#!/usr/bin/env python3

# rot - encode/decode using rot13, rot18 or rot47

# Copyright (c) 2026 640kb.neocities.org
# This script is licensed under the Blue Oak Model License 1.0.0.
# https://blueoakcouncil.org/license/1.0.0
#
# 05aug2026: Initial release

import sys
import argparse

if sys.version_info < (3, 6):
    sys.exit("Error: This script requires Python 3.6 or newer.")

def show_help():
    print("""
rot - encode/decode using rot13, rot18 or rot47. The script can read from a
      file or from standard input (stdin) - ideal for pipelines.

USAGE:
    rot -r13 [file]     ROT13 (rotates letters by 13 positions)
    rot -r18 [file]     ROT18 (ROT13 on letters + ROT5 on digits)
    rot -r47 [file]     ROT47 (rotates letters, numbers and punctuation)
    rot -h | --help     Show this help

EXAMPLES:
    # encode/decode a file to stdout
    rot -r13 file.txt
    rot -r13 file.txt > out.txt

    # read from stdin (to stdout)
    rot -r18 < file.txt
    rot -r18 < file.txt > out.txt

    # read from stdin via pipe (outputs to stdout)
    cat file.txt | rot -r47
    cat file.txt | rot -r47 > out.txt

    # read as stdin from finger, outputs encode/decode to stdout
    finger user@domain.com | rot -r13

""")


def rot13(text):
    result = []
    for char in text:
        if 'a' <= char <= 'z':
            result.append(chr((ord(char) - ord('a') + 13) % 26 + ord('a')))
        elif 'A' <= char <= 'Z':
            result.append(chr((ord(char) - ord('A') + 13) % 26 + ord('A')))
        else:
            result.append(char)
    return ''.join(result)

def rot18(text):
    result = []
    for char in text:
        if 'a' <= char <= 'z':
            result.append(chr((ord(char) - ord('a') + 13) % 26 + ord('a')))
        elif 'A' <= char <= 'Z':
            result.append(chr((ord(char) - ord('A') + 13) % 26 + ord('A')))
        elif '0' <= char <= '9':
            result.append(chr((ord(char) - ord('0') + 5) % 10 + ord('0')))
        else:
            result.append(char)
    return ''.join(result)

def rot47(text):
    result = []
    for char in text:
        if 33 <= ord(char) <= 126:
            result.append(chr(33 + ((ord(char) - 33 + 47) % 94)))
        else:
            result.append(char)
    return ''.join(result)

def main():

    if len(sys.argv) == 1 or sys.argv[1] in ("-h", "--help"):
        show_help()
        sys.exit(0)

    parser = argparse.ArgumentParser(description='ROT cipher encoder/decoder', add_help=False)
    parser.add_argument('file', nargs='?', help='Input file (optional, reads from stdin if not provided)')
    group = parser.add_mutually_exclusive_group(required=True)
    group.add_argument('-r13', action='store_true', help='ROT13 encode/decode')
    group.add_argument('-r18', action='store_true', help='ROT18 encode/decode')
    group.add_argument('-r47', action='store_true', help='ROT47 encode/decode')
    
    args = parser.parse_args()
    
    # Read input from file or stdin
    if args.file:
        try:
            with open(args.file, 'r', encoding='utf-8') as f:
                text = f.read()
        except FileNotFoundError:
            print(f"Error: File '{args.file}' not found", file=sys.stderr)
            sys.exit(1)
        except Exception as e:
            print(f"Error reading file: {e}", file=sys.stderr)
            sys.exit(1)
    else:
        # Check if stdin is a terminal (no piped input)
        if sys.stdin.isatty():
            show_help()
            sys.exit(1)
        text = sys.stdin.read()
    
    if args.r13:
        output = rot13(text)
    elif args.r18:
        output = rot18(text)
    elif args.r47:
        output = rot47(text)
    
    sys.stdout.write(output)

if __name__ == '__main__':
    main()