package nixfig

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"os/exec"
)

var (
	// Nix is the command to call for nix
	Nix            string
	ErrNixNotFound = errors.New("nix was not found or set. You can set it either with `nixfig.Nix` or the `NIXFIG_NIX` environment variable")
)

func init() {
	nixPath, _ := exec.LookPath("nix")
	Nix = nixPath
	if envPath, ok := os.LookupEnv("NIXFIG_NIX"); ok {
		Nix = envPath
	}
}

// Unmarshal unmarshals a nix expression as JSON into a struct
func Unmarshal(data []byte, v any) error {
	if Nix == "" {
		return ErrNixNotFound
	}

	var stdout, stderr bytes.Buffer
	cmd := exec.Command(Nix, "eval", "--json", "--expr", string(data))
	cmd.Stdout = &stdout
	cmd.Stderr = &stderr
	if err := cmd.Run(); err != nil {
		return fmt.Errorf("could not run %q: %w", Nix, err)
	}
	if stderr.Len() > 0 {
		return errors.New(stderr.String())
	}

	if err := json.Unmarshal(stdout.Bytes(), v); err != nil {
		return fmt.Errorf("could not unmarshal nix config as JSON: %w", err)
	}
	return nil
}

// Marshal marshals a struct into a nix expression
func Marshal(v any) ([]byte, error) {
	if Nix == "" {
		return nil, ErrNixNotFound
	}

	data, err := json.Marshal(v)
	if err != nil {
		return nil, fmt.Errorf("could not marshal JSON: %w", err)
	}

	var stdout, stderr bytes.Buffer
	cmd := exec.Command(Nix, "eval", "--expr", fmt.Sprintf(`(builtins.fromJSON %q)`, string(data)))
	cmd.Stdout = &stdout
	cmd.Stderr = &stderr
	if err := cmd.Run(); err != nil {
		return nil, fmt.Errorf("could not run %q: %w", Nix, err)
	}
	if stderr.Len() > 0 {
		return nil, errors.New(stderr.String())
	}

	return stdout.Bytes(), nil
}