mathtools/cmd/modInverse.go

/*
Copyright © 2025 filifa

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package cmd

import (
	"fmt"
	"math/big"

	"github.com/spf13/cobra"
)

var modInverseG string
var modInverseM string

func modInverse(cmd *cobra.Command, args []string) {
	g, ok := new(big.Int).SetString(modInverseG, 10)
	if !ok {
		cobra.CheckErr("invalid input " + modInverseG)
	}

	n, ok := new(big.Int).SetString(modInverseM, 10)
	if !ok {
		cobra.CheckErr("invalid input " + modInverseM)
	}

	z := new(big.Int)
	res := z.ModInverse(g, n)
	if res == nil {
		cobra.CheckErr("cannot find a modular inverse: " + modInverseG + " is not relatively prime to modulus " + modInverseM)
	} else {
		fmt.Println(z)
	}
}

// modInverseCmd represents the modInverse command
var modInverseCmd = &cobra.Command{
	Use:   "mod-inverse -g N -m N",
	Short: "Compute a modular inverse",
	Long: `Compute a modular inverse.

Given a base g and modulus m, compute a value x such that gx = 1 (mod m). A solution only exists if g and m are coprime.`,
	Run: modInverse,
}

func init() {
	rootCmd.AddCommand(modInverseCmd)

	// Here you will define your flags and configuration settings.

	// Cobra supports Persistent Flags which will work for this command
	// and all subcommands, e.g.:
	// modInverseCmd.PersistentFlags().String("foo", "", "A help for foo")

	// Cobra supports local flags which will only run when this command
	// is called directly, e.g.:
	// modInverseCmd.Flags().BoolP("toggle", "t", false, "Help message for toggle")

	modInverseCmd.Flags().StringVarP(&modInverseG, "base", "g", "", "integer to compute modular inverse of")
	modInverseCmd.MarkFlagRequired("base")

	modInverseCmd.Flags().StringVarP(&modInverseM, "modulus", "m", "", "modulus")
	modInverseCmd.MarkFlagRequired("modulus")
}