package store

import (
	"context"
	"crypto/rand"
	"database/sql"
	"embed"
	"encoding/base64"
	"errors"
	"fmt"
	"io/fs"
	"strings"
	"sync"

	"github.com/urpi/go-bananas/internal/domain"
)

// migrationsFS holds the schema that ships inside the binary, so a fresh
// container needs no migration job and no schema files mounted beside it.
//
//go:embed migrations/*.sql
var migrationsFS embed.FS

// Migration is one applied schema change.
type Migration struct {
	Name string
	// Statements is the number of statements the file contains, reported for
	// logging only.
	Statements int
}

// migrationLock serialises migration across goroutines within a process. SQLite
// already serialises writers across processes, so no cross-process lock is
// needed, but two replicas starting simultaneously would otherwise both log the
// same applied migration.
var migrationLock sync.Mutex

// migrate applies every embedded migration that has not yet been recorded.
//
// Migrations are idempotent by construction: each file uses CREATE ... IF NOT
// EXISTS, so re-running is harmless. The applied set is tracked in a table so
// that a future migration which is not idempotent can still be introduced
// safely.
func (s *Store) migrate(ctx context.Context) error {
	migrationLock.Lock()
	defer migrationLock.Unlock()

	if _, err := s.db.ExecContext(ctx, `
		CREATE TABLE IF NOT EXISTS schema_migrations (
			name       TEXT PRIMARY KEY,
			applied_at TEXT NOT NULL
		)`); err != nil {
		return fmt.Errorf("create schema_migrations: %w", err)
	}

	applied, err := s.appliedMigrations(ctx)
	if err != nil {
		return err
	}

	entries, err := fs.ReadDir(migrationsFS, "migrations")
	if err != nil {
		return fmt.Errorf("read embedded migrations: %w", err)
	}

	for _, entry := range entries {
		if entry.IsDir() || !strings.HasSuffix(entry.Name(), ".sql") {
			continue
		}
		if _, done := applied[entry.Name()]; done {
			continue
		}

		body, err := fs.ReadFile(migrationsFS, "migrations/"+entry.Name())
		if err != nil {
			return fmt.Errorf("read migration %s: %w", entry.Name(), err)
		}

		tx, err := s.db.BeginTx(ctx, nil)
		if err != nil {
			return fmt.Errorf("begin migration %s: %w", entry.Name(), err)
		}

		if _, err := tx.ExecContext(ctx, string(body)); err != nil {
			tx.Rollback()
			return fmt.Errorf("apply migration %s: %w", entry.Name(), err)
		}

		if _, err := tx.ExecContext(ctx,
			`INSERT INTO schema_migrations (name, applied_at) VALUES (?, ?)`,
			entry.Name(), s.now().UTC().Format(domain.TimeFormat),
		); err != nil {
			tx.Rollback()
			return fmt.Errorf("record migration %s: %w", entry.Name(), err)
		}

		if err := tx.Commit(); err != nil {
			return fmt.Errorf("commit migration %s: %w", entry.Name(), err)
		}
	}
	return nil
}

// appliedMigrations returns the set of recorded migration names.
func (s *Store) appliedMigrations(ctx context.Context) (map[string]struct{}, error) {
	rows, err := s.db.QueryContext(ctx, `SELECT name FROM schema_migrations`)
	if err != nil {
		return nil, fmt.Errorf("read schema_migrations: %w", err)
	}
	defer rows.Close()

	applied := make(map[string]struct{})
	for rows.Next() {
		var name string
		if err := rows.Scan(&name); err != nil {
			return nil, fmt.Errorf("scan schema_migrations: %w", err)
		}
		applied[name] = struct{}{}
	}
	return applied, rows.Err()
}

// Seed reconciles the built-in themes with what is stored.
//
// The rules are:
//   - a missing built-in is inserted;
//   - a built-in still at its seed version is refreshed, so an upgrade picks up
//     palette and schema fixes;
//   - a built-in an operator has edited (version above the seed) is left alone;
//   - a non-built-in theme is never touched.
//
// It returns how many themes were inserted and how many were refreshed.
func (s *Store) Seed(ctx context.Context, builtins []*domain.Theme) (inserted, refreshed int, err error) {
	if len(builtins) == 0 {
		return 0, 0, nil
	}

	tx, err := s.db.BeginTx(ctx, nil)
	if err != nil {
		return 0, 0, fmt.Errorf("begin seed: %w", err)
	}
	defer func() {
		if err != nil {
			tx.Rollback()
		}
	}()

