package postgres

import (
	"context"
	"database/sql"
	"database/sql/driver"
	"errors"
	"fmt"
	"io/fs"
	"strings"
	"sync"
	"time"

	"github.com/pressly/goose/v3"

	"scrabble/backend/internal/postgres/migrations"
)

// schemaName is the Postgres schema owned by the backend service. Every backend
// table lives here, and the DSN pins search_path to it.
const schemaName = "backend"

// migrationRetryAttempts and migrationRetryBackoff bound the transient-error
// retry around ApplyMigrations. A freshly started Postgres — notably a test
// container — can reset a pooled connection moments after it reports ready,
// which surfaces as "bad connection" mid-migration; a handful of quick retries
// ride over that without masking real failures.
const (
	migrationRetryAttempts = 5
	migrationRetryBackoff  = 250 * time.Millisecond
)

// gooseMu serialises access to goose's package-level filesystem state so a
// second caller in the same process cannot race on goose.SetBaseFS.
var gooseMu sync.Mutex

// ApplyMigrations runs every pending Up migration embedded in the backend
// binary against db. The schema is created upfront so goose's bookkeeping table
// (`goose_db_version`, scoped to the DSN search_path) has somewhere to land
// before the first migration runs; migration 00001_init.sql re-asserts the
// schema with IF NOT EXISTS, so the double-create is idempotent.
//
// The apply is retried on transient connection errors. Both steps are
// idempotent, so a retry after a dropped connection resumes from the last
// committed migration.
func ApplyMigrations(ctx context.Context, db *sql.DB) error {
	return retryOnTransient(ctx, migrationRetryAttempts, migrationRetryBackoff, func() error {
		if _, err := db.ExecContext(ctx, "CREATE SCHEMA IF NOT EXISTS "+schemaName); err != nil {
			return fmt.Errorf("ensure backend schema: %w", err)
		}
		if err := runMigrations(ctx, db, migrations.Migrations(), "."); err != nil {
			return fmt.Errorf("apply backend migrations: %w", err)
		}
		return nil
	})
}

// runMigrations applies every pending Up migration found under dir inside fsys
// against db. The PostgreSQL dialect is forced; goose's package-level base FS is
// restored on the way out so a second caller in the same process is safe. dir
// is "." when the migration files sit at the embed root.
func runMigrations(ctx context.Context, db *sql.DB, fsys fs.FS, dir string) error {
	if db == nil {
		return errors.New("run migrations: nil db")
	}
	if fsys == nil {
		return errors.New("run migrations: nil fs")
	}

	gooseMu.Lock()
	defer gooseMu.Unlock()

	goose.SetBaseFS(fsys)
	defer goose.SetBaseFS(nil)

	if err := goose.SetDialect("postgres"); err != nil {
		return fmt.Errorf("run migrations: set dialect: %w", err)
	}
	if err := goose.UpContext(ctx, db, dir); err != nil {
		return fmt.Errorf("run migrations: %w", err)
	}
	return nil
}

// retryOnTransient runs op up to attempts times, retrying only when op fails
// with a transient connection error — a dropped, reset, or refused connection,
// as opposed to a deterministic SQL error. It waits backoff between attempts and
// stops early if ctx is cancelled.
func retryOnTransient(ctx context.Context, attempts int, backoff time.Duration, op func() error) error {
	var err error
	for attempt := 1; attempt <= attempts; attempt++ {
		if err = op(); err == nil {
			return nil
		}
		if attempt == attempts || !isTransientConnError(err) {
			return err
		}
		select {
		case <-ctx.Done():
			return errors.Join(err, ctx.Err())
		case <-time.After(backoff):
		}
	}
	return err
}

// isTransientConnError reports whether err is a transient connection-level
// failure worth retrying, leaving deterministic SQL errors (syntax, constraint
// violations) to fail fast.
func isTransientConnError(err error) bool {
	if err == nil {
		return false
	}
	if errors.Is(err, driver.ErrBadConn) {
		return true
	}
	msg := strings.ToLower(err.Error())
	for _, s := range []string{
		"bad connection",
		"connection refused",
		"connection reset",
		"broken pipe",
		"server closed the connection",
	} {
		if strings.Contains(msg, s) {
			return true
		}
	}
	return false
}