/*
Copyright 2016 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package cidrset

import (
	"encoding/binary"
	"errors"
	"fmt"
	"math/big"
	"math/bits"
	"net"
	"sync"
)

// CidrSet manages a set of CIDR ranges from which blocks of IPs can
// be allocated from.
type CidrSet struct {
	sync.Mutex
	// clusterCIDR is the CIDR assigned to the cluster
	clusterCIDR *net.IPNet
	// clusterMaskSize is the mask size, in bits, assigned to the cluster
	// caches the mask size to avoid the penalty of calling clusterCIDR.Mask.Size()
	clusterMaskSize int
	// nodeMask is the network mask assigned to the nodes
	nodeMask net.IPMask
	// nodeMaskSize is the mask size, in bits,assigned to the nodes
	// caches the mask size to avoid the penalty of calling nodeMask.Size()
	nodeMaskSize int
	// maxCIDRs is the maximum number of CIDRs that can be allocated
	maxCIDRs int
	// allocatedCIDRs counts the number of CIDRs allocated
	allocatedCIDRs int
	// nextCandidate points to the next CIDR that should be free
	nextCandidate int
	// used is a bitmap used to track the CIDRs allocated
	used big.Int
	// label is used to identify the metrics
	label string
}

const (
	// The subnet mask size cannot be greater than 16 more than the cluster mask size
	// TODO: https://github.com/kubernetes/kubernetes/issues/44918
	// clusterSubnetMaxDiff limited to 16 due to the uncompressed bitmap
	// Due to this limitation the subnet mask for IPv6 cluster cidr needs to be >= 48
	// as default mask size for IPv6 is 64.
	clusterSubnetMaxDiff = 16
	// halfIPv6Len is the half of the IPv6 length
	halfIPv6Len = net.IPv6len / 2
)

var (
	// ErrCIDRRangeNoCIDRsRemaining occurs when there is no more space
	// to allocate CIDR ranges.
	ErrCIDRRangeNoCIDRsRemaining = errors.New(
		"CIDR allocation failed; there are no remaining CIDRs left to allocate in the accepted range")
	// ErrCIDRSetSubNetTooBig occurs when the subnet mask size is too
	// big compared to the CIDR mask size.
	ErrCIDRSetSubNetTooBig = errors.New(
		"New CIDR set failed; the node CIDR size is too big")
)

// NewCIDRSet creates a new CidrSet.
func NewCIDRSet(clusterCIDR *net.IPNet, subNetMaskSize int) (*CidrSet, error) {
	clusterMask := clusterCIDR.Mask
	clusterMaskSize, bits := clusterMask.Size()

	if (clusterCIDR.IP.To4() == nil) && (subNetMaskSize-clusterMaskSize > clusterSubnetMaxDiff) {
		return nil, ErrCIDRSetSubNetTooBig
	}

	// register CidrSet metrics
	registerCidrsetMetrics()

	maxCIDRs := getMaxCIDRs(subNetMaskSize, clusterMaskSize)
	cidrSet := &CidrSet{
		clusterCIDR:     clusterCIDR,
		nodeMask:        net.CIDRMask(subNetMaskSize, bits),
		clusterMaskSize: clusterMaskSize,
		maxCIDRs:        maxCIDRs,
		nodeMaskSize:    subNetMaskSize,
		label:           clusterCIDR.String(),
	}
	cidrSetMaxCidrs.WithLabelValues(cidrSet.label).Set(float64(maxCIDRs))

	return cidrSet, nil
}

func (s *CidrSet) indexToCIDRBlock(index int) *net.IPNet {
	var ip []byte
	switch /*v4 or v6*/ {
	case s.clusterCIDR.IP.To4() != nil:
		{
			j := uint32(index) << uint32(32-s.nodeMaskSize)
			ipInt := (binary.BigEndian.Uint32(s.clusterCIDR.IP)) | j
			ip = make([]byte, net.IPv4len)
			binary.BigEndian.PutUint32(ip, ipInt)
		}
	case s.clusterCIDR.IP.To16() != nil:
		{
			// leftClusterIP      |     rightClusterIP
			// 2001:0DB8:1234:0000:0000:0000:0000:0000
			const v6NBits = 128
			const halfV6NBits = v6NBits / 2
			leftClusterIP := binary.BigEndian.Uint64(s.clusterCIDR.IP[:halfIPv6Len])
			rightClusterIP := binary.BigEndian.Uint64(s.clusterCIDR.IP[halfIPv6Len:])