	now := s.now().UTC().Format(domain.TimeFormat)

	for _, theme := range builtins {
		tokensJSON, err := marshalJSON(theme.Tokens)
		if err != nil {
			return 0, 0, fmt.Errorf("marshal tokens for %s: %w", theme.Slug, err)
		}
		metaJSON, err := marshalJSON(theme.Meta)
		if err != nil {
			return 0, 0, fmt.Errorf("marshal meta for %s: %w", theme.Slug, err)
		}

		var existingVersion int64
		err = tx.QueryRowContext(ctx,
			`SELECT version FROM themes WHERE id = ? OR slug = ?`, theme.ID, theme.Slug,
		).Scan(&existingVersion)

		switch {
		case errors.Is(err, sql.ErrNoRows):
			if err := insertTheme(ctx, tx, theme, tokensJSON, metaJSON, now); err != nil {
				return 0, 0, fmt.Errorf("insert built-in theme %s: %w", theme.Slug, err)
			}
			if err := insertVersion(ctx, tx, theme, tokensJSON, "built-in seed", "system", now); err != nil {
				return 0, 0, fmt.Errorf("insert built-in version %s: %w", theme.Slug, err)
			}
			inserted++

		case err != nil:
			return 0, 0, fmt.Errorf("look up built-in theme %s: %w", theme.Slug, err)

		default:
			// Refresh only when the stored copy predates this seed. An equal
			// version means the theme is already current; a higher version means
			// an operator has edited it, and their work wins either way.
			if existingVersion >= theme.Version {
				continue
			}
			if err := refreshTheme(ctx, tx, theme, tokensJSON, metaJSON, now); err != nil {
				return 0, 0, fmt.Errorf("refresh built-in theme %s: %w", theme.Slug, err)
			}
			if err := insertVersion(ctx, tx, theme, tokensJSON, "refreshed by Go Bananas!! upgrade", "system", now); err != nil {
				return 0, 0, fmt.Errorf("insert built-in version %s: %w", theme.Slug, err)
			}
			refreshed++
		}
	}

	if err = tx.Commit(); err != nil {
		return 0, 0, fmt.Errorf("commit seed: %w", err)
	}
	return inserted, refreshed, nil
}

// insertTheme writes a new theme row.
func insertTheme(ctx context.Context, tx *sql.Tx, theme *domain.Theme, tokensJSON, metaJSON, now string) error {
	_, err := tx.ExecContext(ctx, `
		INSERT INTO themes (
			id, slug, name, description, status, default_appearance,
			tokens_json, meta_json, is_system, version,
			created_at, updated_at, created_by, updated_by
		) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
		theme.ID, theme.Slug, theme.Name, theme.Description, string(theme.Status), string(theme.DefaultAppearance),
		tokensJSON, metaJSON, boolToInt(theme.System), theme.Version,
		now, now, theme.CreatedBy, theme.UpdatedBy,
	)
	return translateError(err)
}

// refreshTheme overwrites an untouched built-in theme in place.
func refreshTheme(ctx context.Context, tx *sql.Tx, theme *domain.Theme, tokensJSON, metaJSON, now string) error {
	_, err := tx.ExecContext(ctx, `
		UPDATE themes SET
			name = ?, description = ?, status = ?, default_appearance = ?,
			tokens_json = ?, meta_json = ?, version = ?, updated_at = ?, updated_by = ?
		WHERE slug = ?`,
		theme.Name, theme.Description, string(theme.Status), string(theme.DefaultAppearance),
		tokensJSON, metaJSON, theme.Version, now, "system",
		theme.Slug,
	)
	return translateError(err)
}

// insertVersion appends an immutable snapshot to a theme's history.
func insertVersion(ctx context.Context, tx *sql.Tx, theme *domain.Theme, tokensJSON, message, author, now string) error {
	_, err := tx.ExecContext(ctx, `
		INSERT INTO theme_versions (
			theme_id, version, slug, name, description, status,
			default_appearance, tokens_json, message, author, created_at
		) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
		theme.ID, theme.Version, theme.Slug, theme.Name, theme.Description, string(theme.Status),
		string(theme.DefaultAppearance), tokensJSON, message, author, now,
	)
	return translateError(err)
}

// randomID returns a URL-safe random identifier.
func randomID() (string, error) {
	buf := make([]byte, 16)
	if _, err := rand.Read(buf); err != nil {
		return "", fmt.Errorf("read random: %w", err)
	}
	return base64.RawURLEncoding.EncodeToString(buf), nil
}