Base91.java

package cryptography.encoding.base91;

import java.io.ByteArrayOutputStream;

import cryptography.Mode;

public class Base91 {

	private static final byte[] ENCODING_TABLE;
	private static final byte[] DECODING_TABLE;
	private static final int BASE;
	private static final float AVERAGE_ENCODING_RATIO = 1.2297f;

	public static void main(String[] args) {
	}

	static {
		String ts = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!#$%&()*+,./:;<=>?@[]^_`{|}~\"";
		ENCODING_TABLE = ts.getBytes();
		BASE = ENCODING_TABLE.length;
		DECODING_TABLE = new byte[256];
		for (int i = 0; i < 256; ++i) {
			DECODING_TABLE[i] = -1;
		}
		for (int i = 0; i < BASE; ++i) {
			DECODING_TABLE[ENCODING_TABLE[i]] = (byte) i;
		}
	}

	public static String base91(String input, Mode mode) {

		StringBuilder sb = new StringBuilder();
		byte[] data = input.getBytes();

		if (mode == Mode.ENCODE) {
			try {
				int estimatedSize = (int) Math.ceil(data.length * AVERAGE_ENCODING_RATIO);
				ByteArrayOutputStream output = new ByteArrayOutputStream(estimatedSize);
				int ebq = 0;
				int en = 0;
				for (int i = 0; i < data.length; ++i) {
					ebq |= (data[i] & 255) << en;
					en += 8;
					if (en > 13) {
						int ev = ebq & 8191;
						if (ev > 88) {
							ebq >>= 13;
							en -= 13;
						} else {
							ev = ebq & 16383;
							ebq >>= 14;
							en -= 14;
						}
						output.write(ENCODING_TABLE[ev % BASE]);
						output.write(ENCODING_TABLE[ev / BASE]);
					}
				}
				if (en > 0) {
					output.write(ENCODING_TABLE[ebq % BASE]);
					if (en > 7 || ebq > 90) {
						output.write(ENCODING_TABLE[ebq / BASE]);
					}
				}

				sb.append(new String(output.toByteArray()));
			} catch (ArrayIndexOutOfBoundsException e) {
				return "Error; " + e.toString();
			}
		}

		if (mode == Mode.DECODE) {
			try {
				int dbq = 0;
				int dn = 0;
				int dv = -1;
				int estimatedSize = Math.round(data.length / AVERAGE_ENCODING_RATIO);
				ByteArrayOutputStream output = new ByteArrayOutputStream(estimatedSize);
				for (int i = 0; i < data.length; ++i) {
					if (DECODING_TABLE[data[i]] == -1)
						continue;
					if (dv == -1)
						dv = DECODING_TABLE[data[i]];
					else {
						dv += DECODING_TABLE[data[i]] * BASE;
						dbq |= dv << dn;
						dn += (dv & 8191) > 88 ? 13 : 14;
						do {
							output.write((byte) dbq);
							dbq >>= 8;
							dn -= 8;
						} while (dn > 7);
						dv = -1;
					}
				}
				if (dv != -1) {
					output.write((byte) (dbq | dv << dn));
				}

				sb.append(new String(output.toByteArray()));
			} catch (ArrayIndexOutOfBoundsException e) {
				return "Error; " + e.toString();
			}
		}

		return sb.toString();
	}

}