package store

import (
	"context"
	"database/sql"
	"embed"
	"errors"
	"fmt"
	"io/fs"

	"github.com/pressly/goose/v3"
)

// Each file holds both directions, marked with -- +goose Up / -- +goose Down.
// Create new ones with: goose -dir internal/store/migrations create NAME sql
//
//go:embed migrations/*.sql
var migrationsFS embed.FS

// ErrNeedsMigration is returned by Open when the database has pending
// migrations. Apply them with the migrate command, or pass
// WithAutoMigrate.
var ErrNeedsMigration = errors.New("store: database needs migrating")

// ErrSchemaTooNew is returned by Open when the database has migrations
// this build doesn't know about, i.e. it was migrated by a newer build.
var ErrSchemaTooNew = errors.New("store: database schema is newer than this build")

// ErrWouldEmptyDatabase is returned by StepDown when the migration would drop
// all of the tables and thus empty the database.
var ErrWouldEmptyDatabase = errors.New(
	"migration cancelled; database would be emptied by undoing all migrations",
)

// NewMigrator returns a goose Provider over the store's embedded migrations,
// for applying, rolling back or inspecting them. Each migration runs in its
// own transaction, so a failed migration leaves the database at the previous
// version (goose has no "dirty" state).
func NewMigrator(db *sql.DB) (*goose.Provider, error) {
	fsys, err := fs.Sub(migrationsFS, "migrations")
	if err != nil {
		return nil, err
	}

	return goose.NewProvider(goose.DialectSQLite3, db, fsys)
}

// LatestVersion is the highest migration version in this build.
func LatestVersion(p *goose.Provider) int64 {
	sources := p.ListSources() // sorted ascending
	if len(sources) == 0 {
		return 0
	}
	return sources[len(sources)-1].Version
}

// checkVersion returns ErrNeedsMigration or ErrSchemaTooNew unless the
// database is exactly at this build's schema.
func checkVersion(ctx context.Context, p *goose.Provider) error {
	current, err := p.GetDBVersion(ctx)
	if err != nil {
		return err
	}

	latest := LatestVersion(p)
	if current > latest {
		return fmt.Errorf("%w (database is at version %d, this build knows up to %d)",
			ErrSchemaTooNew, current, latest)
	}

	pending, err := p.HasPending(ctx)
	if err != nil {
		return err
	}

	if pending {
		return fmt.Errorf("%w (database is at version %d, latest is %d)",
			ErrNeedsMigration, current, latest)
	}

	return nil
}

// checkNotNewer refuses to work on a database migrated by a newer build:
// this build has neither the newer migrations nor their down steps.
func checkNotNewer(ctx context.Context, m *goose.Provider) error {
	current, err := m.GetDBVersion(ctx)
	if err != nil {
		return err
	}
	if latest := LatestVersion(m); current > latest {
		return fmt.Errorf(
			"database is at version %d but this build only knows up to %d; use a newer build",
			current, latest)
	}
	return nil
}

// prepareSchema migrates (with WithAutoMigrate) or verifies the schema
// version.
func prepareSchema(ctx context.Context, db *sql.DB, o options) error {
	p, err := NewMigrator(db)
	if err != nil {
		return err
	}

	if o.autoMigrate {
		if _, err := p.Up(ctx); err != nil {
			return fmt.Errorf("store: migrating database: %w", err)
		}
	}

	return checkVersion(ctx, p)
}

// StepUp applies steps pending migrations, or all of them when steps is 0.
func StepUp(
	ctx context.Context, m *goose.Provider, steps int,
) ([]*goose.MigrationResult, error) {
	var results []*goose.MigrationResult

	if err := checkNotNewer(ctx, m); err != nil {
		return results, err
	}

	var err error
	if steps == 0 {
		results, err = m.Up(ctx)
	} else {
		for range steps {
			var r *goose.MigrationResult
			r, err = m.UpByOne(ctx)
			if errors.Is(err, goose.ErrNoNextVersion) {
				err = nil
				break
			}
			if err != nil {
				break
			}
			results = append(results, r)
		}
	}

	return results, err
}

// StepDown rolls back steps applied migrations, or all of them when steps
// is 0. Rolling everything back deletes all data, so confirmDrop is checked
// first in that case.
func StepDown(
	ctx context.Context, m *goose.Provider, steps int, confirmDrop bool,
) ([]*goose.MigrationResult, error) {
	var results []*goose.MigrationResult

	if err := checkNotNewer(ctx, m); err != nil {
		return results, err
	}

	current, err := m.GetDBVersion(ctx)
	if err != nil {
		return results, err
	}
	if current == 0 {
		return results, nil
	}

	statuses, err := m.Status(ctx)
	if err != nil {
		return results, err
	}
	applied := 0
	for _, s := range statuses {
		if s.State == goose.StateApplied {
			applied++
		}
	}
	all := steps == 0 || steps >= applied
	if all && !confirmDrop {
		return results, ErrWouldEmptyDatabase
	}

	if all {
		results, err = m.DownTo(ctx, 0)
	} else {
		for range steps {
			var r *goose.MigrationResult
			r, err = m.Down(ctx)
			if errors.Is(err, goose.ErrNoNextVersion) {
				err = nil
				break
			}
			if err != nil {
				break
			}
			results = append(results, r)
		}
	}

	return results, err
}