// Package ratelimit counts each client's requests over a minute, an hour
// and a day, as the "Counting method" section of SPEC.md describes, and
// tells when a request takes a client over a rate limit. The counts are
// kept in memory only, for at most 20,000 clients.
package ratelimit

import (
	"net/netip"
	"sync"
	"time"

	"github.com/hashicorp/golang-lru/v2/simplelru"
)

// maxClients is how many clients are kept. Past it, the least recently
// seen client is dropped, and starts afresh if it comes back.
const maxClients = 20000

const day = 24 * time.Hour

// Limits are the most requests a client may make in a minute, an hour and
// a day. Zero is no limit.
type Limits struct {
	PerMinute int64
	PerHour   int64
	PerDay    int64
}

// Limiter counts each client's requests against the limits. It is safe
// for concurrent use.
type Limiter struct {
	windows [3]window

	mu sync.Mutex
	// clients holds each client's buckets, one pair for each of windows,
	// in the same order.
	clients *simplelru.LRU[netip.Prefix, *[3]buckets]
}

// New returns a Limiter for limits, with no client counted yet.
func New(limits Limits) *Limiter {
	clients, err := simplelru.NewLRU[netip.Prefix, *[3]buckets](maxClients, nil)
	if err != nil {
		panic(err) // NewLRU fails only for a size below one
	}

	return &Limiter{
		windows: [3]window{
			{name: "minute", length: time.Minute, limit: limits.PerMinute},
			{name: "hour", length: time.Hour, limit: limits.PerHour},
			{name: "day", length: day, limit: limits.PerDay},
		},
		clients: clients,
	}
}

// Count counts a request from client at now, in every window, whether or
// not it is refused. It returns the window whose limit the request takes
// the client over, "minute", "hour" or "day", the shortest if it is over
// several, or "" if it is within every limit.
func (l *Limiter) Count(client netip.Prefix, now time.Time) string {
	l.mu.Lock()
	defer l.mu.Unlock()

	counts, seen := l.clients.Get(client)
	if !seen {
		counts = &[3]buckets{}
		l.clients.Add(client, counts)
	}

	limitHit := ""

	for i, w := range l.windows {
		requests := counts[i].add(now, w.length)
		if limitHit == "" && w.limit > 0 && requests > float64(w.limit) {
			limitHit = w.name
		}
	}

	return limitHit
}

// window is a length of time over which requests are counted, and the
// most requests a client may make in it.
type window struct {
	name   string
	length time.Duration
	limit  int64
}

// buckets are a client's two buckets in one window: the requests in the
// bucket under way, which began at start, and in the bucket before it.
type buckets struct {
	start    time.Time
	current  int64
	previous int64
}

// add counts a request at now in a window of length, and returns the
// client's requests in the window that ends at now: those in the bucket
// under way, and those in the bucket before it weighted by how much of
// that bucket the window still covers.
//
// Concurrent requests can be counted out of order, so now can be before
// the bucket under way began; such a request is counted in that bucket.
func (b *buckets) add(now time.Time, length time.Duration) float64 {
	start := now.Truncate(length)
	if start.After(b.start) {
		if start.Equal(b.start.Add(length)) {
			b.previous = b.current
		} else {
			b.previous = 0
		}

		b.start = start
		b.current = 0
	}

	b.current++

	elapsed := max(now.Sub(b.start), 0)
	covered := 1 - float64(elapsed)/float64(length)

	return float64(b.previous)*covered + float64(b.current)
}