package store

import (
	"bytes"
	"context"
	"crypto/sha256"
	"database/sql"
	"errors"
	"fmt"
	"math"
	"time"

	"github.com/klauspost/compress/zstd"
	rvboxv1 "github.com/rvbox/rvbox/gen/go/rvbox/v1"
	"github.com/rvbox/rvbox/internal/domain"
	"google.golang.org/protobuf/proto"
)

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
	ScriptPresent   bool
	ScriptContent   []byte
}

// PendingScript is the server-owned immutable script body for a command that
// has already crossed the queued boundary. It is used to reconstruct the
// session-local transfer window after a daemon or server reconnect; the
// command row remains the source of truth for whether the work is still
// non-terminal.
type PendingScript struct {
	IssueUUID domain.UUID
	Body      []byte
	Digest    [sha256.Size]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
	}
	script, scriptPresent, err := loadDispatchScript(ctx, tx, issue, spec)
	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, ScriptPresent: scriptPresent, ScriptContent: script}
	if expiry.Valid {
		value := time.Unix(0, expiry.Int64).UTC()
		candidate.QueueExpiryTime = &value
	}
	return candidate, nil
}

func (store *Store) PendingScriptDispatches(ctx context.Context, clientID string) ([]PendingScript, error) {
	if clientID == "" {
		return nil, errors.New("client ID is required")
	}
	database, err := store.openDatabase()
	if err != nil {
		return nil, err
	}
	rows, err := database.QueryContext(ctx, `SELECT issue_uuid, execution_spec, execution_spec_raw_bytes
FROM commands WHERE client_id = ? AND lifecycle BETWEEN 2 AND 4 ORDER BY issue_time, issue_uuid`, clientID)
	if err != nil {
		return nil, err
	}
	defer rows.Close()
	type pendingRow struct {
		issue       domain.UUID
		storedSpec  []byte
		rawSpecSize uint64
	}
	var candidates []pendingRow
	for rows.Next() {
		var encodedIssue, stored []byte
		var rawBytes uint64
		if err := rows.Scan(&encodedIssue, &stored, &rawBytes); err != nil {
			return nil, err
		}
		if len(encodedIssue) != 16 {
			return nil, ErrInvalidSegmentRecord
		}
		var issue domain.UUID
		copy(issue[:], encodedIssue)
		candidates = append(candidates, pendingRow{issue: issue, storedSpec: bytes.Clone(stored), rawSpecSize: rawBytes})
	}
	if err := rows.Close(); err != nil {
		return nil, err
	}
	var pending []PendingScript
	for _, candidate := range candidates {
		spec, err := decompressCommandSpec(candidate.storedSpec, candidate.rawSpecSize)
		if err != nil {
			return nil, err
		}
		body, present, err := loadDispatchScript(ctx, database, candidate.issue, spec)
		if err != nil {
			return nil, err
		}
		if !present {
			continue
		}
		pending = append(pending, PendingScript{IssueUUID: candidate.issue, Body: body, Digest: sha256.Sum256(body)})
	}
	if err := rows.Err(); err != nil {
		return nil, err
	}
	return pending, nil
}

type queryRower interface {
	QueryRowContext(context.Context, string, ...any) *sql.Row
}

func loadDispatchScript(ctx context.Context, tx queryRower, issue domain.UUID, encodedSpec []byte) ([]byte, bool, error) {
	var spec rvboxv1.ExecutionSpec
	if err := proto.Unmarshal(encodedSpec, &spec); err != nil {
		// Existing store callers may use opaque test bytes. They cannot carry a
		// script descriptor, so there is no payload to load.
		return nil, false, nil
	}
	descriptor := spec.GetScript()
	if descriptor == nil {
		return nil, false, nil
	}
	var stored []byte
	var rawBytes, storedBytes uint64
	var compression uint32
	var digest []byte
	if err := tx.QueryRowContext(ctx, `SELECT inline_data, raw_bytes, stored_bytes, compression, sha256
FROM command_payloads WHERE issue_uuid = ? AND kind = 'script'`, issue[:]).Scan(&stored, &rawBytes, &storedBytes, &compression, &digest); err != nil {
		return nil, false, fmt.Errorf("load persisted script payload: %w", err)
	}
	if compression != 2 || storedBytes != uint64(len(stored)) || rawBytes != descriptor.GetSizeBytes() || len(digest) != 32 || !bytes.Equal(digest, descriptor.GetSha256()) {
		return nil, false, ErrInvalidSegmentRecord
	}
	body, err := decompressStoredPayload(stored, rawBytes, maxStoredScriptBytes)
	if err != nil {
		return nil, false, err
	}
	computed := sha256.Sum256(body)
	if !bytes.Equal(computed[:], descriptor.GetSha256()) {
		return nil, false, ErrInvalidSegmentRecord
	}
	return body, true, nil
}

func decompressStoredPayload(stored []byte, rawBytes, maximum uint64) ([]byte, error) {
	if rawBytes > maximum || rawBytes > uint64(math.MaxInt) {
		return nil, ErrInvalidSegmentRecord
	}
	decoder, err := zstd.NewReader(nil, zstd.WithDecoderConcurrency(1), zstd.WithDecoderMaxMemory(maximum+1))
	if err != nil {
		return nil, ErrInvalidSegmentRecord
	}
	defer decoder.Close()
	decoded, err := decoder.DecodeAll(stored, nil)
	if err != nil || uint64(len(decoded)) != rawBytes || uint64(len(decoded)) > maximum {
		return nil, ErrInvalidSegmentRecord
	}
	return bytes.Clone(decoded), 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
}