			ip = make([]byte, net.IPv6len)

			if s.nodeMaskSize <= halfV6NBits {
				// We only care about left side IP
				leftClusterIP |= uint64(index) << uint(halfV6NBits-s.nodeMaskSize)
			} else {
				if s.clusterMaskSize < halfV6NBits {
					// see how many bits are needed to reach the left side
					btl := uint(s.nodeMaskSize - halfV6NBits)
					indexMaxBit := uint(64 - bits.LeadingZeros64(uint64(index)))
					if indexMaxBit > btl {
						leftClusterIP |= uint64(index) >> btl
					}
				}
				// the right side will be calculated the same way either the
				// subNetMaskSize affects both left and right sides
				rightClusterIP |= uint64(index) << uint(v6NBits-s.nodeMaskSize)
			}
			binary.BigEndian.PutUint64(ip[:halfIPv6Len], leftClusterIP)
			binary.BigEndian.PutUint64(ip[halfIPv6Len:], rightClusterIP)
		}
	}
	return &net.IPNet{
		IP:   ip,
		Mask: s.nodeMask,
	}
}

// AllocateNext allocates the next free CIDR range. This will set the range
// as occupied and return the allocated range.
func (s *CidrSet) AllocateNext() (*net.IPNet, error) {
	s.Lock()
	defer s.Unlock()

	if s.allocatedCIDRs == s.maxCIDRs {
		return nil, ErrCIDRRangeNoCIDRsRemaining
	}
	candidate := s.nextCandidate
	var i int
	for i = 0; i < s.maxCIDRs; i++ {
		if s.used.Bit(candidate) == 0 {
			break
		}
		candidate = (candidate + 1) % s.maxCIDRs
	}

	s.nextCandidate = (candidate + 1) % s.maxCIDRs
	s.used.SetBit(&s.used, candidate, 1)
	s.allocatedCIDRs++
	// Update metrics
	cidrSetAllocations.WithLabelValues(s.label).Inc()
	cidrSetAllocationTriesPerRequest.WithLabelValues(s.label).Observe(float64(i))
	cidrSetUsage.WithLabelValues(s.label).Set(float64(s.allocatedCIDRs) / float64(s.maxCIDRs))

	return s.indexToCIDRBlock(candidate), nil
}

func (s *CidrSet) getBeginningAndEndIndices(cidr *net.IPNet) (begin, end int, err error) {
	if cidr == nil {
		return -1, -1, fmt.Errorf("error getting indices for cluster cidr %v, cidr is nil", s.clusterCIDR)
	}
	begin, end = 0, s.maxCIDRs-1
	cidrMask := cidr.Mask
	maskSize, _ := cidrMask.Size()
	var ipSize int

	if !s.clusterCIDR.Contains(cidr.IP.Mask(s.clusterCIDR.Mask)) && !cidr.Contains(s.clusterCIDR.IP.Mask(cidr.Mask)) {
		return -1, -1, fmt.Errorf("cidr %v is out the range of cluster cidr %v", cidr, s.clusterCIDR)
	}

