package dhcpd

import (
	"net/netip"
	"time"

	"steward/internal/model"
)

// neverUsed picks a pool address that was never handed out (or whose
// lease was deleted), in the subnet's order (s.mu held): the pools listed
// or at random, and inside a pool sequential, reverse or random
// (docs/subnets.md, "Address order").
func (s *Server) neverUsed(cfg *config, own map[netip.Addr]bool, sub *subnet, cl client, now time.Time) (netip.Addr, bool) {
	ok := func(ip netip.Addr) bool { return s.leases[ip] == nil && s.usable(cfg, own, sub, ip, cl, now) }
	order := func(p model.Pool) string {
		switch {
		case p.Order != "":
			return p.Order
		case sub.AddressOrder != "":
			return sub.AddressOrder
		}
		return cfg.settings.EffectiveAddressOrder()
	}
	if sub.PoolOrder != model.PoolsRandom {
		for _, p := range sub.Pools {
			if ip, found := s.pickIn(p, order(p), ok); found {
				return ip, true
			}
		}
		return netip.Addr{}, false
	}
	// A pool at random, weighted by its never-used addresses.
	counts := make([]int, len(sub.Pools))
	total := 0
	for i, p := range sub.Pools {
		counts[i] = countIn(p, ok)
		total += counts[i]
	}
	if total == 0 {
		return netip.Addr{}, false
	}
	n := s.randN(total)
	for i, p := range sub.Pools {
		if n < counts[i] {
			return s.pickIn(p, order(p), ok)
		}
		n -= counts[i]
	}
	return netip.Addr{}, false
}

// pickIn picks an address of the pool that ok accepts, in an order.
func (s *Server) pickIn(p model.Pool, order string, ok func(netip.Addr) bool) (netip.Addr, bool) {
	var found netip.Addr
	switch order {
	case model.OrderRandom:
		n := countIn(p, ok)
		if n == 0 {
			return netip.Addr{}, false
		}
		k := s.randN(n)
		eachIn(p, false, func(ip netip.Addr) bool {
			if !ok(ip) {
				return false
			}
			if k == 0 {
				found = ip
				return true
			}
			k--
			return false
		})
	default:
		eachIn(p, order == model.OrderReverse, func(ip netip.Addr) bool {
			if ok(ip) {
				found = ip
				return true
			}
			return false
		})
	}
	return found, found.IsValid()
}

// countIn counts the pool's addresses that ok accepts.
func countIn(p model.Pool, ok func(netip.Addr) bool) int {
	n := 0
	eachIn(p, false, func(ip netip.Addr) bool {
		if ok(ip) {
			n++
		}
		return false
	})
	return n
}

// eachIn calls fn for the pool's addresses, lowest first (highest first
// when reverse), until it returns true.
func eachIn(p model.Pool, reverse bool, fn func(netip.Addr) bool) {
	if !p.Start.Is4() || !p.End.Is4() || p.End.Less(p.Start) {
		return
	}
	lo, hi := model.U32(p.Start), model.U32(p.End)
	if reverse {
		for v := hi; ; v-- {
			if fn(model.FromU32(v)) || v == lo {
				return
			}
		}
	}
	for v := lo; ; v++ {
		if fn(model.FromU32(v)) || v == hi {
			return
		}
	}
}