package sampling

import (
	"math"
	"time"

	"qbank/internal/models"
)

const (
	FloorWeight      = 0.15 // mastered questions still appear at ~15% base rate
	RecencyCapDays   = 30.0 // days until recency multiplier saturates
	RecencyMaxMult   = 2.0  // peak recency multiplier
	UnseenBaseWeight = 0.5  // base weight for questions with no stats row
)

// ComputeWeight returns the sampling weight for a question given its per-user
// stat. A nil stat means the question has never been seen.
func ComputeWeight(stat *models.UserQuestionStat, now time.Time) float64 {
	if stat == nil {
		// Unseen: mid-range base + full recency = 1.0
		return UnseenBaseWeight * RecencyMaxMult
	}

	s := float64(stat.TimesSeen)
	c := float64(stat.TimesCorrect)

	// Laplace-smoothed error rate dampens noise from small samples.
	errorRate := (s - c + 1) / (s + 2)
	base := math.Max(FloorWeight, errorRate)

	var daysSince float64
	if stat.LastSeenAt.Valid {
		daysSince = now.Sub(stat.LastSeenAt.Time).Hours() / 24
	} else {
		daysSince = RecencyCapDays
	}
	recency := 1 + math.Min(daysSince/RecencyCapDays, 1.0)

	return base * recency
}