	if s.clusterMaskSize < maskSize {

		ipSize = net.IPv4len
		if cidr.IP.To4() == nil {
			ipSize = net.IPv6len
		}
		begin, err = s.getIndexForIP(cidr.IP.Mask(s.nodeMask))
		if err != nil {
			return -1, -1, err
		}
		ip := make([]byte, ipSize)
		if cidr.IP.To4() != nil {
			ipInt := binary.BigEndian.Uint32(cidr.IP) | (^binary.BigEndian.Uint32(cidr.Mask))
			binary.BigEndian.PutUint32(ip, ipInt)
		} else {
			// ipIntLeft          |         ipIntRight
			// 2001:0DB8:1234:0000:0000:0000:0000:0000
			ipIntLeft := binary.BigEndian.Uint64(cidr.IP[:net.IPv6len/2]) | (^binary.BigEndian.Uint64(cidr.Mask[:net.IPv6len/2]))
			ipIntRight := binary.BigEndian.Uint64(cidr.IP[net.IPv6len/2:]) | (^binary.BigEndian.Uint64(cidr.Mask[net.IPv6len/2:]))
			binary.BigEndian.PutUint64(ip[:net.IPv6len/2], ipIntLeft)
			binary.BigEndian.PutUint64(ip[net.IPv6len/2:], ipIntRight)
		}
		end, err = s.getIndexForIP(net.IP(ip).Mask(s.nodeMask))
		if err != nil {
			return -1, -1, err
		}
	}
	return begin, end, nil
}

// Release releases the given CIDR range.
func (s *CidrSet) Release(cidr *net.IPNet) error {
	begin, end, err := s.getBeginningAndEndIndices(cidr)
	if err != nil {
		return err
	}
	s.Lock()
	defer s.Unlock()
	for i := begin; i <= end; i++ {
		// Only change the counters if we change the bit to prevent
		// double counting.
		if s.used.Bit(i) != 0 {
			s.used.SetBit(&s.used, i, 0)
			s.allocatedCIDRs--
			cidrSetReleases.WithLabelValues(s.label).Inc()
		}
	}

	cidrSetUsage.WithLabelValues(s.label).Set(float64(s.allocatedCIDRs) / float64(s.maxCIDRs))
	return nil
}

// Occupy marks the given CIDR range as used. Occupy succeeds even if the CIDR
// range was previously used.
func (s *CidrSet) Occupy(cidr *net.IPNet) (err error) {
	begin, end, err := s.getBeginningAndEndIndices(cidr)
	if err != nil {
		return err
	}
	s.Lock()
	defer s.Unlock()
	for i := begin; i <= end; i++ {
		// Only change the counters if we change the bit to prevent
		// double counting.
		if s.used.Bit(i) == 0 {
			s.used.SetBit(&s.used, i, 1)
			s.allocatedCIDRs++
			cidrSetAllocations.WithLabelValues(s.label).Inc()
		}
	}

	cidrSetUsage.WithLabelValues(s.label).Set(float64(s.allocatedCIDRs) / float64(s.maxCIDRs))
	return nil
}

func (s *CidrSet) getIndexForIP(ip net.IP) (int, error) {
	if ip.To4() != nil {
		cidrIndex := (binary.BigEndian.Uint32(s.clusterCIDR.IP) ^ binary.BigEndian.Uint32(ip.To4())) >> uint32(32-s.nodeMaskSize)
		if cidrIndex >= uint32(s.maxCIDRs) {
			return 0, fmt.Errorf("CIDR: %v/%v is out of the range of CIDR allocator", ip, s.nodeMaskSize)
		}
		return int(cidrIndex), nil
	}
	if ip.To16() != nil {
		bigIP := big.NewInt(0).SetBytes(s.clusterCIDR.IP)
		bigIP = bigIP.Xor(bigIP, big.NewInt(0).SetBytes(ip))
		cidrIndexBig := bigIP.Rsh(bigIP, uint(net.IPv6len*8-s.nodeMaskSize))
		cidrIndex := cidrIndexBig.Uint64()
		if cidrIndex >= uint64(s.maxCIDRs) {
			return 0, fmt.Errorf("CIDR: %v/%v is out of the range of CIDR allocator", ip, s.nodeMaskSize)
		}
		return int(cidrIndex), nil
	}

	return 0, fmt.Errorf("invalid IP: %v", ip)
}

// getMaxCIDRs returns the max number of CIDRs that can be obtained by subdividing a mask of size `clusterMaskSize`
// into subnets with mask of size `subNetMaskSize`.
func getMaxCIDRs(subNetMaskSize, clusterMaskSize int) int {
	return 1 << uint32(subNetMaskSize-clusterMaskSize)
}