package session

import (
	"context"
	"log/slog"
	"sync"
	"time"

	"git.yeij.top/AskaEth/Cyrene_v2/internal/channel/adapter"
	"git.yeij.top/AskaEth/Cyrene_v2/internal/logger"
)

// Proactive push per-platform defaults (design/08 §3): weixin-ilink allows push
// only within 24h of the last inbound message; OneBot allows push but must be
// rate-limited to avoid wind-control flags; telegram/webhook push freely.
const (
	// iLinkWindow is the weixin-ilink proactive push window.
	iLinkWindow = 24 * time.Hour
	// oneBotBurstLimit is the maximum OneBot pushes within the window.
	oneBotBurstLimit = 3
	// oneBotBurstWindow is the OneBot rate-limit window.
	oneBotBurstWindow = 10 * time.Second
)

// ProactiveOptions configures a ProactivePolicy.
type ProactiveOptions struct {
	// Window is the inactivity window after which a push is refused. Zero
	// disables the window check (unrestricted channels).
	Window time.Duration
	// RateLimit is the maximum push count within RateWindow. Zero disables the
	// window check and rate limiting together.
	RateLimit int
	// RateWindow is the sliding window over which RateLimit is enforced. Zero
	// disables rate limiting.
	RateWindow time.Duration
}

// ProactivePolicy decides whether a proactive push to a channel is allowed. It
// encodes the per-platform rules from design/08 §3: a capability gate (the
// adapter must declare Capabilities.Proactive), an optional inactivity window
// (weixin-ilink), and an optional burst rate limit (OneBot). The policy is
// immutable after construction and safe for concurrent use.
type ProactivePolicy struct {
	log     *slog.Logger
	window  time.Duration
	limiter *RateLimiter
}

// NewProactivePolicy builds a policy from opts. A zero Window and a zero
// RateLimit yield an unrestricted policy (telegram/webhook). WithLogger can be
// chained to tag denial records with the channel/session source.
func NewProactivePolicy(opts ProactiveOptions) *ProactivePolicy {
	p := &ProactivePolicy{window: opts.Window}
	if opts.RateLimit > 0 && opts.RateWindow > 0 {
		p.limiter = NewRateLimiter(opts.RateLimit, opts.RateWindow)
	}
	return p
}

// ProactivePolicyForPlatform returns the default proactive policy for a known
// platform identifier, following design/08 §3. Unknown platforms get an
// unrestricted policy, which is the conservative-outcome of "strong" channels.
func ProactivePolicyForPlatform(platform string) *ProactivePolicy {
	switch platform {
	case "weixin-ilink":
		return NewProactivePolicy(ProactiveOptions{Window: iLinkWindow})
	case "qq-obv11", "qq-onebot", "onebot":
		return NewProactivePolicy(ProactiveOptions{RateLimit: oneBotBurstLimit, RateWindow: oneBotBurstWindow})
	default:
		return NewProactivePolicy(ProactiveOptions{})
	}
}

// WithLogger returns a copy of p that logs denials through l with the
// channel/session source tag. It does not mutate p.
func (p *ProactivePolicy) WithLogger(l *slog.Logger) *ProactivePolicy {
	if p == nil || l == nil {
		return p
	}
	cp := *p
	cp.log = logger.WithSource(l, "channel/session")
	return &cp
}

// Allow reports whether a proactive push may be sent now. It fails closed: a nil
// policy, a canceled ctx, or an adapter that does not declare proactive support
// all deny. Then it applies the configured inactivity window and burst limit.
func (p *ProactivePolicy) Allow(ctx context.Context, caps adapter.Capabilities, lastMessageAt time.Time) bool {
	if p == nil {
		return false
	}
	if err := ctx.Err(); err != nil {
		p.logDeny("context canceled", err.Error())
		return false
	}
	if !caps.Proactive {
		p.logDeny("capability off", "capture.proactive=false")
		return false
	}
	if p.window > 0 {
		if lastMessageAt.IsZero() || time.Since(lastMessageAt) > p.window {
			p.logDeny("outside window", "last_message_at="+lastMessageAt.UTC().Format(time.RFC3339))
			return false
		}
	}
	if p.limiter != nil && !p.limiter.Allow() {
		p.logDeny("rate limited", "")
		return false
	}
	return true
}

func (p *ProactivePolicy) logDeny(reason, detail string) {
	if p == nil || p.log == nil {
		return
	}
	args := []any{"reason", reason}
	if detail != "" {
		args = append(args, "detail", detail)
	}
	p.log.Warn("proactive push denied by policy", args...)
}

// RateLimiter is a concurrency-safe sliding-window rate limiter. It keeps event
// timestamps in memory and reports whether a new event may proceed. It is
// independent of any bus: the sliding window is a purely in-process counter that
// is safe to share across goroutines.
type RateLimiter struct {
	mu     sync.Mutex
	limit  int
	window time.Duration
	events []time.Time
}

// NewRateLimiter builds a limiter that allows at most limit events per window.
// A limit <= 0 or window <= 0 yields an unlimited (always-allow) limiter, which
// is the safe "no rule configured" default.
func NewRateLimiter(limit int, window time.Duration) *RateLimiter {
	return &RateLimiter{limit: limit, window: window}
}

// Allow records a new event if the sliding window has capacity, returning true
// when the event may proceed, and false when it would exceed the limit. Events
// older than window are dropped before counting.
func (l *RateLimiter) Allow() bool {
	if l == nil || l.limit <= 0 || l.window <= 0 {
		return true
	}
	now := time.Now()
	cutoff := now.Add(-l.window)

	l.mu.Lock()
	defer l.mu.Unlock()

	keep := 0
	for _, t := range l.events {
		if t.After(cutoff) {
			l.events[keep] = t
			keep++
		}
	}
	l.events = l.events[:keep]

	if len(l.events) >= l.limit {
		return false
	}
	l.events = append(l.events, now)
	return true
}