take numbered files, sort them, assure equal length, rename (made for ffmpeg frames)

shifd.go

package main

import (
	"errors"
	"fmt"
	"os"
	"slices"
	"strconv"
	"strings"
)

func Rename(oldName, newName string, dryRun bool) error {
	if dryRun {
		_, _ = os.Stderr.WriteString(fmt.Sprintf("rename %s -> %s\n", oldName, newName))
		if _, err := os.Stat(oldName); err != nil {
			os.Stderr.WriteString(err.Error() + "\n")
		}
		return nil
	}
	err := os.Rename(oldName, newName)
	if err != nil {
		return err
	}
	if _, err = os.Stat(newName); err != nil {
		return err
	}
	return nil
}

func usage(err ...error) {
	if len(err) == 1 {
		_, _ = os.Stderr.WriteString(err[0].Error() + "\n")
		defer func() {
			os.Exit(1)
		}()
	}
	if err == nil || len(err) == 0 {
		defer func() {
			os.Exit(0)
		}()
	}
	_, _ = os.Stderr.WriteString("\n" +
		"syntax:\n\t" + os.Args[0] + " [-nh] <prefix> <suffix> [start]\n\n" +
		"\t-n\tdry run\n" +
		"\t-h\thelp\n\n" +
		"example:\n\t" + os.Args[0] + " 'img_' '.png' 1\n\n",
	)
}

func RenumberFilesInDir(path string, prefix string, suffix string, start int, dryRun bool) error {
	var err error
	var fm []os.DirEntry

	fm, err = os.ReadDir(path)
	if err != nil {
		_, _ = os.Stderr.WriteString(err.Error())
		return err
	}

	oldNums := make([]int, 0, len(fm))

	maxLen := 0

	numsToNames := make(map[int]string)

	for _, f := range fm {
		key := f.Name()
		if prefix != "" && !strings.HasPrefix(key, prefix) {
			continue
		}
		if suffix != "" && !strings.HasSuffix(key, suffix) {
			continue
		}
		numString := strings.TrimPrefix(key, prefix)
		numString = strings.TrimSuffix(numString, suffix)
		num, err := strconv.Atoi(numString)
		if err != nil {
			return err
		}
		oldNums = append(oldNums, num)
		numsToNames[num] = key
		if len(numString) > maxLen {
			maxLen = len(numString)
		}
	}

	slices.Sort(oldNums)

	for i := start; i != len(oldNums)-1; i++ {
		newFile := fmt.Sprintf("%s%0*d%s", prefix, maxLen, i, suffix)
		Rename(numsToNames[oldNums[i]], newFile, dryRun)
	}

	return nil
}

func main() {
	var err error

	dryRun := false
	var args = make([]string, 0, len(os.Args))

	for i, arg := range os.Args {
		switch {
		case i == 0:
			continue
		case arg == "-h":
			usage()
			return
		case arg == "--dry-run", arg == "-n":
			dryRun = true
			_, _ = os.Stderr.WriteString(
				"dry run enabled, no file operations will occur\n",
			)
		default:
			args = append(args, arg)
		}
	}

	start := 1
	if len(args) > 2 {
		start, err = strconv.Atoi(args[2])
		if err != nil {
			usage(err)
		}
	}

	if len(args) < 2 {
		usage(errors.New("bad syntax: not enough arguments"))
	}

	if err = RenumberFilesInDir("./", args[0], args[1], start, dryRun); err != nil {
		_, _ = os.Stderr.WriteString(err.Error())
		return
	}
}