A quick and dirty decompressor for Firefly Zero carts (Which are Zip files compressed using ZSTD) - https://docs.fireflyzero.com/internal/formats/#rom

unzipzstd.go

package main

import (
	"archive/zip"
	"flag"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"strings"

	"github.com/klauspost/compress/zstd"
)

func main() {
	if err := run(os.Args); err != nil {
		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
		os.Exit(1)
	}
}

func run(args []string) error {
	in, err := parse(args)
	if err != nil {
		return err
	}

	r, err := zip.OpenReader(in.name)
	if err != nil {
		return err
	}

	r.RegisterDecompressor(zstd.ZipMethodWinZip, zstd.ZipDecompressor())

	base := strings.TrimSuffix(in.name, ".zip")

	if err := os.Mkdir(base, 0o755); err != nil {
		return err
	}

	for _, f := range r.File {
		fn := filepath.Join(base, f.Name)

		ff, err := r.Open(f.Name)
		if err != nil {
			return err
		}

		data, err := io.ReadAll(ff)
		if err != nil {
			return err
		}

		if err := os.WriteFile(fn, data, 0o755); err != nil {
			return err
		}
	}

	return nil
}

type input struct {
	exe  string
	name string
}

func parse(args []string) (input, error) {
	var in input

	if len(args) == 0 {
		return in, fmt.Errorf("no arguments")
	}

	in.exe = args[0]

	flags := flag.NewFlagSet(in.exe, flag.ExitOnError)

	if err := flags.Parse(args[1:]); err != nil {
		return in, err
	}

	if rest := flags.Args(); len(rest) > 0 {
		in.name = rest[0]
	}

	if in.name == "" {
		return in, fmt.Errorf("no zip file specified")
	}

	return in, nil
}

Tip

Note that you can unpack a Firefly ROM using the firefly_cli

$ firefly_cli import lux.snek.zip 
new device name: goofy-ruff1es
⚠️  verification failed: read key from ROM
✅ installed: /home/peter/.local/share/firefly/roms/lux/snek