package store

import (
	"bytes"
	"context"
	"database/sql"
	"errors"
	"fmt"
	"math"
	"time"

	"github.com/klauspost/compress/zstd"
	"github.com/rvbox/rvbox/internal/domain"
)

const maxStoredExecutionSpecBytes = 1 << 20

// DispatchCandidate is the immutable work reconstructed after an atomic
// queued-to-dispatched transition. The session writer must use the supplied
// generation and call RequeueDispatch if its network write never starts.
type DispatchCandidate struct {
	IssueUUID       domain.UUID
	Revision        uint64
	IssueTime       time.Time
	QueueExpiryTime *time.Time
	ImmutableSHA256 [32]byte
	ExecutionSpec   []byte
}

// ClaimNextDispatch expires old queued rows and atomically assigns the oldest
// remaining row to one live client-session generation. It deliberately records
// the dispatch before the wire write so uncertain writes are reconciled rather
// than accidentally delivered twice.
func (store *Store) ClaimNextDispatch(ctx context.Context, clientID string, generation uint64, now time.Time) (*DispatchCandidate, error) {
	if clientID == "" || generation == 0 || now.IsZero() {
		return nil, errors.New("invalid dispatch claim")
	}
	store.writeMu.Lock()
	defer store.writeMu.Unlock()
	database, err := store.openDatabase()
	if err != nil {
		return nil, err
	}
	tx, err := database.BeginTx(ctx, nil)
	if err != nil {
		return nil, err
	}
	defer tx.Rollback()
	// Expiry is terminal. The revision advance distinguishes it from a delayed
	// queued snapshot or a late dispatch attempt.
	if _, err := tx.ExecContext(ctx, `UPDATE commands SET lifecycle = 10, terminal_time = ?, revision = revision + 1
WHERE client_id = ? AND lifecycle = 1 AND queue_expiry_time IS NOT NULL AND queue_expiry_time <= ?`, now.UTC().UnixNano(), clientID, now.UTC().UnixNano()); err != nil {
		return nil, err
	}
	var encodedIssue, stored, immutableHash []byte
	var revision uint64
	var issueTime int64
	var expiry sql.NullInt64
	var rawBytes uint64
	err = tx.QueryRowContext(ctx, `SELECT issue_uuid, revision, issue_time, queue_expiry_time, immutable_request_sha256, execution_spec, execution_spec_raw_bytes
FROM commands WHERE client_id = ? AND lifecycle = 1 ORDER BY issue_time, issue_uuid LIMIT 1`, clientID).Scan(&encodedIssue, &revision, &issueTime, &expiry, &immutableHash, &stored, &rawBytes)
	if errors.Is(err, sql.ErrNoRows) {
		if err := tx.Commit(); err != nil {
			return nil, err
		}
		return nil, nil
	}
	if err != nil {
		return nil, err
	}
	if len(encodedIssue) != 16 || len(immutableHash) != 32 {
		return nil, ErrInvalidSegmentRecord
	}
	var issue domain.UUID
	copy(issue[:], encodedIssue)
	var immutable [32]byte
	copy(immutable[:], immutableHash)
	spec, err := decompressCommandSpec(stored, rawBytes)
	if err != nil {
		return nil, err
	}
	result, err := tx.ExecContext(ctx, `UPDATE commands SET lifecycle = 2, target_session_generation = ? WHERE issue_uuid = ? AND client_id = ? AND lifecycle = 1`, generation, issue[:], clientID)
	if err != nil {
		return nil, err
	}
	changed, err := result.RowsAffected()
	if err != nil {
		return nil, err
	}
	if changed != 1 {
		return nil, errors.New("queued command changed during dispatch claim")
	}
	if err := tx.Commit(); err != nil {
		return nil, err
	}
	candidate := &DispatchCandidate{IssueUUID: issue, Revision: revision, IssueTime: time.Unix(0, issueTime).UTC(), ImmutableSHA256: immutable, ExecutionSpec: spec}
	if expiry.Valid {
		value := time.Unix(0, expiry.Int64).UTC()
		candidate.QueueExpiryTime = &value
	}
	return candidate, nil
}

// RequeueDispatch reverses only a known pre-acceptance write failure. A later
// session generation, acceptance, or reconciliation decision cannot be rolled
// back by a stale writer.
func (store *Store) RequeueDispatch(ctx context.Context, issueUUID domain.UUID, clientID string, generation uint64) (bool, error) {
	if isZeroUUID([16]byte(issueUUID)) || clientID == "" || generation == 0 {
		return false, errors.New("invalid dispatch requeue")
	}
	store.writeMu.Lock()
	defer store.writeMu.Unlock()
	database, err := store.openDatabase()
	if err != nil {
		return false, err
	}
	result, err := database.ExecContext(ctx, `UPDATE commands SET lifecycle = 1, target_session_generation = NULL
WHERE issue_uuid = ? AND client_id = ? AND lifecycle = 2 AND target_session_generation = ?`, issueUUID[:], clientID, generation)
	if err != nil {
		return false, err
	}
	changed, err := result.RowsAffected()
	return changed == 1, err
}

func decompressCommandSpec(stored []byte, rawBytes uint64) ([]byte, error) {
	if rawBytes == 0 || rawBytes > maxStoredExecutionSpecBytes || rawBytes > uint64(math.MaxInt) {
		return nil, errors.New("invalid stored execution spec size")
	}
	decoder, err := zstd.NewReader(nil, zstd.WithDecoderConcurrency(1), zstd.WithDecoderMaxMemory(maxStoredExecutionSpecBytes+1))
	if err != nil {
		return nil, err
	}
	defer decoder.Close()
	decoded, err := decoder.DecodeAll(stored, nil)
	if err != nil || uint64(len(decoded)) != rawBytes || uint64(len(decoded)) > maxStoredExecutionSpecBytes {
		return nil, fmt.Errorf("invalid stored execution spec")
	}
	return bytes.Clone(decoded), nil
}