// Package database owns the sqlite connection and the schema. The
// schema is embedded in the binary, so a deployment is one file: there
// is no migrations directory to ship alongside it and no version of it
// that can be out of step with the code that expects it.
package database

import (
	"context"
	"database/sql"
	"embed"
	"fmt"
	"log/slog"
	"os"
	"path/filepath"
	"time"

	"go.uber.org/fx"
	"sneak.berlin/go/simplexcalc/internal/config"
	"sneak.berlin/go/simplexcalc/internal/logger"

	// modernc.org/sqlite is the pure-Go driver: no cgo, so the binary
	// links statically and the container needs no libc.
	_ "modernc.org/sqlite"
)

// schemaFS carries the migrations into the binary.
//
//go:embed schema/*.sql
var schemaFS embed.FS

// dirPerm is the mode for the data directory: owner-only, because it
// holds the database.
const dirPerm = 0o700

// pragmas are applied to every connection. WAL is what makes concurrent
// reads not block on a write; busy_timeout is what turns the remaining
// contention into a short wait rather than an immediate SQLITE_BUSY;
// foreign_keys is off by default in sqlite and has to be asked for.
const pragmas = `
PRAGMA journal_mode = WAL;
PRAGMA busy_timeout = 5000;
PRAGMA foreign_keys = ON;
PRAGMA synchronous = NORMAL;
`

// Params defines dependencies for Database.
type Params struct {
	fx.In

	Config *config.Config
	Logger *logger.Logger
}

// Database is the handle to the application's sqlite database.
type Database struct {
	db  *sql.DB
	log *slog.Logger
}

// New opens the database, applies the embedded migrations, and
// registers a close hook. Migrations run during OnStart rather than
// lazily on first use: a schema that cannot be applied is a failure to
// start, and the process says so before it accepts a request.
func New(lc fx.Lifecycle, params Params) (*Database, error) {
	d := &Database{log: params.Logger.Get()}

	err := os.MkdirAll(filepath.Dir(params.Config.DBPath), dirPerm)
	if err != nil {
		return nil, fmt.Errorf("creating data directory: %w", err)
	}

	// New runs during graph construction, which has no request or
	// lifecycle context of its own; the pragmas are a handful of
	// in-process statements against a file that was just created.
	db, err := Open(context.Background(), params.Config.DBPath)
	if err != nil {
		return nil, err
	}

	d.db = db

	lc.Append(fx.Hook{
		OnStart: func(ctx context.Context) error {
			return d.Migrate(ctx)
		},
		OnStop: func(_ context.Context) error {
			d.log.Info("closing database")

			closeErr := d.db.Close()
			if closeErr != nil {
				return fmt.Errorf("closing database: %w", closeErr)
			}

			return nil
		},
	})

	return d, nil
}

// Open opens a sqlite database at path and applies the connection
// pragmas. Exported so tests can open a scratch database without the
// fx graph.
func Open(ctx context.Context, path string) (*sql.DB, error) {
	db, err := sql.Open("sqlite", path)
	if err != nil {
		return nil, fmt.Errorf("opening database %s: %w", path, err)
	}

	// sqlite tolerates exactly one writer. Holding the pool to a
	// single connection makes that limit explicit here rather than
	// intermittent under load, and WAL keeps readers off the writer's
	// back anyway.
	db.SetMaxOpenConns(1)
	db.SetConnMaxLifetime(time.Hour)

	_, err = db.ExecContext(ctx, pragmas)
	if err != nil {
		_ = db.Close()

		return nil, fmt.Errorf("applying pragmas: %w", err)
	}

	return db, nil
}

// NewForTest opens a scratch database in dir and migrates it. Test
// helper, exported so that a project seeded from this template can use
// it from any package's tests.
func NewForTest(ctx context.Context, dir string) (*Database, error) {
	d := &Database{log: slog.New(slog.DiscardHandler)}

	db, err := Open(ctx, filepath.Join(dir, "test.db"))
	if err != nil {
		return nil, err
	}

	d.db = db

	err = d.Migrate(ctx)
	if err != nil {
		_ = db.Close()

		return nil, err
	}

	return d, nil
}

// Migrate applies every embedded migration that has not been applied to
// this database yet.
func (d *Database) Migrate(ctx context.Context) error {
	set := migrationSet{fsys: schemaFS, dir: "schema"}

	err := set.apply(ctx, d.db, d.log)
	if err != nil {
		return fmt.Errorf("applying migrations: %w", err)
	}

	return nil
}

// DB exposes the underlying handle for packages that need to query it.
func (d *Database) DB() *sql.DB {
	return d.db
}

// AppliedVersions returns the migration versions recorded as applied,
// ascending. The healthcheck reports the highest of them, so an
// operator can see which schema a running instance is on without
// shelling into it.
func (d *Database) AppliedVersions(ctx context.Context) ([]int, error) {
	rows, err := d.db.QueryContext(ctx,
		"SELECT version FROM schema_migrations ORDER BY version",
	)
	if err != nil {
		return nil, fmt.Errorf("reading applied migrations: %w", err)
	}
	defer func() { _ = rows.Close() }()

	var versions []int

	for rows.Next() {
		var v int

		scanErr := rows.Scan(&v)
		if scanErr != nil {
			return nil, fmt.Errorf("scanning migration version: %w", scanErr)
		}

		versions = append(versions, v)
	}

	err = rows.Err()
	if err != nil {
		return nil, fmt.Errorf("iterating applied migrations: %w", err)
	}

	return versions, nil
}

// Close releases the handle. Production uses the fx OnStop hook; tests
// call this.
func (d *Database) Close() error {
	err := d.db.Close()
	if err != nil {
		return fmt.Errorf("closing database: %w", err)
	}

	return nil
}