// Package customschema: Milestone 3 (reset-metadata) service
// implementation. Lives in its own file so Milestone 3 owns a disjoint
// partition from Milestone 2's service_metadata.go (see tracking §2.9).
package customschema

import (
	"context"
	"errors"
	"fmt"
	"time"

	"queryorchestration/internal/database/repository"

	"github.com/google/uuid"
	"github.com/jackc/pgx/v5"
)

// ResetDocumentMetadata wipes a document's custom metadata history and
// clears its custom_schema_id binding in a single transaction. The
// operation is idempotent: calling it on a document that has no schema
// bound and no metadata rows succeeds with MetadataVersionsDeleted == 0
// and all PreviousSchema* fields nil.
//
// Transaction sequence (single tx, in exact order):
//
//  1. LockDocumentForReset — FOR UPDATE on the documents row.
//     ErrDocumentNotFound when the row is missing.
//  2. HasFieldExtraction — mutual-exclusivity guard. A document carrying
//     any legacy field extraction rows is rejected with
//     ErrMutualExclusivityViolation before any DELETE runs.
//  3. GetDocumentSchemaBindingForReset — capture PreviousSchema* fields
//     for the response.
//  4. CountDocumentCustomMetadataVersions — capture count for the
//     response under the same lock so it is authoritative at commit.
//  5. DeleteDocumentCustomMetadataForReset — DELETE every metadata row.
//  6. NullifyDocumentCustomSchemaIdForReset — UPDATE custom_schema_id
//     to NULL. Trigger 1 allows this because step 5 already cleared
//     metadata and step 2 already verified no legacy extractions.
//  7. Post-commit audit entry (metadata.reset).
//
// The actor argument is the authenticated Cognito subject and is the
// only source of the ResetBy field on the result; the handler never
// reads an actor from the request body.
func (s *Service) ResetDocumentMetadata(ctx context.Context, documentID uuid.UUID, actor string) (*ResetMetadataResult, error) {
	if documentID == uuid.Nil {
		return nil, fmt.Errorf("customschema: document id is required")
	}

	result := &ResetMetadataResult{
		DocumentID: documentID,
		ResetBy:    actor,
	}

	var docClientID string

	txErr := s.cfg.ExecuteDBTransaction(ctx, func(txCtx context.Context, q *repository.Queries) error {
		clientID, err := s.resetDocumentMetadataTx(txCtx, q, documentID, result)
		if err != nil {
			return err
		}
		docClientID = clientID
		return nil
	})
	if txErr != nil {
		return nil, txErr
	}

	// Populate ResetAt at commit time so the response timestamp and the
	// audit entry agree. This value is also written into the audit
	// record below.
	result.ResetAt = time.Now().UTC()

	// Post-commit audit. Resource id is the document id (the thing being
	// mutated). Details carry previous_schema_id (nilable) and the
	// version count so downstream log aggregation can compute "how much
	// history was discarded" without re-reading the tracking plan.
	details := map[string]any{
		"metadata_versions_deleted": result.MetadataVersionsDeleted,
	}
	if result.PreviousSchemaID != nil {
		details["previous_schema_id"] = result.PreviousSchemaID.String()
	} else {
		details["previous_schema_id"] = nil
	}
	if err := s.audit.Record(ctx, AuditRecord{
		Actor:      actor,
		Action:     AuditActionMetadataReset,
		ResourceID: documentID,
		ClientID:   docClientID,
		Timestamp:  result.ResetAt,
		Details:    details,
	}); err != nil {
		return nil, fmt.Errorf("customschema: record metadata.reset audit: %w", err)
	}
	return result, nil
}

// resetDocumentMetadataTx runs the transactional body of
// ResetDocumentMetadata. Pulled out so the top-level method stays under
// 100 lines and the transaction sequence reads top-to-bottom. Mutates
// `result` in place so captured fields survive after the closure
// returns. Returns the document's client id so the caller can populate
// the post-commit audit record.
func (s *Service) resetDocumentMetadataTx(
	txCtx context.Context,
	q *repository.Queries,
	documentID uuid.UUID,
	result *ResetMetadataResult,
) (string, error) {
	// 1. Parent-row lock. Must be the first DML inside the tx.
	lockRow, err := q.LockDocumentForReset(txCtx, documentID)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return "", ErrDocumentNotFound
		}
		return "", fmt.Errorf("customschema: lock document for reset: %w", err)
	}

	docClientID := lockRow.Clientid

	// 2. Mutual-exclusivity guard. A document with any legacy extraction
	//    rows is not a custom-schema document and must not be reset.
	hasLegacy, err := q.HasFieldExtraction(txCtx, documentID)
	if err != nil {
		return "", fmt.Errorf("customschema: check legacy extractions: %w", err)
	}
	if hasLegacy {
		return "", ErrMutualExclusivityViolation
	}

	// 3. Capture previousSchema* for the response. The LEFT JOIN returns
	//    NULL name/version when no schema is bound — propagate as nil.
	binding, err := q.GetDocumentSchemaBindingForReset(txCtx, documentID)
	if err != nil {
		return "", fmt.Errorf("customschema: read document schema binding: %w", err)
	}
	if binding.CustomSchemaID != nil {
		idCopy := *binding.CustomSchemaID
		result.PreviousSchemaID = &idCopy
	}
	if binding.SchemaName != nil {
		nameCopy := *binding.SchemaName
		result.PreviousSchemaName = &nameCopy
	}
	if binding.SchemaVersion != nil {
		v := int(*binding.SchemaVersion)
		result.PreviousSchemaVersion = &v
	}

	// 4. Count metadata rows BEFORE the delete so the caller learns how
	//    many versions were discarded.
	count, err := q.CountDocumentCustomMetadataVersions(txCtx, documentID)
	if err != nil {
		return "", fmt.Errorf("customschema: count metadata versions: %w", err)
	}
	result.MetadataVersionsDeleted = int(count)

	// 5. Delete every metadata row for this document.
	if err := q.DeleteDocumentCustomMetadataForReset(txCtx, documentID); err != nil {
		return "", fmt.Errorf("customschema: delete metadata rows: %w", err)
	}

	// 6. Clear the schema binding. Trigger 1 fires on this UPDATE — see
	//    the function-level comment for why both EXISTS checks inside
	//    the trigger return false at this point.
	if err := q.NullifyDocumentCustomSchemaIdForReset(txCtx, documentID); err != nil {
		return "", fmt.Errorf("customschema: nullify custom_schema_id: %w", err)
	}
	return docClientID, nil
}