex.Unlock()

	if proto != "tcp" && proto != "udp" {
		return 0, ErrUnknownProtocol
	}

	if ip == nil {
		ip = defaultIP
	}
	ipstr := ip.String()
	protomap, ok := p.ipMap[ipstr]
	if !ok {
		protomap = protoMap{
			"tcp": p.newPortMap(),
			"udp": p.newPortMap(),
		}

		p.ipMap[ipstr] = protomap
	}
	mapping := protomap[proto]
	if port > 0 {
		if _, ok := mapping.p[port]; !ok {
			mapping.p[port] = struct{}{}
			return port, nil
		}
		return 0, newErrPortAlreadyAllocated(ipstr, port)
	}

	port, err := mapping.findPort()
	if err != nil {
		return 0, err
	}
	return port, nil
}

// ReleasePort releases port from global ports pool for specified ip and proto.
func (p *PortAllocator) ReleasePort(ip net.IP, proto string, port int) error {
	p.mutex.Lock()
	defer p.mutex.Unlock()

	if ip == nil {
		ip = defaultIP
	}
	protomap, ok := p.ipMap[ip.String()]
	if !ok {
		return nil
	}
	delete(protomap[proto].p, port)
	return nil
}

func (p *PortAllocator) newPortMap() *portMap {
	return &portMap{
		p:     map[int]struct{}{},
		begin: p.Begin,
		end:   p.End,
		last:  p.End,
	}
}

// ReleaseAll releases all ports for all ips.
func (p *PortAllocator) ReleaseAll() error {
	p.mutex.Lock()
	p.ipMap = ipMapping{}
	p.mutex.Unlock()
	return nil
}

func (pm *portMap) findPort() (int, error) {
	port := pm.last
	for i := 0; i <= pm.end-pm.begin; i++ {
		port++
		if port > pm.end {
			port = pm.begin
		}

		if _, ok := pm.p[port]; !ok {
			pm.p[port] = struct{}{}
			pm.last = port
			return port, nil
		}
	}
	return 0, ErrAllPortsAllocated
}