package routetable_test

import (
	"fmt"
	"math/rand/v2"
	"net/netip"
	"runtime"
	"testing"
	"time"

	"git.eeqj.de/sneak/routewatch/internal/routetable"
)

// Parameters for BenchmarkApplyMixedBatch.
const (
	// benchPeers is how many peers there are, about as many as RIS has.
	benchPeers = 1400
	// benchPrefixes is how many IPv4 /24 prefixes the table holds, and
	// benchPeersPerPrefix how many peers, picked at random, carry each.
	benchPrefixes       = 100000
	benchPeersPerPrefix = 20
	// benchBatchSize is how many updates the timed batch holds, as many as
	// the prefix handler's largest batch (prefixBatchMaxSize in
	// internal/routewatch).
	benchBatchSize = 250000
	// In the timed batch every benchWithdrawalEvery-th update is a
	// withdrawal, and every benchNewOriginEvery-th announcement has an
	// origin AS other than the prefix's.
	benchWithdrawalEvery = 5
	benchNewOriginEvery  = 7
	// benchOrigins is how many origin ASes the prefixes are spread over,
	// from benchFirstASN on: 20 prefixes each, about as many as an origin AS
	// has on average in the full table.
	benchOrigins  = 5000
	benchFirstASN = 64500
	// benchUpdatesPerSecond is how many updates share one time.
	benchUpdatesPerSecond = 1000
	benchMaskLength       = 24
)

// benchUpdate is one update of the benchmark.
type benchUpdate struct {
	prefix   netip.Prefix
	origin   int
	route    routetable.Route
	withdraw bool
}

// BenchmarkApplyMixedBatch times one batch of 250,000 updates of routes picked
// at random: announcements, withdrawals and announcements with a new origin AS.
// The table it applies them to holds 100,000 IPv4 prefixes from 5,000 origin
// ASes, each carried by 20 of 1,400 peers. It reports the heap bytes that table
// takes per prefix and origin AS pair. It is run by hand, not by make test, for
// example in the test image:
//
//	go test -run '^$' -bench MixedBatch -benchtime 10x ./internal/routetable/
func BenchmarkApplyMixedBatch(b *testing.B) {
	//nolint:gosec // The benchmark needs the same updates every run.
	rng := rand.New(rand.NewPCG(1, 2))
	peers := make([]string, benchPeers)

	for i := range peers {
		peers[i] = fmt.Sprintf("198.18.%d.%d", i/256, i%256)
	}

	before := heapBytes()
	table := routetable.New(routetable.Params{})
	updates := 0

	for prefix := range benchPrefixes {
		for range benchPeersPerPrefix {
			peer := peers[rng.IntN(benchPeers)]
			apply(table, benchAnnouncement(rng, prefix, peer, updates, false))
			updates++
		}
	}

	// The record of changes is handed over every second while routewatch
	// runs, so it is not part of the table's size.
	table.TakeChanges()

	bytesPerPair := float64(heapBytes()-before) / benchPrefixes

	batch := make([]benchUpdate, 0, benchBatchSize)
	for i := range benchBatchSize {
		update := benchAnnouncement(rng, rng.IntN(benchPrefixes),
			peers[rng.IntN(benchPeers)], updates+i, i%benchNewOriginEvery == 0)
		update.withdraw = i%benchWithdrawalEvery == 0
		batch = append(batch, update)
	}

	for b.Loop() {
		for _, update := range batch {
			apply(table, update)
		}
	}

	// After the loop: b.Loop drops the metrics reported before it starts.
	b.ReportMetric(bytesPerPair, "bytes/pair")
}

// benchAnnouncement returns the announcement of the prefix numbered prefix, a
// /24 from 1.0.0.0/24 on, by peer. It is the updates-th update of the
// benchmark, and its AS path holds four ASes from the peer's AS to the origin
// AS. newOrigin gives it an origin AS other than the prefix's.
func benchAnnouncement(
	rng *rand.Rand, prefix int, peer string, updates int, newOrigin bool,
) benchUpdate {
	origin := benchFirstASN + prefix%benchOrigins
	if newOrigin {
		origin += benchOrigins
	}

	start := time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC)

	return benchUpdate{
		prefix: netip.MustParsePrefix(fmt.Sprintf("%d.%d.%d.0/%d",
			1+prefix>>16, (prefix>>8)%256, prefix%256, benchMaskLength)),
		origin: origin,
		route: routetable.Route{
			Peer: peer,
			ASPath: []int{
				benchFirstASN + rng.IntN(benchOrigins),
				benchFirstASN + rng.IntN(benchOrigins),
				benchFirstASN + rng.IntN(benchOrigins),
				origin,
			},
			Time: start.Add(time.Duration(updates/benchUpdatesPerSecond) *
				time.Second),
		},
	}
}

// apply applies update to table.
func apply(table *routetable.Table, update benchUpdate) {
	if update.withdraw {
		table.Withdraw(update.prefix, update.route.Peer)

		return
	}

	table.Announce(update.prefix, update.origin, update.route)
}

// heapBytes returns the bytes of the live heap objects, after a collection.
func heapBytes() uint64 {
	runtime.GC()

	var stats runtime.MemStats

	runtime.ReadMemStats(&stats)

	return stats.HeapAlloc
}