// SPDX-License-Identifier: MPL-2.0

// Package mailsqlite provides an encrypted transactional mail outbox. Callers
// own schema-version journals, authorization, rate limits and connection opening.
package mailsqlite

import (
	"context"
	"crypto/aes"
	"crypto/cipher"
	"crypto/hmac"
	"crypto/rand"
	"crypto/sha256"
	"database/sql"
	"encoding/json"
	"errors"
	"time"

	"gamertan.com/web/mail"
)

var (
	ErrInvalid  = errors.New("mailsqlite: invalid outbox operation")
	ErrConflict = errors.New("mailsqlite: message identity or lease conflict")
	ErrFull     = errors.New("mailsqlite: pending outbox limit reached")
)

const maxAttempts = 5
const leaseDuration = time.Minute

type Options struct {
	// EncryptionKey is a separate 32-byte application-managed secret. Back it up
	// separately from SQLite. Losing/changing it makes pending messages unreadable.
	EncryptionKey []byte
	MaxPending    int
	Now           func() time.Time
}

type Queue struct {
	db         *sql.DB
	aead       cipher.AEAD
	digestKey  []byte
	maxPending int
	now        func() time.Time
}

func (*Queue) String() string         { return "mailsqlite.Queue{keys:redacted}" }
func (queue *Queue) GoString() string { return queue.String() }

func derive(key []byte, label string) []byte {
	mac := hmac.New(sha256.New, key)
	mac.Write([]byte(label))
	return mac.Sum(nil)
}

func New(db *sql.DB, options Options) (*Queue, error) {
	if db == nil || len(options.EncryptionKey) != 32 {
		return nil, ErrInvalid
	}
	if options.MaxPending == 0 {
		options.MaxPending = 1000
	}
	if options.MaxPending < 1 || options.MaxPending > 10000 {
		return nil, ErrInvalid
	}
	if options.Now == nil {
		options.Now = time.Now
	}
	block, err := aes.NewCipher(derive(options.EncryptionKey, "gwf.mail.encryption.v1"))
	if err != nil {
		return nil, err
	}
	aead, err := cipher.NewGCM(block)
	if err != nil {
		return nil, err
	}
	return &Queue{db: db, aead: aead, digestKey: derive(options.EncryptionKey, "gwf.mail.deduplication.v1"), maxPending: options.MaxPending, now: options.Now}, nil
}

// CreateSchema is called inside the application's explicit migration, never
// request startup. A caller-owned transaction also makes schema creation atomic.
func CreateSchema(ctx context.Context, tx *sql.Tx) error {
	if tx == nil {
		return ErrInvalid
	}
	for _, statement := range []string{
		`CREATE TABLE IF NOT EXISTS gwf_mail_outbox (
 id TEXT PRIMARY KEY,state TEXT NOT NULL CHECK(state IN ('queued','sending','accepted','failed','expired')),
 payload BLOB,digest BLOB NOT NULL CHECK(length(digest)=32),created_at INTEGER NOT NULL,expires_at INTEGER NOT NULL,
 next_attempt_at INTEGER NOT NULL,attempts INTEGER NOT NULL DEFAULT 0,lease_hash BLOB,lease_until INTEGER NOT NULL DEFAULT 0,
 last_stage TEXT NOT NULL DEFAULT '',smtp_code INTEGER NOT NULL DEFAULT 0,updated_at INTEGER NOT NULL,
 CHECK(expires_at>created_at),CHECK(attempts>=0),CHECK(lease_hash IS NULL OR length(lease_hash)=32))`,
		`CREATE INDEX IF NOT EXISTS gwf_mail_outbox_ready ON gwf_mail_outbox(state,next_attempt_at,created_at,id)`,
		`CREATE INDEX IF NOT EXISTS gwf_mail_outbox_expiry ON gwf_mail_outbox(expires_at)`,
	} {
		if _, err := tx.ExecContext(ctx, statement); err != nil {
			return err
		}
	}
	return nil
}

type payload struct {
	Version   int
	Message   mail.Message
	ExpiresAt time.Time
}

