package auth

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"fmt"
	"time"

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

// RateLimiter implements per-scope sliding-window rate limiting and failure
// counters on top of Redis. It stores nothing but short-lived counters — no
// connection or behaviour logs — per the no-log baseline (doc/06 §4).
//
// Sliding-window keys are ZSETs named `rl:{scope}:{key}` whose members are
// timestamped attempts; failure counters are plain integer keys.
type RateLimiter struct {
	rdb *redis.Client
	now func() time.Time
}

// NewRateLimiter builds a RateLimiter. now may be nil (defaults to time.Now).
func NewRateLimiter(rdb *redis.Client, now func() time.Time) *RateLimiter {
	if now == nil {
		now = time.Now
	}
	return &RateLimiter{rdb: rdb, now: now}
}

// slidingWindow is a single atomic Lua script:
//   - drops members older than the window,
//   - if the live count already reached the limit, returns {0, oldestScore},
//   - otherwise records the attempt and returns {1, 0}.
//
// All times are unix-milliseconds.
var slidingWindow = redis.NewScript(`
local key    = KEYS[1]
local now    = tonumber(ARGV[1])
local window = tonumber(ARGV[2])
local limit  = tonumber(ARGV[3])
local member = ARGV[4]
redis.call('ZREMRANGEBYSCORE', key, 0, now - window)
local count = redis.call('ZCARD', key)
if count >= limit then
  local oldest = redis.call('ZRANGE', key, 0, 0, 'WITHSCORES')
  return {0, oldest[2]}
end
redis.call('ZADD', key, now, member)
redis.call('PEXPIRE', key, window)
return {1, 0}
`)

// rlKey builds the canonical `rl:{scope}:{key}` Redis key.
func rlKey(scope, key string) string {
	return fmt.Sprintf("rl:%s:%s", scope, key)
}

// Allow records an attempt in the sliding window for (scope, key). It returns
// allowed=false together with the Retry-After duration when the limit within
// window has been reached. limit is the maximum number of attempts permitted
// inside window.
func (rl *RateLimiter) Allow(ctx context.Context, scope, key string, limit int, window time.Duration) (allowed bool, retryAfter time.Duration, err error) {
	nowMs := rl.now().UnixMilli()
	winMs := window.Milliseconds()

	member, err := uniqueMember(nowMs)
	if err != nil {
		return false, 0, err
	}

	res, err := slidingWindow.Run(ctx, rl.rdb, []string{rlKey(scope, key)},
		nowMs, winMs, limit, member).Result()
	if err != nil {
		return false, 0, fmt.Errorf("auth: ratelimit run: %w", err)
	}

	vals, ok := res.([]interface{})
	if !ok || len(vals) != 2 {
		return false, 0, fmt.Errorf("auth: ratelimit unexpected result %v", res)
	}
	ok1, _ := vals[0].(int64)
	if ok1 == 1 {
		return true, 0, nil
	}

	// Denied: compute how long until the oldest attempt leaves the window.
	oldest := toInt64(vals[1])
	ra := time.Duration(oldest+winMs-nowMs) * time.Millisecond
	if ra < time.Second {
		ra = time.Second
	}
	return false, ra, nil
}

// failKey builds the failure-counter key, e.g. `rl:login:{email}`.
func failKey(scope, key string) string {
	return rlKey(scope, key)
}

// RecordFailure increments the failure counter for (scope, key), (re)setting
// its TTL to window on every increment so the lock slides forward while abuse
// continues. It returns the new count.
func (rl *RateLimiter) RecordFailure(ctx context.Context, scope, key string, window time.Duration) (int64, error) {
	k := failKey(scope, key)
	pipe := rl.rdb.Pipeline()
	incr := pipe.Incr(ctx, k)
	pipe.Expire(ctx, k, window)
	if _, err := pipe.Exec(ctx); err != nil {
		return 0, fmt.Errorf("auth: record failure: %w", err)
	}
	return incr.Val(), nil
}

// FailureCount returns the current failure count and the remaining TTL (lock
// window) for (scope, key). count is 0 when no counter exists.
func (rl *RateLimiter) FailureCount(ctx context.Context, scope, key string) (count int64, ttl time.Duration, err error) {
	k := failKey(scope, key)
	pipe := rl.rdb.Pipeline()
	get := pipe.Get(ctx, k)
	pttl := pipe.PTTL(ctx, k)
	if _, err := pipe.Exec(ctx); err != nil && err != redis.Nil {
		return 0, 0, fmt.Errorf("auth: failure count: %w", err)
	}
	n, _ := get.Int64()
	d := pttl.Val()
	if d < 0 {
		d = 0
	}
	return n, d, nil
}

// ClearFailures removes the failure counter after a successful authentication.
func (rl *RateLimiter) ClearFailures(ctx context.Context, scope, key string) error {
	return rl.rdb.Del(ctx, failKey(scope, key)).Err()
}

// uniqueMember returns a ZSET member that is unique even for attempts that share
// the same millisecond timestamp (timestamp prefix keeps ordering stable).
func uniqueMember(nowMs int64) (string, error) {
	var b [8]byte
	if _, err := rand.Read(b[:]); err != nil {
		return "", fmt.Errorf("auth: ratelimit member: %w", err)
	}
	return fmt.Sprintf("%d-%s", nowMs, hex.EncodeToString(b[:])), nil
}

// toInt64 coerces a Redis Lua return value (which may be int64 or string) to int64.
func toInt64(v interface{}) int64 {
	switch t := v.(type) {
	case int64:
		return t
	case string:
		var n int64
		_, _ = fmt.Sscanf(t, "%d", &n)
		return n
	default:
		return 0
	}
}