SixBit.java

package au.gov.amsa.util;

public final class SixBit {

	/** Precompiled list of int to six bit mappings. */
	private static final int[] INT_TO_SIX_BIT = createIntToSixBit();

	private static int[] createIntToSixBit() {
		int[] toSixbit = new int[256 * 256]; // we actually only use 256, but we
		                                     // parse chars around instead of
		                                     // bytes
		for (int chr = 0; chr < toSixbit.length; chr++) {
			if (chr < 48 || chr > 119 || chr > 87 && chr < 96) {
				toSixbit[chr] = -1;
			} else if (chr < 0x60) {
				toSixbit[chr] = chr - 48 & 0x3F;
			} else {
				toSixbit[chr] = chr - 56 & 0x3F;
			}
		}
		return toSixbit;
	}

	/**
	 * Converts sixBit string characters to bits (boolean values in the array
	 * bitSet) but just between the bit range {@code from} to {@code to}
	 * exclusive.
	 * 
	 * @param str
	 * @param padBits
	 * @param bitSet
	 * @param calculated
	 * @param from
	 * @param to
	 */
	public static void convertSixBitToBits(String str, int padBits, boolean bitSet[],
	        boolean[] calculated, int from, int to) {
		if (str.length() == 0)
			return;
		int index = from - from % 6;
		int strFrom = from / 6;
		int slen = str.length() - 1;
		int strTo = to / 6 + 1;
		for (int i = strFrom; i < Math.min(strTo, slen); i++) {
			if (!calculated[i]) {
				char chr = str.charAt(i);
				int binVal = INT_TO_SIX_BIT[chr];
				if (binVal == -1) {
					throw new SixBitException("Illegal sixbit ascii char: " + chr);
				}
				bitSet[index] = (binVal & 32) > 0;
				bitSet[index + 1] = (binVal & 16) > 0;
				bitSet[index + 2] = (binVal & 8) > 0;
				bitSet[index + 3] = (binVal & 4) > 0;
				bitSet[index + 4] = (binVal & 2) > 0;
				bitSet[index + 5] = (binVal & 1) > 0;

				calculated[i] = true;
			}

			index += 6;
		}
		if (strTo > slen) {
			// Process the last char which might be padded
			char chr = str.charAt(slen);
			int binVal = INT_TO_SIX_BIT[chr];
			if (binVal == -1) {
				throw new SixBitException("Illegal sixbit ascii char: " + chr);
			}
			int bits = 6 - padBits;

			switch (bits) {
			case 6:
				bitSet[index + 5] = (binVal & 1) > 0;
			case 5:
				bitSet[index + 4] = (binVal & 2) > 0;
			case 4:
				bitSet[index + 3] = (binVal & 4) > 0;
			case 3:
				bitSet[index + 2] = (binVal & 8) > 0;
			case 2:
				bitSet[index + 1] = (binVal & 16) > 0;
			case 1:
				bitSet[index] = (binVal & 32) > 0;
			}

			calculated[slen] = true;
		}
	}

	public static long getValue(int from, int to, boolean[] bitSet) {
		if (to > bitSet.length) {
			throw new SixBitException(bitSet.length + " is not enough bits. At least " + to
			        + " expected.");
		}
		long val = 0;
		long powMask = 1;
		for (int i = to - 1; i >= from; i--) {
			if (bitSet[i]) {
				val += powMask;
			}
			powMask <<= 1;
		}
		return val;
	}

	public static long getSignedValue(int from, int to, boolean[] bitSet) {
		if (to > bitSet.length) {
			throw new SixBitException(bitSet.length + " is not enough bits. At least " + to
			        + " expected.");
		}
		long val = 0;
		long powMask = 1;

		for (int i = to - 1; i >= from; i--) {
			if (bitSet[i]) {
				val += powMask;
			}
			powMask <<= 1;
		}
		if (bitSet[from]) {
			val = val - powMask;
		}
		return val;
	}

	public static String getString(int from, int to, boolean[] bitSet) {
		int len = (to - from) / 6;
		char[] resStr = new char[len];
		int pos = from;
		int i;
		for (i = 0; i < len; i++) {
			// Note that SixBit.getValue() should never return a value > 63
			// because it is only using 6 bits so intToAscii should never throw
			// a SixBitException.
			char ch = (char) intToAscii((char) SixBit.getValue(pos, pos + 6, bitSet));
			// stops at the first instance of @ character
			if (ch == '@') {
				len = i;
				break;
			}
			resStr[i] = ch;
			pos += 6;
		}
		// remove trailing @ characters and spaces
		while (len > 0 && (resStr[len - 1] == ' '))
			len -= 1;
		return new String(resStr, 0, len);
	}

	/**
	 * Convert six bit int value to character
	 *
	 * @param val
	 * @return
	 */
	private static int intToAscii(int val) {
		if (val > 63) {
			throw new SixBitException("Char value " + val + " not allowed");
		} else if (val < 32) {
			return val + 64;
		} else {
			return val;
		}
	}

}