package main

import (
	"bufio"
	"flag"
	"fmt"
	"io"
	"os"
	"strconv"
	"strings"
	"unicode"
)

// Function to decode number pairs into characters
func decodePair(pair string) rune {
	alphabet := " ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
	num, err := strconv.Atoi(pair)
	if err != nil || num < 0 || num >= len(alphabet) {
		return rune(-1) // Return -1 if pair is not valid
	}
	return rune(alphabet[num])
}

// Function to apply the reverse shift and wrap around
func applyReverseShift(value, shift int) int {
	shifted := (value - shift) % 37
	if shifted < 0 {
		shifted += 37
	}
	return shifted
}

func main() {
	// Define command-line flags
	shift := flag.Int("s", 0, "Shift amount for the cipher")

	flag.Usage = func() {
		fmt.Fprintf(os.Stderr, "Usage: %s [options] <<< \"String\"\n", os.Args[0])
		fmt.Fprintf(os.Stderr, "       %s [options] < file.txt\n", os.Args[0])
		fmt.Fprintf(os.Stderr, "       echo \"string\" | %s [options]\n", os.Args[0])
		fmt.Fprintf(os.Stderr, "\nDescription:\n")
		fmt.Fprintf(os.Stderr, "  A companion program to numberstation, intended to make easy work of decrypting very basic ciphers for numbers station projects using the logic of nsencode.\n")
		fmt.Fprintf(os.Stderr, "\nOptions:\n")
		flag.PrintDefaults() // Print the default flag help information
		fmt.Fprintf(os.Stderr, "\nExamples:\n")
		fmt.Fprintf(os.Stderr, "  %s -s 2 <<< \"22101 12102 11210 20302 22072 122\"\n", os.Args[0])
		fmt.Fprintf(os.Stderr, "  %s -s 16 < file.txt\n", os.Args[0])
	}

	flag.Parse()

	// Check if there is input from stdin
	info, _ := os.Stdin.Stat()
	if (info.Mode() & os.ModeCharDevice) != 0 {
		flag.Usage()
		os.Exit(1)
	}

	// Read all input from stdin
	reader := bufio.NewReader(os.Stdin)
	inputBytes, _ := io.ReadAll(reader)
	input := string(inputBytes)

	// Remove all spaces and line breaks
	input = strings.ReplaceAll(input, " ", "")
	input = strings.ReplaceAll(input, "\n", "")
	input = strings.TrimSpace(input)

	var decoded strings.Builder

	// Decode the input string
	for i := 0; i < len(input); i += 2 {
		if i+1 >= len(input) {
			break // If there's an odd number of characters, break out
		}

		pair := input[i : i+2]
		num, err := strconv.Atoi(pair)
		if err != nil {
			continue // Skip invalid pairs
		}

		shiftedValue := applyReverseShift(num, *shift)
		char := decodePair(fmt.Sprintf("%02d", shiftedValue))
		if !unicode.IsPrint(char) || char == rune(-1) {
			continue // Skip invalid characters
		}

		decoded.WriteRune(char)
	}

	// Output the decoded string
	fmt.Println(decoded.String())
}