package ipcohort

import (
	"encoding/binary"
	"encoding/csv"
	"fmt"
	"io"
	"log"
	"net/netip"
	"os"
	"slices"
	"sort"
	"strings"
)

// Either a subnet or single address (subnet with /32 CIDR prefix)
type IPv4Net struct {
	networkBE uint32
	prefix    uint8
	shift     uint8
}

func NewIPv4Net(ip4be uint32, prefix uint8) IPv4Net {
	return IPv4Net{
		networkBE: ip4be,
		prefix:    prefix,
		shift:     32 - prefix,
	}
}

func (r IPv4Net) Contains(ip uint32) bool {
	mask := uint32(0xFFFFFFFF << (r.shift))
	return (ip & mask) == r.networkBE
}

func New() *Cohort {
	cohort := &Cohort{ranges: []IPv4Net{}}
	return cohort
}

func Parse(prefixList []string) (*Cohort, error) {
	var ranges []IPv4Net
	for _, raw := range prefixList {
		ipv4net, err := ParseIPv4(raw)
		if err != nil {
			log.Printf("skipping invalid entry: %q", raw)
			continue
		}
		ranges = append(ranges, ipv4net)
	}

	sizedList := make([]IPv4Net, len(ranges))
	copy(sizedList, ranges)
	sortRanges(ranges)

	cohort := &Cohort{ranges: sizedList}
	return cohort, nil
}

func ParseIPv4(raw string) (ipv4net IPv4Net, err error) {
	var ippre netip.Prefix
	var ip netip.Addr
	if strings.Contains(raw, "/") {
		ippre, err = netip.ParsePrefix(raw)
		if err != nil {
			return ipv4net, err
		}
	} else {
		ip, err = netip.ParseAddr(raw)
		if err != nil {
			return ipv4net, err
		}
		ippre = netip.PrefixFrom(ip, 32)
	}

	ip4 := ippre.Addr().As4()
	prefix := uint8(ippre.Bits()) // 0-32
	return NewIPv4Net(
		binary.BigEndian.Uint32(ip4[:]),
		prefix,
	), nil
}

func LoadFile(path string, unsorted bool) (*Cohort, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, fmt.Errorf("could not load %q: %v", path, err)
	}
	defer f.Close()

	return ParseCSV(f, unsorted)
}

func ParseCSV(f io.Reader, unsorted bool) (*Cohort, error) {
	r := csv.NewReader(f)
	r.FieldsPerRecord = -1

	return ReadAll(r, unsorted)
}

func ReadAll(r *csv.Reader, unsorted bool) (*Cohort, error) {
	var ranges []IPv4Net
	for {
		record, err := r.Read()
		if err == io.EOF {
			break
		}
		if err != nil {
			return nil, fmt.Errorf("csv read error: %w", err)
		}

		if len(record) == 0 {
			continue
		}

		raw := strings.TrimSpace(record[0])

		// Skip comments/empty
		if raw == "" || strings.HasPrefix(raw, "#") {
			continue
		}

		// skip IPv6
		if strings.Contains(raw, ":") {
			continue
		}

		ipv4net, err := ParseIPv4(raw)
		if err != nil {
			log.Printf("skipping invalid entry: %q", raw)
			continue
		}
		ranges = append(ranges, ipv4net)
	}

	if unsorted {
		sortRanges(ranges)
	}

	sizedList := make([]IPv4Net, len(ranges))
	copy(sizedList, ranges)

	cohort := &Cohort{ranges: sizedList}
	return cohort, nil
}

func sortRanges(ranges []IPv4Net) {
	// Sort by network address (required for binary search)
	sort.Slice(ranges, func(i, j int) bool {
		// Note: we could also sort by prefix (largest first)
		return ranges[i].networkBE < ranges[j].networkBE
	})

	// Note: we could also merge ranges here
}

type Cohort struct {
	ranges []IPv4Net
}

func (c *Cohort) Size() int {
	return len(c.ranges)
}

func (c *Cohort) Contains(ipStr string) bool {
	ip, err := netip.ParseAddr(ipStr)
	if err != nil {
		return true
	}
	ip4 := ip.As4()
	ipU32 := binary.BigEndian.Uint32(ip4[:])

	idx, found := slices.BinarySearchFunc(c.ranges, ipU32, func(r IPv4Net, target uint32) int {
		if r.networkBE < target {
			return -1
		}
		if r.networkBE > target {
			return 1
		}
		return 0
	})
	if found {
		return true
	}

	// Check the range immediately before the insertion point
	if idx > 0 {
		if c.ranges[idx-1].Contains(ipU32) {
			return true
		}
	}

	return false
}