package main

import (
	// "bufio"
	"fmt"
	"io"
	"os"
)

type CopyOp struct {
	Bytes            int64
	Original, Backup *os.File
}

func copy(src, dst string) (CopyOp, error) {
	var co CopyOp
	sourceFileStat, err := os.Stat(src)
	if err != nil {
		return co, err
	}

	if !sourceFileStat.Mode().IsRegular() {
		return co, fmt.Errorf("%s is not a regular file", src)
	}

	source, err := os.Open(src)
	if err != nil {
		return co, err
	}
	co.Original = source
	defer source.Close()

	destination, err := os.Create(dst)
	if err != nil {
		return co, err
	}
	defer destination.Close()
	co.Backup = destination

	nBytes, err := io.Copy(destination, source)
	co.Bytes = nBytes
	return co, err
}

func restoreBackup(fn string) error {
	err := os.Remove(fn)
	if err != nil {
		// removing the original has failed.
		return fmt.Errorf("error removing modified file[%s] : %w", fn, err)
	}

	// copy the backup back to the original
	_, err = copy(fn+".bak", fn)
	if err != nil {
		return fmt.Errorf("error copying backup into original location[%s]: %w", fn, err)
	}

	// remove the backup
	err = os.Remove(fn + ".bak")
	if err != nil {
		// removing the backup has failed.
		return fmt.Errorf("error removing backup file[%s] : %w", fn+".bak", err)
	}

	return nil
}

// Update modifies the contents of a file in-place.
func Update(fn string, newContents io.Reader) error {
	// create a backup copy
	copyOp, err := copy(fn, fn+".bak")
	if err != nil {
		return fmt.Errorf("error backing up file[%s]: %w", fn, err)
	}

	// truncate the original
	err = os.Truncate(fn, 0)
	if err != nil {
		err := restoreBackup(fn)
		if err != nil {
			return fmt.Errorf("error restoring backup: %w", err)
		}
		return fmt.Errorf("error truncating original file[%s]: %w", fn, err)
	}

	// copy new contents into original
	_, err = io.Copy(copyOp.Original, newContents)
	if err != nil {
		// the copy has failed. we need to remove the original
		err := restoreBackup(fn)
		if err != nil {
			return fmt.Errorf("error restoring backup: %w", err)
		}

		return fmt.Errorf("error writing new contents to file[%s] : %w", fn, err)
	}

	return nil
}