package youtube

import (
	"context"
	"sync/atomic"
	"time"

	"golang.org/x/time/rate"
)

// globalFetchGate is the process-wide rate limiter for outbound timedtext/caption
// fetches. A single instance is shared by ALL Adapter instances (the scheduler's
// per-user runners + the web click-path) so they cannot collectively exceed the
// per-egress-IP cap. ADR-014 item 2: the gate serialises/limits concurrent caption
// fetches regardless of how many users or goroutines are upstream. The 429 is per
// IP, not per user — so the gate is process-wide, not per-withUser, not per-video.
//
// Default 2s/req (burst 1): the first fetch passes immediately, subsequent fetches
// are spaced at least 2s apart. Production overrides via SetFetchRate from config.
var globalFetchGate = rate.NewLimiter(rate.Every(2*time.Second), 1)

// SetFetchRate replaces the process-wide gate's rate with one token per interval.
// Call once at startup from config (TAPIR_FETCH_RATE). A non-positive interval
// installs an unlimited gate (rate.Inf) — used in dev/tests so nothing throttles.
func SetFetchRate(interval time.Duration) {
	if interval <= 0 {
		globalFetchGate = rate.NewLimiter(rate.Inf, 1)
		return
	}
	globalFetchGate = rate.NewLimiter(rate.Every(interval), 1)
}

// foregroundPending counts in-flight foreground (user-initiated) caption fetches.
// The background sweep yields the gate while this is non-zero so a human waiting
// on a click gets the next slot — and, on a near-throttled IP, the pre-429 window
// — instead of competing equally with the firehose (ADR-026, Pillar A). Clicks are
// rare and bursty, so background barely notices; the win to the click is large.
var foregroundPending atomic.Int64

// fgCtxKey marks a context as foreground (user-initiated). Unexported; set via
// ForegroundContext and read via isForeground so only this package owns the key.
type fgCtxKey struct{}

// ForegroundContext marks ctx as a user-initiated (foreground) fetch so the gate
// gives it priority. The web "Summarize"/paste/retry path wraps its context with
// this; the background scheduler leaves it unset.
func ForegroundContext(ctx context.Context) context.Context {
	return context.WithValue(ctx, fgCtxKey{}, true)
}

func isForeground(ctx context.Context) bool {
	v, _ := ctx.Value(fgCtxKey{}).(bool)
	return v
}

// fgYieldPoll is how often a background waiter re-checks whether a foreground
// fetch is still pending. Short enough to feel immediate, long enough not to spin.
const fgYieldPoll = 200 * time.Millisecond

// WaitFetchGate blocks until the process-wide gate allows one timedtext fetch,
// respecting ctx cancellation. Called from httpDo before every live outbound
// caption fetch so the scheduler and the click-path share the same egress budget.
//
// Foreground (user-initiated) fetches take priority: they register as pending and
// acquire a token immediately. Background fetches first yield — they wait until no
// foreground fetch is pending — so a live click is never stuck behind the
// background sweep and gets the cleaner slot against the per-IP limit (ADR-026).
func WaitFetchGate(ctx context.Context) error {
	if isForeground(ctx) {
		foregroundPending.Add(1)
		defer foregroundPending.Add(-1)
		return globalFetchGate.Wait(ctx)
	}

	// Background: defer to any pending foreground fetch before taking a token.
	for foregroundPending.Load() > 0 {
		select {
		case <-ctx.Done():
			return ctx.Err()
		case <-time.After(fgYieldPoll):
		}
	}
	return globalFetchGate.Wait(ctx)
}