// EnqueueTx commits mail intent alongside the caller's domain change and audit.
// It never commits, sends SMTP or logs plaintext. Reusing an ID is idempotent only
// for the same message/expiry; a completed ID cannot send a second time.
func (queue *Queue) EnqueueTx(ctx context.Context, tx *sql.Tx, message mail.Message, expiresAt time.Time) error {
	if tx == nil || message.Validate() != nil {
		return ErrInvalid
	}
	now := queue.now().UTC()
	expiresAt = expiresAt.UTC().Truncate(time.Second)
	message.CreatedAt = message.CreatedAt.UTC().Truncate(time.Second)
	if !expiresAt.After(now) || expiresAt.Sub(now) > 24*time.Hour || !expiresAt.After(message.CreatedAt) || message.CreatedAt.After(now.Add(time.Minute)) {
		return ErrInvalid
	}
	raw, err := json.Marshal(payload{Version: 1, Message: message, ExpiresAt: expiresAt})
	if err != nil {
		return err
	}
	mac := hmac.New(sha256.New, queue.digestKey)
	mac.Write(raw)
	digest := mac.Sum(nil)
	nonce := make([]byte, queue.aead.NonceSize())
	if _, err = rand.Read(nonce); err != nil {
		return err
	}
	sealed := append([]byte{1}, nonce...)
	sealed = queue.aead.Seal(sealed, nonce, raw, []byte("gwf.mail.v1:"+message.ID))
	// First statement acquires SQLite's writer lock, avoiding read/write upgrade
	// races when multiple requests enqueue or compete for the bounded capacity.
	var inserted string
	err = tx.QueryRowContext(ctx, `INSERT INTO gwf_mail_outbox(id,state,payload,digest,created_at,expires_at,next_attempt_at,updated_at)
 SELECT ?,'queued',?,?,?,?,?,? WHERE (SELECT COUNT(*) FROM gwf_mail_outbox WHERE payload IS NOT NULL)<?
 ON CONFLICT(id) DO NOTHING RETURNING id`, message.ID, sealed, digest, message.CreatedAt.Unix(), expiresAt.Unix(), now.Unix(), now.Unix(), queue.maxPending).Scan(&inserted)
	if err == nil {
		return nil
	}
	if !errors.Is(err, sql.ErrNoRows) {
		return err
	}
	var existing []byte
	if err = tx.QueryRowContext(ctx, `SELECT digest FROM gwf_mail_outbox WHERE id=?`, message.ID).Scan(&existing); errors.Is(err, sql.ErrNoRows) {
		return ErrFull
	} else if err != nil {
		return err
	}
	if !hmac.Equal(existing, digest) {
		return ErrConflict
	}
	return nil
}

func (queue *Queue) Enqueue(ctx context.Context, message mail.Message, expiresAt time.Time) error {
	tx, err := queue.db.BeginTx(ctx, nil)
	if err != nil {
		return err
	}
	defer tx.Rollback()
	if err = queue.EnqueueTx(ctx, tx, message, expiresAt); err != nil {
		return err
	}
	return tx.Commit()
}

// Record is safe operational metadata; it contains no recipient/body/token.
// Expose it only through the application's current operator authority.
type Record struct {
	ID, State, FailureStage             string
	SMTPCode, Attempts                  int
	CreatedAt, ExpiresAt, NextAttemptAt time.Time
}

func (queue *Queue) Recent(ctx context.Context, limit int) ([]Record, error) {
	if limit < 1 || limit > 100 {
		return nil, ErrInvalid
	}
	rows, err := queue.db.QueryContext(ctx, `SELECT id,state,last_stage,smtp_code,attempts,created_at,expires_at,next_attempt_at FROM gwf_mail_outbox ORDER BY created_at DESC,id DESC LIMIT ?`, limit)
	if err != nil {
		return nil, err
	}
	defer rows.Close()
	var records []Record
	for rows.Next() {
		var record Record
		var created, expires, next int64
		if err = rows.Scan(&record.ID, &record.State, &record.FailureStage, &record.SMTPCode, &record.Attempts, &created, &expires, &next); err != nil {
			return nil, err
		}
		record.CreatedAt = time.Unix(created, 0).UTC()
		record.ExpiresAt = time.Unix(expires, 0).UTC()
		if next > 0 && record.State == "queued" {
			record.NextAttemptAt = time.Unix(next, 0).UTC()
		}
		records = append(records, record)
	}
	return records, rows.Err()
}

// Sweep clears sensitive payloads for expired/exhausted work in batches of 100.
// A running, unexpired lease is left alone. Metadata/deduplication tombstones stay.
func (queue *Queue) Sweep(ctx context.Context) error {
	now := queue.now().UTC().Unix()
	_, err := queue.db.ExecContext(ctx, `UPDATE gwf_mail_outbox SET state=CASE WHEN expires_at<=? THEN 'expired' ELSE 'failed' END,
 payload=NULL,lease_hash=NULL,lease_until=0,next_attempt_at=0,updated_at=?,last_stage=CASE WHEN expires_at<=? THEN 'expired' ELSE 'attempt-limit' END
 WHERE id IN (SELECT id FROM gwf_mail_outbox WHERE payload IS NOT NULL AND (expires_at<=? OR attempts>=?) AND (state<>'sending' OR lease_until<=?) ORDER BY expires_at,id LIMIT 100)`, now, now, now, now, maxAttempts, now)
	return err
}

func (queue *Queue) open(id string, sealed []byte, expires int64) (mail.Message, error) {
	n := queue.aead.NonceSize()
	if len(sealed) < 1+n+queue.aead.Overhead() || sealed[0] != 1 {
		return mail.Message{}, ErrInvalid
	}
	raw, err := queue.aead.Open(nil, sealed[1:1+n], sealed[1+n:], []byte("gwf.mail.v1:"+id))
	if err != nil {
		return mail.Message{}, ErrInvalid
	}
	var body payload
	if json.Unmarshal(raw, &body) != nil || body.Version != 1 || body.Message.ID != id || body.ExpiresAt.Unix() != expires || body.Message.Validate() != nil {
		return mail.Message{}, ErrInvalid
	}
	return body.Message, nil
}

