package nixmsg

import (
	"math/rand"
	"sync"
	"time"
)

// reconnectBackoff 按 DEVELOPMENT 第 9 节：1s 起、加倍、上限 30s、±30% 抖动；
// 稳定在线 60s 后恢复到 1s。
type reconnectBackoff struct {
	mu       sync.Mutex
	base     time.Duration
	onlineAt time.Time
	online   bool
	stable   bool
	timer    *time.Timer
}

func newReconnectBackoff() *reconnectBackoff {
	return &reconnectBackoff{base: time.Second}
}

func (b *reconnectBackoff) Func(attempt int) time.Duration {
	b.mu.Lock()
	defer b.mu.Unlock()
	if attempt <= 0 {
		return 0
	}
	d := b.base
	for i := 1; i < attempt; i++ {
		d *= 2
		if d > 30*time.Second {
			d = 30 * time.Second
			break
		}
	}
	return withJitter(d)
}

func (b *reconnectBackoff) MarkOnline() {
	b.mu.Lock()
	defer b.mu.Unlock()
	b.online = true
	b.onlineAt = time.Now()
	b.stable = false
	if b.timer != nil {
		b.timer.Stop()
	}
	b.timer = time.AfterFunc(60*time.Second, func() {
		b.mu.Lock()
		defer b.mu.Unlock()
		if b.online {
			b.stable = true
			b.base = time.Second
		}
	})
}

func (b *reconnectBackoff) MarkOffline() {
	b.mu.Lock()
	defer b.mu.Unlock()
	if b.timer != nil {
		b.timer.Stop()
		b.timer = nil
	}
	wasOnline := b.online
	b.online = false
	if !wasOnline {
		// 连接尚未成功就失败：在 Func 内已按 attempt 加倍，这里把 base 提到下次周期的起点。
		next := b.base * 2
		if next > 30*time.Second {
			next = 30 * time.Second
		}
		if next < time.Second {
			next = time.Second
		}
		b.base = next
		return
	}
	if b.stable || time.Since(b.onlineAt) >= 60*time.Second {
		b.base = time.Second
		b.stable = false
		return
	}
	next := b.base * 2
	if next > 30*time.Second {
		next = 30 * time.Second
	}
	b.base = next
	b.stable = false
}

func (b *reconnectBackoff) Base() time.Duration {
	b.mu.Lock()
	defer b.mu.Unlock()
	return b.base
}

func withJitter(d time.Duration) time.Duration {
	// ±30%
	f := 0.7 + rand.Float64()*0.6
	return time.Duration(float64(d) * f)
}

// computeBackoffDelay 供单测：无 attempt 与 base 计算无抖动前的标称延迟。
func computeBackoffDelay(base time.Duration, attempt int) time.Duration {
	if attempt <= 0 {
		return 0
	}
	d := base
	for i := 1; i < attempt; i++ {
		d *= 2
		if d > 30*time.Second {
			return 30 * time.Second
		}
	}
	if d > 30*time.Second {
		return 30 * time.Second
	}
	return d
}