/* ====================================================================
 * Copyright (c) 2006 J.T. Beetstra
 *
 * Permission is hereby granted, free of charge, to any person obtaining 
 * a copy of this software and associated documentation files (the 
 * "Software"), to deal in the Software without restriction, including 
 * without limitation the rights to use, copy, modify, merge, publish, 
 * distribute, sublicense, and/or sell copies of the Software, and to 
 * permit persons to whom the Software is furnished to do so, subject to 
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be 
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 
 * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY 
 * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, 
 * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE 
 * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 * ====================================================================
 */
package com.beetstra.jutf7;

import java.util.Arrays;

/**
 * <p>Represent a base 64 mapping. The 64 characters used in the encoding can be specified, 
 * since modified-UTF-7 uses other characters than UTF-7 (',' instead of '/').</p>
 * 
 * <p>The exact type of the arguments and result values is adapted to the needs of the 
 * encoder and decoder, as opposed to following a strict interpretation of base 64.</p> 
 * <p>Base 64, as specified in RFC 2045, is an encoding used to encode bytes as characters. 
 * In (modified-)UTF-7 however, it is used to encode characters as bytes, using some 
 * intermediate steps:</p>
 * <ol>
 * <li>Encode all characters as a 16-bit (UTF-16) integer value</li>
 * <li>Write this as stream of bytes (most-significant first)</li> 
 * <li>Encode these bytes using (modified) base 64 encoding</li>
 * <li>Write the thus formed stream of characters as a stream of bytes, using ASCII encoding</li>
 * </ol>
 * 
 * @author Jaap Beetstra
 */
class Base64Util {
	private static final int ALPHABET_LENGTH = 64;
	private final char[] alphabet;
	private final int[] inverseAlphabet;

	/**
	 * Initializes the class with the specified encoding/decoding alphabet. 
	 * 
	 * @param alphabet
	 * @throws IllegalArgumentException if alphabet is not 64 characters long or 
	 *  contains characters which are not 7-bit ASCII
	 */
	Base64Util(final String alphabet) {
		this.alphabet = alphabet.toCharArray();
		if (alphabet.length() != ALPHABET_LENGTH)
			throw new IllegalArgumentException("alphabet has incorrect length (should be 64, not "
					+ alphabet.length() + ")");
		inverseAlphabet = new int[128];
		Arrays.fill(inverseAlphabet, -1);
		for (int i = 0; i < this.alphabet.length; i++) {
			final char ch = this.alphabet[i];
			if (ch >= 128)
				throw new IllegalArgumentException("invalid character in alphabet: " + ch);
			inverseAlphabet[ch] = i;
		}
	}

	/**
	 * Returns the integer value of the six bits represented by the specified character.
	 * 
	 * @param ch The character, as a ASCII encoded byte 
	 * @return The six bits, as an integer value, or -1 if the byte is not in the alphabet
	 */
	int getSextet(final byte ch) {
		if (ch >= 128)
			return -1;
		return inverseAlphabet[ch];
	}

	/**
	 * Tells whether the alphabet contains the specified character.
	 * 
	 * @param ch The character 
	 * @return true if the alphabet contains <code>ch</code>, false otherwise
	 */
	boolean contains(final char ch) {
		if (ch >= 128)
			return false;
		return inverseAlphabet[ch] >= 0;
	}

	/**
	 * Encodes the six bit group as a character.
	 * 
	 * @param sextet The six bit group to be encoded
	 * @return The ASCII value of the character
	 */
	byte getChar(final int sextet) {
		return (byte) alphabet[sextet];
	}
}