// ProcessOne takes one bounded lease, commits it, then contacts SMTP without a
// database write lock. It records safe status and clears terminal payloads. A
// crash/uncertain acceptance can duplicate delivery; stable IDs aid diagnosis.
// false means no ready work. A delivery failure is a Record, not a raw SMTP error.
func (queue *Queue) ProcessOne(ctx context.Context, transport mail.Transport) (Record, bool, error) {
	if transport == nil {
		return Record{}, false, ErrInvalid
	}
	if err := queue.Sweep(ctx); err != nil {
		return Record{}, false, err
	}
	now := queue.now().UTC()
	lease := make([]byte, 32)
	if _, err := rand.Read(lease); err != nil {
		return Record{}, false, err
	}
	var record Record
	var sealed []byte
	var created, expires int64
	err := queue.db.QueryRowContext(ctx, `UPDATE gwf_mail_outbox SET state='sending',attempts=attempts+1,lease_hash=?,lease_until=?,updated_at=?
 WHERE id=(SELECT id FROM gwf_mail_outbox WHERE payload IS NOT NULL AND expires_at>? AND next_attempt_at<=? AND attempts<?
 AND (state='queued' OR (state='sending' AND lease_until<=?)) ORDER BY created_at,id LIMIT 1)
 RETURNING id,payload,attempts,created_at,expires_at`, lease, now.Add(leaseDuration).Unix(), now.Unix(), now.Unix(), now.Unix(), maxAttempts, now.Unix()).Scan(&record.ID, &sealed, &record.Attempts, &created, &expires)
	if errors.Is(err, sql.ErrNoRows) {
		return Record{}, false, nil
	}
	if err != nil {
		return Record{}, false, err
	}
	record.CreatedAt = time.Unix(created, 0).UTC()
	record.ExpiresAt = time.Unix(expires, 0).UTC()
	message, sendErr := queue.open(record.ID, sealed, expires)
	payloadInvalid := sendErr != nil
	if sendErr == nil {
		// End transmission before the lease expires. Also stop when the message's
		// own deadline expires; no stale verification link starts after expiry.
		duration := min(30*time.Second, record.ExpiresAt.Sub(queue.now().UTC()))
		if duration <= 0 {
			sendErr = &mail.Error{Stage: "expired"}
		} else {
			sendContext, cancel := context.WithTimeout(ctx, duration)
			sendErr = transport.Send(sendContext, message)
			cancel()
		}
	}
	finished := queue.now().UTC()
	record.State = "accepted"
	if payloadInvalid {
		// A wrong key is an operational fault, not a delivery attempt. Preserve
		// ciphertext until expiry so fixing configuration can recover pending
		// messages. The same bounded delay also covers corrupt/invalid payloads;
		// never pass them to SMTP or persist the decryption error.
		record.Attempts--
		record.State, record.FailureStage = "queued", "payload"
		record.NextAttemptAt = finished.Add(5 * time.Minute)
		if !record.ExpiresAt.After(finished) {
			record.State = "expired"
		}
	} else if sendErr != nil {
		stage, code, retry := "transport", 0, true
		var failure *mail.Error
		if errors.As(sendErr, &failure) {
			stage, code, retry = failure.Stage, failure.Code, failure.Retryable
		}
		// Only bounded known stages may enter durable diagnostics, even for a
		// caller-provided Transport. Never persist arbitrary error strings.
		switch stage {
		case "connect", "tls", "greeting", "starttls-required", "authentication", "sender", "recipient", "data", "acceptance", "payload", "expired":
		default:
			stage = "transport"
		}
		if code < 0 || code > 599 {
			code = 0
		}
		record.FailureStage, record.SMTPCode = stage, code
		record.State = "failed"
		record.NextAttemptAt = finished.Add(time.Minute * time.Duration(1<<(record.Attempts-1)))
		if !record.ExpiresAt.After(finished) || stage == "expired" {
			record.State = "expired"
		} else if retry && record.Attempts < maxAttempts && record.NextAttemptAt.Before(record.ExpiresAt) {
			record.State = "queued"
		}
	}
	var retain []byte
	var nextAttempt int64
	if record.State == "queued" {
		retain = sealed
		nextAttempt = record.NextAttemptAt.Unix()
	} else {
		record.NextAttemptAt = time.Time{}
	}
	result, err := queue.db.ExecContext(ctx, `UPDATE gwf_mail_outbox SET state=?,payload=?,last_stage=?,smtp_code=?,attempts=?,next_attempt_at=?,lease_hash=NULL,lease_until=0,updated_at=? WHERE id=? AND state='sending' AND lease_hash=? AND lease_until>?`, record.State, retain, record.FailureStage, record.SMTPCode, record.Attempts, nextAttempt, finished.Unix(), record.ID, lease, finished.Unix())
	if err != nil {
		return record, true, err
	}
	changed, err := result.RowsAffected()
	if err != nil {
		return record, true, err
	}
	if changed != 1 {
		return record, true, ErrConflict
	}
	return record, true, nil
}