package store

import (
	"context"
	"crypto/rand"
	"database/sql"
	"fmt"
	"time"

	"github.com/cosmic-clash/cosmic-clash/server/domain"
)

// ProbeNonceBytes is the challenge size. It only needs to be unguessable
// within the freshness window, not long-lived key material.
const ProbeNonceBytes = 16

const (
	ProbeChallengeUpsertSQL = `INSERT INTO probe_challenges (player_id, region, nonce, issued_at)
VALUES ($1, $2, $3, $4)
ON CONFLICT (player_id, region) DO UPDATE
SET nonce = EXCLUDED.nonce, issued_at = EXCLUDED.issued_at`
	// Consuming deletes in the same statement: a challenge is single-use, so a
	// captured probe response cannot be replayed to refresh a stale RTT.
	ProbeChallengeConsumeSQL = `DELETE FROM probe_challenges
WHERE player_id = $1 AND region = $2
RETURNING nonce, issued_at`
	ProbeChallengePurgeSQL = `DELETE FROM probe_challenges WHERE issued_at < $1`
)

// IssueProbeChallenge mints and stores a fresh nonce for one player and
// region. It is durable rather than per-process because any control-plane
// replica may serve the follow-up submission.
func IssueProbeChallenge(ctx context.Context, db *sql.DB, playerID, region string, now time.Time) ([]byte, error) {
	if db == nil || playerID == "" || (region != "EU" && region != "NA") || now.IsZero() {
		return nil, fmt.Errorf("invalid probe challenge arguments")
	}
	nonce := make([]byte, ProbeNonceBytes)
	if _, err := rand.Read(nonce); err != nil {
		return nil, err
	}
	if _, err := db.ExecContext(ctx, ProbeChallengeUpsertSQL, playerID, region, nonce, now); err != nil {
		return nil, err
	}
	return nonce, nil
}

// ConsumeProbeChallenge returns the outstanding nonce and when it was issued,
// removing it so it cannot be reused.
func ConsumeProbeChallenge(ctx context.Context, db *sql.DB, playerID, region string) ([]byte, time.Time, error) {
	if db == nil || playerID == "" || (region != "EU" && region != "NA") {
		return nil, time.Time{}, fmt.Errorf("invalid probe challenge arguments")
	}
	var nonce []byte
	var issuedAt time.Time
	err := db.QueryRowContext(ctx, ProbeChallengeConsumeSQL, playerID, region).Scan(&nonce, &issuedAt)
	if err == sql.ErrNoRows {
		return nil, time.Time{}, domain.ErrInvalidProbe
	}
	if err != nil {
		return nil, time.Time{}, err
	}
	return nonce, issuedAt, nil
}

// PurgeExpiredProbeChallenges drops challenges that can no longer be answered
// within the freshness window, so an abandoned probe cannot accumulate.
func PurgeExpiredProbeChallenges(ctx context.Context, db *sql.DB, now time.Time) (int64, error) {
	if db == nil || now.IsZero() {
		return 0, fmt.Errorf("invalid probe challenge purge arguments")
	}
	result, err := db.ExecContext(ctx, ProbeChallengePurgeSQL, now.Add(-domain.ProbeFreshness))
	if err != nil {
		return 0, err
	}
	return result.RowsAffected()
}

// ProbeEvidenceFromChallenge is the production ProbeProvider. The RTT is
// derived entirely from backend timestamps -- the interval between issuing the
// challenge and receiving the answer -- so no client-reported latency
// influences placement, which is the property docs/MATCHMAKING.md §4 requires.
func ProbeEvidenceFromChallenge(ctx context.Context, db *sql.DB, playerID, region string, opaqueLocation, nonce []byte, receivedAt time.Time) (domain.ProbeEvidence, []byte, error) {
	expectedNonce, issuedAt, err := ConsumeProbeChallenge(ctx, db, playerID, region)
	if err != nil {
		return domain.ProbeEvidence{}, nil, err
	}
	rtt := receivedAt.Sub(issuedAt)
	if rtt < 0 {
		// Clock skew between replicas; treat as immediate rather than letting
		// a negative duration through to placement.
		rtt = 0
	}
	return domain.ProbeEvidence{
		OpaqueLocation: opaqueLocation,
		Nonce:          nonce,
		IssuedAt:       issuedAt,
		Region:         region,
		ServerRTT:      rtt,
	}, expectedNonce, nil
}