// Package ratelimit provides a Redis-backed fixed-window rate limiter and
// lightweight per-key usage counters for the public API.
//
// All state lives in Redis so it is shared across API replicas and visible to
// the admin console, and so the public server keeps its read-only contract
// against PostgreSQL. Every operation fails open: if Redis is unavailable the
// limiter allows the request rather than taking the API down.
package ratelimit

import (
	"context"
	"fmt"
	"log"
	"time"

	"github.com/redis/go-redis/v9"
)

// Limiter throttles callers and records usage. A nil-backed Limiter (when Redis
// could not be configured) disables limiting and usage tracking.
type Limiter struct {
	rdb *redis.Client
}

// Result describes the outcome of an Allow check and the headers to surface.
type Result struct {
	Allowed   bool
	Limit     int
	Remaining int
	ResetUnix int64
}

// UsageStat is the aggregated usage for a single API key.
type UsageStat struct {
	Total      int64  `json:"total"`
	Today      int64  `json:"today"`
	LastUsedAt *int64 `json:"last_used_at,omitempty"`
}

// New builds a Limiter from a redis:// URL. On a parse error it logs and returns
// a fail-open limiter (Redis disabled) so the server still boots.
func New(redisURL string) *Limiter {
	opt, err := redis.ParseURL(redisURL)
	if err != nil {
		log.Printf("ratelimit: invalid redis url %q: %v (rate limiting disabled)", redisURL, err)
		return &Limiter{}
	}
	return &Limiter{rdb: redis.NewClient(opt)}
}

// Enabled reports whether a Redis backend is configured.
func (l *Limiter) Enabled() bool { return l != nil && l.rdb != nil }

// Ping verifies the Redis backend is reachable. Returns an error if disabled or
// unreachable.
func (l *Limiter) Ping(ctx context.Context) error {
	if !l.Enabled() {
		return redis.ErrClosed
	}
	return l.rdb.Ping(ctx).Err()
}

// Allow records a hit for id within a fixed window and reports whether the
// caller is under limit. Fails open (Allowed=true) on any Redis error.
func (l *Limiter) Allow(ctx context.Context, id string, limit int, window time.Duration) Result {
	reset := func() int64 {
		win := int64(window / time.Second)
		if win < 1 {
			win = 1
		}
		return (time.Now().Unix()/win + 1) * win
	}
	if !l.Enabled() {
		return Result{Allowed: true, Limit: limit, Remaining: limit, ResetUnix: reset()}
	}
	win := int64(window / time.Second)
	if win < 1 {
		win = 1
	}
	bucket := time.Now().Unix() / win
	key := fmt.Sprintf("rl:%s:%d", id, bucket)

	n, err := l.rdb.Incr(ctx, key).Result()
	if err != nil {
		return Result{Allowed: true, Limit: limit, Remaining: limit, ResetUnix: (bucket + 1) * win}
	}
	if n == 1 {
		l.rdb.Expire(ctx, key, time.Duration(win)*time.Second)
	}
	remaining := limit - int(n)
	if remaining < 0 {
		remaining = 0
	}
	return Result{
		Allowed:   int(n) <= limit,
		Limit:     limit,
		Remaining: remaining,
		ResetUnix: (bucket + 1) * win,
	}
}

// RecordUsage increments total/daily counters and stamps last-used for a key.
// Best-effort: errors are ignored.
func (l *Limiter) RecordUsage(ctx context.Context, keyID string) {
	if !l.Enabled() || keyID == "" {
		return
	}
	now := time.Now()
	day := now.Format("20060102")
	pipe := l.rdb.Pipeline()
	pipe.Incr(ctx, "usage:total:"+keyID)
	dayKey := "usage:day:" + keyID + ":" + day
	pipe.Incr(ctx, dayKey)
	pipe.Expire(ctx, dayKey, 90*24*time.Hour)
	pipe.Set(ctx, "usage:last:"+keyID, now.Unix(), 0)
	_, _ = pipe.Exec(ctx)
}

// IncrTotal increments the lifetime call counter for subject and returns the
// new total. The counter never expires; it is the cumulative number of calls
// attributed to a caller (an API key id, or "ip:<addr>" for anonymous callers).
// Fails open returning 0 on any error so quota enforcement never takes the API
// down.
func (l *Limiter) IncrTotal(ctx context.Context, subject string) int64 {
	if !l.Enabled() || subject == "" {
		return 0
	}
	n, err := l.rdb.Incr(ctx, "usage:total:"+subject).Result()
	if err != nil {
		return 0
	}
	return n
}

// CopyTotal carries a lifetime counter from one subject to another, used when a
// key is regenerated so a caller cannot reset their cumulative quota. Best
// effort: a missing or zero source counter is a no-op.
func (l *Limiter) CopyTotal(ctx context.Context, from, to string) {
	if !l.Enabled() || from == "" || to == "" {
		return
	}
	n, err := l.rdb.Get(ctx, "usage:total:"+from).Int64()
	if err != nil || n == 0 {
		return
	}
	l.rdb.Set(ctx, "usage:total:"+to, n, 0)
}

// TotalUsed reads the lifetime call counter for subject without incrementing.
func (l *Limiter) TotalUsed(ctx context.Context, subject string) int64 {
	if !l.Enabled() || subject == "" {
		return 0
	}
	n, _ := l.rdb.Get(ctx, "usage:total:"+subject).Int64()
	return n
}

// Usage reads aggregated usage for a key. Returns a zero-value stat on error.
func (l *Limiter) Usage(ctx context.Context, keyID string) UsageStat {
	var st UsageStat
	if !l.Enabled() || keyID == "" {
		return st
	}
	day := time.Now().Format("20060102")
	st.Total, _ = l.rdb.Get(ctx, "usage:total:"+keyID).Int64()
	st.Today, _ = l.rdb.Get(ctx, "usage:day:"+keyID+":"+day).Int64()
	if v, err := l.rdb.Get(ctx, "usage:last:"+keyID).Int64(); err == nil {
		st.LastUsedAt = &v
	}
	return st
}