package main

import (
	"compress/gzip"
	"errors"
	"io"
	"io/fs"
	"log/slog"
	"os"
	"os/exec"
	"os/signal"
	"path"
	"strings"
	"syscall"
	"time"
)

const (
	FilenameTemplate = "postgresql-backup-2006-01-02T15:04:05Z07:00.sql.gz"
)

func getStrFromEnv(varName, defaultValue string) string {
	value := os.Getenv(varName)
	if value == "" {
		return defaultValue
	}

	return value
}

func backup(backupsDir, pgdumpBin string, customArgs []string) {
	filename := time.Now().Format(FilenameTemplate)
	filepath := path.Join(backupsDir, filename)
	slog.Info("creating a new backup", "path", filepath)
	backupFile, err := os.Create(filepath)
	if err != nil {
		slog.Error("unable to create backup file", "err", err)
		return
	}
	defer backupFile.Close()

	cmd := exec.Command(pgdumpBin, customArgs...)
	stdout, err := cmd.StdoutPipe()
	if err != nil {
		slog.Error("unable to connect to stdout", "err", err)
		return
	}

	if err := cmd.Start(); err != nil {
		slog.Error("unable to start the program", "err", err)
		return
	}

	gzipWriter, err := gzip.NewWriterLevel(backupFile, gzip.BestCompression)
	if err != nil {
		slog.Error("error creating gzip writer", "err", err)
		return
	}
	defer gzipWriter.Close()

	_, err = io.Copy(gzipWriter, stdout)
	if err != nil {
		slog.Error("error writing to the backup file", "err", err)
		return
	}

	if err := cmd.Wait(); err != nil {
		slog.Error("command failed", "err", err)
		return
	}

	slog.Info("backup is successful")
}

func getLatestBackupTime(backupsDir string) (*time.Time, error) {
	entries, err := os.ReadDir(backupsDir)
	if err != nil {
		return nil, err
	}

	var maxTs *time.Time

	for _, entry := range entries {
		if entry.IsDir() {
			continue
		}

		filename := entry.Name()
		fileTimestamp, err := time.Parse(FilenameTemplate, filename)
		if err != nil {
			continue
		}

		if maxTs == nil || fileTimestamp.After(*maxTs) {
			maxTs = &fileTimestamp
		}
	}

	return maxTs, nil
}

// Delete backups older than retention period.
func cleanup(backupsDir string, retention time.Duration) {
	entries, err := os.ReadDir(backupsDir)
	if err != nil {
		slog.Error("unable to list backup dir contents", "err", err)
		return
	}

	for _, entry := range entries {
		if entry.IsDir() {
			continue
		}

		filename := entry.Name()
		fileTimestamp, err := time.Parse(FilenameTemplate, filename)
		if err != nil {
			continue
		}

		if time.Since(fileTimestamp) <= retention {
			continue
		}

		fullpath := path.Join(backupsDir, filename)
		err = os.Remove(fullpath)
		if err != nil {
			slog.Error("unable to remove old backup", "path", fullpath, "err", err)
		}
	}
}

func sleepUntilNextBackup(lastBackupTime *time.Time, backupInterval time.Duration) {
	if lastBackupTime == nil {
		return
	}

	timeSinceLastBackup := time.Since(*lastBackupTime)
	if timeSinceLastBackup >= backupInterval {
		return
	}

	time.Sleep(backupInterval - timeSinceLastBackup)
}

func main() {
	backupsDir := getStrFromEnv("BACKUPS_DIR", ".")
	err := os.Mkdir(backupsDir, 0750)
	if err != nil && !errors.Is(err, fs.ErrExist) {
		slog.Error("unable to create backups directory", "err", err)
		os.Exit(-1)
	}

	pgdumpBin := getStrFromEnv("PGDUMP_BINARY", "pg_dumpall")

	var customArgs []string
	customArgsStr := getStrFromEnv("CUSTOM_ARGS", "")
	if customArgsStr != "" {
		customArgs = strings.Split(customArgsStr, " ")
	}

	retention, err := time.ParseDuration(getStrFromEnv("RETENTION", "168h"))
	if err != nil {
		slog.Error("retention is incorrect", "err", err)
		os.Exit(-1)
	}

	backupIntervalStr := getStrFromEnv("BACKUP_INTERVAL", "24h")
	backupInterval, err := time.ParseDuration(backupIntervalStr)
	if err != nil {
		slog.Error("backup interval is incorrect", "err", err)
		os.Exit(-1)
	}

	exit := make(chan os.Signal, 1)
	signal.Notify(exit, os.Interrupt, syscall.SIGTERM)

	lastTimestamp, err := getLatestBackupTime(backupsDir)
	if err != nil {
		slog.Error("unable to list existing backups", "err", err)
		os.Exit(-1)
	}

	sleepUntilNextBackup(lastTimestamp, backupInterval)

	backup(backupsDir, pgdumpBin, customArgs)
	cleanup(backupsDir, retention)

	timer := time.NewTicker(backupInterval)

forloop:
	for {
		select {
		case <-timer.C:
			backup(backupsDir, pgdumpBin, customArgs)
			cleanup(backupsDir, retention)
		case <-exit:
			break forloop
		}
	}
}