package migration

import (
	"context"
	"crypto/sha256"
	"encoding/base64"
	"errors"
	"fmt"
	"github.com/jmoiron/sqlx"
	"io/fs"
	"log/slog"
	"sort"
	"strings"
	"time"
)

const migrationsTable = `
CREATE TABLE IF NOT EXISTS migration (
    filename varchar,
    hash varchar,
    created timestamp,
    should_validate boolean
);`
const fetchMigrationsTable = `SELECT * FROM "migration";`
const createMigration = `INSERT INTO "migration" (filename, hash, created, should_validate) VALUES  ($1, $2, current_timestamp, TRUE);`

type Migration struct {
	Filename       string    `db:"filename"`
	Hash           string    `db:"hash"`
	Created        time.Time `db:"created"`
	ShouldValidate bool      `db:"should_validate"`
}

func InitializeMigrations(db *sqlx.DB, migrationFiles fs.FS) error {
	// create table if it doesn't exist already
	if _, err := db.Exec(migrationsTable); err != nil {
		return err
	}

	// fetch current migrations
	var entries []Migration
	if err := db.Select(&entries, fetchMigrationsTable); err != nil {
		return err
	}

	migrations := map[string]Migration{}
	for _, entry := range entries {
		migrations[entry.Filename] = entry
	}
	return validateMigrations(db, migrations, migrationFiles)
}

type version struct {
	major int
	minor int
}

func validateMigrations(db *sqlx.DB, migrations map[string]Migration, migrationFiles fs.FS) error {
	scripts := map[string]string{}
	var scriptNames []string

	// fetch scripts
	err := fs.WalkDir(migrationFiles, ".", func(path string, d fs.DirEntry, outerErr error) error {
		if !d.IsDir() {
			if bytearr, err := fs.ReadFile(migrationFiles, path); err == nil {
				scripts[d.Name()] = strings.TrimSpace(string(bytearr))
				scriptNames = append(scriptNames, d.Name())
			}
		}
		return nil
	})
	if err != nil {
		return err
	}

	// sort scripts by version
	sort.Slice(scriptNames, func(i, j int) bool {
		return isLessThan(parseVersion(scriptNames[i]), parseVersion(scriptNames[j]))
	})

	for _, name := range scriptNames {
		if _, exists := migrations[name]; exists {
			if err := validateMigration(name, migrations[name], scripts[name]); err != nil {
				return err
			}
		} else {
			if err := executeMigration(db, name, scripts[name]); err != nil {
				return err
			}
		}
	}
	return nil
}

func executeMigration(db *sqlx.DB, name string, script string) error {
	logger := slog.Default().With(slog.String("script", name))
	logger.Info("migrations - executing")
	tx := db.MustBeginTx(context.Background(), nil)
	var err error = nil
	if _, e := tx.Exec(script); e != nil {
		err = e
	}
	if _, e := tx.Exec(createMigration, name, hash(script)); e != nil {
		err = e
	}
	if err != nil {
		logger.Error("migrations - failed executing", slog.String("err", err.Error()))
		tx.Rollback()
	} else {
		logger.Info("migrations - successfully executed")
		tx.Commit()
	}
	return err
}

func validateMigration(name string, migration Migration, script string) error {
	if !migration.ShouldValidate {
		return nil
	}

	calculatedHash := hash(script)

	if calculatedHash != migration.Hash {
		err := errors.New(fmt.Sprintf("migrations - mismatch in hash for %s (expected '%s', calculated '%s')", name, migration.Hash, calculatedHash))
		slog.Error("migrations - failed validation", slog.String("script", name), slog.String("err", err.Error()))
		return err
	}
	return nil
}

func hash(script string) string {
	hash := sha256.New()
	hash.Write([]byte(script))
	return base64.StdEncoding.EncodeToString(hash.Sum(nil))
}

func isLessThan(v1 version, v2 version) bool {
	if v1.major > v2.major {
		return false
	}
	if v1.major == v2.major && v1.minor > v2.minor {
		return false
	}
	return true
}

func parseVersion(filename string) version {
	ver := version{}
	if _, err := fmt.Sscanf(filename, "v%d_%d", &ver.major, &ver.minor); err != nil {
		panic(err)
	}
	return ver
}