package Conversions;

import java.util.Arrays;
import java.util.HashSet;
import java.util.InputMismatchException;
import java.util.Scanner;

/**
 * Class for converting from "any" base to "any" other base, when "any" means from 2-36.
 * Works by going from base 1 to decimal to base 2. Includes auxiliary method for
 * determining whether a number is valid for a given base.
 *
 * @author Michael Rolland
 * @version 2017年10月10日
 */
public class AnyBaseToAnyBase {

 /**
 * Smallest and largest base you want to accept as valid input
 */
 static final int MINIMUM_BASE = 2;
 static final int MAXIMUM_BASE = 36;

 public static void main(String[] args) {
 Scanner in = new Scanner(System.in);
 String n;
 int b1, b2;
 while (true) {
 try {
 System.out.print("Enter number: ");
 n = in.next();
 System.out.print("Enter beginning base (between " + MINIMUM_BASE + " and " + MAXIMUM_BASE + "): ");
 b1 = in.nextInt();
 if (b1 > MAXIMUM_BASE || b1 < MINIMUM_BASE) {
 System.out.println("Invalid base!");
 continue;
 }
 if (!validForBase(n, b1)) {
 System.out.println("The number is invalid for this base!");
 continue;
 }
 System.out.print("Enter end base (between " + MINIMUM_BASE + " and " + MAXIMUM_BASE + "): ");
 b2 = in.nextInt();
 if (b2 > MAXIMUM_BASE || b2 < MINIMUM_BASE) {
 System.out.println("Invalid base!");
 continue;
 }
 break;
 } catch (InputMismatchException e) {
 System.out.println("Invalid input.");
 in.next();
 }
 }
 System.out.println(base2base(n, b1, b2));
 in.close();
 }

 /**
 * Checks if a number (as a String) is valid for a given base.
 */
 public static boolean validForBase(String n, int base) {
 char[] validDigits = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E',
 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V',
 'W', 'X', 'Y', 'Z'};
 // digitsForBase contains all the valid digits for the base given
 char[] digitsForBase = Arrays.copyOfRange(validDigits, 0, base);

 // Convert character array into set for convenience of contains() method
 HashSet<Character> digitsList = new HashSet<>();
 for (int i = 0; i < digitsForBase.length; i++)
 digitsList.add(digitsForBase[i]);

 // Check that every digit in n is within the list of valid digits for that base.
 for (char c : n.toCharArray())
 if (!digitsList.contains(c))
 return false;

 return true;
 }

 /**
 * Method to convert any integer from base b1 to base b2. Works by converting from b1 to decimal,
 * then decimal to b2.
 *
 * @param n The integer to be converted.
 * @param b1 Beginning base.
 * @param b2 End base.
 * @return n in base b2.
 */
 public static String base2base(String n, int b1, int b2) {
 // Declare variables: decimal value of n,
 // character of base b1, character of base b2,
 // and the string that will be returned.
 int decimalValue = 0, charB2;
 char charB1;
 String output = "";
 // Go through every character of n
 for (int i = 0; i < n.length(); i++) {
 // store the character in charB1
 charB1 = n.charAt(i);
 // if it is a non-number, convert it to a decimal value >9 and store it in charB2
 if (charB1 >= 'A' && charB1 <= 'Z')
 charB2 = 10 + (charB1 - 'A');
 // Else, store the integer value in charB2
 else
 charB2 = charB1 - '0';
 // Convert the digit to decimal and add it to the
 // decimalValue of n
 decimalValue = decimalValue * b1 + charB2;
 }

 // Converting the decimal value to base b2:
 // A number is converted from decimal to another base
 // by continuously dividing by the base and recording
 // the remainder until the quotient is zero. The number in the
 // new base is the remainders, with the last remainder
 // being the left-most digit.
 if (0 == decimalValue)
 	return "0";
 // While the quotient is NOT zero:
 while (decimalValue != 0) {
 // If the remainder is a digit < 10, simply add it to
 // the left side of the new number.
 if (decimalValue % b2 < 10)
 output = Integer.toString(decimalValue % b2) + output;
 // If the remainder is >= 10, add a character with the
 // corresponding value to the new number. (A = 10, B = 11, C = 12, ...)
 else
 output = (char) ((decimalValue % b2) + 55) + output;
 // Divide by the new base again
 decimalValue /= b2;
 }
 return output;
 }
}