import java.util.Scanner;
import java.io.PrintWriter;
import java.io.File;
import java.io.FileNotFoundException;

/**
 * Performs a traditional Caesar cipher using text files for input and output.
 * @author Sarah Larkin
 * CS3090, Spring 2018
 * Date LastModified: March 3, 2018
 *
 */
public class TraditionalCaesarCipher {

	public static void main(String[] args) {
		TraditionalCaesarCipher self = new TraditionalCaesarCipher();
		String original = self.readFile("TradCaesarCipherABC1Orig.txt");
		String encrypted = self.shiftCipher(original, 3);
		self.printFile(encrypted, "TradCaesarCipherABC1Coded.txt");
	}
	
	/**
	 * Reads in a file and returns its contents as a string
	 * @param filename the name of the file to read in, including its type
	 * @return the contents string
	 */
	public String readFile(String filename) {
		String input = "";
		try(Scanner scan = new Scanner(new File(filename))) {
			while(scan.hasNext()) {
				input += scan.nextLine() + "\n";
			}
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		}
		return input;
	}
	
	
	/**
	 * Performs a shift cipher on a string, using modulus to wrap around the 
	 * letters at the end of the alphabet.
	 * @param input the string to cipher
	 * @param shift the cipher amount
	 * @return the cipher shifted string
	 */
	public String shiftCipher(String input, int shift) {
		String coded = "";
		char [] message = input.toCharArray();
		for (int i = 0; i < message.length; i++) {
			char letter = message[i];
			letter = (char) cipherUppercase(letter, shift);
			letter = (char) cipherLowercase(letter, shift);
			message[i] = letter;
		}
		for (int i = 0; i < message.length; i++) {
			coded += message[i];
		}
		return coded;
	}
	
	
	/**
	 * Perform the cipher on an uppercase letter, keeping it in the letter range.
	 * If the character given is not an uppercase letter, return the character unchanged.
	 * @param letter 	the character to cipher
	 * @param shift		the amount by which to shift the letter
	 * @return code		the letter shifted
	 */
	private int cipherUppercase(char letter, int shift) {
		final int a = 65; // ASCII value for 'A'
		final int z = 90; // ASCII value for 'Z'
		int coded = letter;
		if ((a <= letter) && (letter <= z)) {
				// Get the value of the letter on a scale of 0 to 25
				int azLetter = letter - a; 
				int newLetter = azLetter + shift; // Add the shift
				int remainder = newLetter % 26; // Get the remainder
				coded = a + remainder; // Return to ASCII scale
		}
		return coded;
	}
	
	/**
	 * Perform the cipher on an lowercase letter, keeping it in the letter range.
	 * If the character given is not an lowercase letter, return the character unchanged.
	 * @param letter 	the character to cipher
	 * @param shift		the amount by which to shift the letter
	 * @return code		the letter shifted
	 */
	private int cipherLowercase(char letter, int shift) {
		final int a = 97; // ASCII value for 'a'
		final int z = 122; // ASCII value for 'z'
		int coded = letter;
		if ((a <= letter) && (letter <= z)) {
				// Get the value of the letter on a scale of 0 to 25
				int azLetter = letter - a; 
				int newLetter = azLetter + shift; // Add the shift
				int remainder = newLetter % 26; // Get the remainder
				coded = a + remainder; // Return to ASCII scale
		}
		return coded;
	}
	
	/**
	 * Prints a string to the specified file and the console.
	 * @param fileOutput 	the string to print
	 * @param filename 		the file to which to print the string
	 */
	public void printFile(String fileOutput, String filename) {
		try(PrintWriter print = new PrintWriter(new File(filename))) {
			print.print(fileOutput);
			System.out.println(fileOutput);
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		}
	}

}