// Package migrations holds the schema of each kind of SQLite file
// webhooker writes, as numbered SQL files embedded in the binary, and
// applies it when a file is opened.
//
// Each kind of file has its own directory. In each, 000_migration.sql
// creates only the schema_migrations table, and 001_schema.sql holds
// the whole schema. Before 1.0 a schema change is an edit to
// 001_schema.sql, never a new file.
package migrations

import (
	"context"
	"database/sql"
	"embed"
	"fmt"
	"io/fs"
	"path"
	"strconv"
	"strings"
)

// The directories, one for each kind of database file.
const (
	// Main is the main database, webhooker.db.
	Main = "main"

	// Events is a webhook's event database, events-{uuid}.db.
	Events = "events"

	// Archive is a database target's archive file.
	Archive = "archive"
)

//go:embed main/*.sql events/*.sql archive/*.sql
var files embed.FS

// Apply runs each SQL file in dir, which is Main, Events or Archive,
// that the database has not had yet, in the order of the number its
// name starts with, and records that number in schema_migrations.
// 000_migration.sql creates that table and records itself, so it runs
// first, and only when the table is missing.
//
// It all runs in one transaction, so a failure leaves the file as it
// was.
func Apply(ctx context.Context, db *sql.DB, dir string) error {
	tx, err := db.BeginTx(ctx, nil)
	if err != nil {
		return fmt.Errorf("starting migrations: %w", err)
	}

	// Does nothing once Commit has succeeded.
	defer func() { _ = tx.Rollback() }()

	var tables int

	err = tx.QueryRowContext(ctx,
		"SELECT count(*) FROM sqlite_master "+
			"WHERE type = 'table' AND name = 'schema_migrations'",
	).Scan(&tables)
	if err != nil {
		return fmt.Errorf("looking for schema_migrations: %w", err)
	}

	if tables == 0 {
		err = run(ctx, tx, path.Join(dir, "000_migration.sql"))
		if err != nil {
			return err
		}
	}

	// fs.Glob lists the files in name order, which is number order.
	names, err := fs.Glob(files, path.Join(dir, "*.sql"))
	if err != nil {
		return fmt.Errorf("listing migrations: %w", err)
	}

	for _, name := range names {
		err = applyOnce(ctx, tx, name)
		if err != nil {
			return err
		}
	}

	err = tx.Commit()
	if err != nil {
		return fmt.Errorf("committing migrations: %w", err)
	}

	return nil
}

// applyOnce runs the migration file name and records its number,
// unless schema_migrations already has that number.
func applyOnce(ctx context.Context, tx *sql.Tx, name string) error {
	number, _, _ := strings.Cut(path.Base(name), "_")

	version, err := strconv.Atoi(number)
	if err != nil {
		return fmt.Errorf("numbering migration %s: %w", name, err)
	}

	var applied int

	err = tx.QueryRowContext(ctx,
		"SELECT count(*) FROM schema_migrations WHERE version = ?",
		version,
	).Scan(&applied)
	if err != nil {
		return fmt.Errorf("checking migration %s: %w", name, err)
	}

	if applied > 0 {
		return nil
	}

	err = run(ctx, tx, name)
	if err != nil {
		return err
	}

	_, err = tx.ExecContext(ctx,
		"INSERT INTO schema_migrations (version) VALUES (?)",
		version,
	)
	if err != nil {
		return fmt.Errorf("recording migration %s: %w", name, err)
	}

	return nil
}

// run executes every statement in the migration file name.
func run(ctx context.Context, tx *sql.Tx, name string) error {
	script, err := files.ReadFile(name)
	if err != nil {
		return fmt.Errorf("reading migration %s: %w", name, err)
	}

	_, err = tx.ExecContext(ctx, string(script))
	if err != nil {
		return fmt.Errorf("applying migration %s: %w", name, err)
	}

	return nil
}