// Copyright 2016 Google Inc.  All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package uuid

import (
	"net"
	"sync"
)

var (
	nodeMu     sync.Mutex
	interfaces []net.Interface // cached list of interfaces
	ifname     string          // name of interface being used
	nodeID     [6]byte         // hardware for version 1 UUIDs
	zeroID     [6]byte         // nodeID with only 0's
)

// NodeInterface returns the name of the interface from which the NodeID was
// derived.  The interface "user" is returned if the NodeID was set by
// SetNodeID.
func NodeInterface() string {
	defer nodeMu.Unlock()
	nodeMu.Lock()
	return ifname
}

// SetNodeInterface selects the hardware address to be used for Version 1 UUIDs.
// If name is "" then the first usable interface found will be used or a random
// Node ID will be generated.  If a named interface cannot be found then false
// is returned.
//
// SetNodeInterface never fails when name is "".
func SetNodeInterface(name string) bool {
	defer nodeMu.Unlock()
	nodeMu.Lock()
	return setNodeInterface(name)
}

func setNodeInterface(name string) bool {
	if interfaces == nil {
		var err error
		interfaces, err = net.Interfaces()
		if err != nil && name != "" {
			return false
		}
	}

	for _, ifs := range interfaces {
		if len(ifs.HardwareAddr) >= 6 && (name == "" || name == ifs.Name) {
			copy(nodeID[:], ifs.HardwareAddr)
			ifname = ifs.Name
			return true
		}
	}

	// We found no interfaces with a valid hardware address.  If name
	// does not specify a specific interface generate a random Node ID
	// (section 4.1.6)
	if name == "" {
		randomBits(nodeID[:])
		return true
	}
	return false
}

// NodeID returns a slice of a copy of the current Node ID, setting the Node ID
// if not already set.
func NodeID() []byte {
	defer nodeMu.Unlock()
	nodeMu.Lock()
	if nodeID == zeroID {
		setNodeInterface("")
	}
	nid := nodeID
	return nid[:]
}

// SetNodeID sets the Node ID to be used for Version 1 UUIDs.  The first 6 bytes
// of id are used.  If id is less than 6 bytes then false is returned and the
// Node ID is not set.
func SetNodeID(id []byte) bool {
	if len(id) < 6 {
		return false
	}
	defer nodeMu.Unlock()
	nodeMu.Lock()
	copy(nodeID[:], id)
	ifname = "user"
	return true
}

// NodeID returns the 6 byte node id encoded in uuid.  It returns nil if uuid is
// not valid.  The NodeID is only well defined for version 1 and 2 UUIDs.
func (uuid UUID) NodeID() []byte {
	if len(uuid) != 16 {
		return nil
	}
	var node [6]byte
	copy(node[:], uuid[10:])
	return node[:]
}