package ratelimiter

import (
	"sync"
	"testing"
	"time"
)

// TokenBucket is a thread-safe token-bucket rate limiter.
type TokenBucket struct {
	mu       sync.Mutex
	rate     float64
	capacity float64
	tokens   float64
	last     time.Time
}

// NewTokenBucket creates a limiter that refills rate tokens per second, up to capacity.
func NewTokenBucket(rate float64, capacity int) *TokenBucket {
	if rate <= 0 {
		panic("rate must be positive")
	}
	if capacity <= 0 {
		panic("capacity must be positive")
	}

	now := time.Now()
	return &TokenBucket{
		rate:     rate,
		capacity: float64(capacity),
		tokens:   float64(capacity),
		last:     now,
	}
}

// Allow reports whether one token is available and consumes it if so.
func (b *TokenBucket) Allow() bool {
	return b.AllowN(1)
}

// AllowN reports whether n tokens are available and consumes them if so.
func (b *TokenBucket) AllowN(n int) bool {
	if n <= 0 {
		return true
	}

	b.mu.Lock()
	defer b.mu.Unlock()

	b.refill(time.Now())

	need := float64(n)
	if need > b.tokens {
		return false
	}

	b.tokens -= need
	return true
}

// Tokens returns the current number of available whole tokens.
func (b *TokenBucket) Tokens() int {
	b.mu.Lock()
	defer b.mu.Unlock()

	b.refill(time.Now())
	return int(b.tokens)
}

func (b *TokenBucket) refill(now time.Time) {
	elapsed := now.Sub(b.last).Seconds()
	if elapsed <= 0 {
		return
	}

	b.tokens += elapsed * b.rate
	if b.tokens > b.capacity {
		b.tokens = b.capacity
	}
	b.last = now
}

func TestTokenBucketConsumesCapacity(t *testing.T) {
	b := NewTokenBucket(10, 2)

	if !b.Allow() {
		t.Fatal("expected first token to be allowed")
	}
	if !b.Allow() {
		t.Fatal("expected second token to be allowed")
	}
	if b.Allow() {
		t.Fatal("expected empty bucket to reject request")
	}
}

func TestTokenBucketRefills(t *testing.T) {
	b := NewTokenBucket(100, 1)

	if !b.Allow() {
		t.Fatal("expected initial token to be allowed")
	}
	if b.Allow() {
		t.Fatal("expected empty bucket to reject request")
	}

	time.Sleep(15 * time.Millisecond)

	if !b.Allow() {
		t.Fatal("expected bucket to refill")
	}
}