NBitInteger.java

package com.firefly.codec.http2.hpack;

import java.nio.ByteBuffer;

public class NBitInteger {
	public static int octectsNeeded(int n, int i) {
		if (n == 8) {
			int nbits = 0xFF;
			i = i - nbits;
			if (i < 0)
				return 1;
			if (i == 0)
				return 2;
			int lz = Integer.numberOfLeadingZeros(i);
			int log = 32 - lz;
			return 1 + (log + 6) / 7;
		}

		int nbits = 0xFF >>> (8 - n);
		i = i - nbits;
		if (i < 0)
			return 0;
		if (i == 0)
			return 1;
		int lz = Integer.numberOfLeadingZeros(i);
		int log = 32 - lz;
		return (log + 6) / 7;
	}

	public static void encode(ByteBuffer buf, int n, int i) {
		if (n == 8) {
			if (i < 0xFF) {
				buf.put((byte) i);
			} else {
				buf.put((byte) 0xFF);

				int length = i - 0xFF;
				while (true) {
					if ((length & ~0x7F) == 0) {
						buf.put((byte) length);
						return;
					} else {
						buf.put((byte) ((length & 0x7F) | 0x80));
						length >>>= 7;
					}
				}
			}
		} else {
			int p = buf.position() - 1;
			int bits = 0xFF >>> (8 - n);

			if (i < bits) {
				buf.put(p, (byte) ((buf.get(p) & ~bits) | i));
			} else {
				buf.put(p, (byte) (buf.get(p) | bits));

				int length = i - bits;
				while (true) {
					if ((length & ~0x7F) == 0) {
						buf.put((byte) length);
						return;
					} else {
						buf.put((byte) ((length & 0x7F) | 0x80));
						length >>>= 7;
					}
				}
			}
		}
	}

	public static int decode(ByteBuffer buffer, int n) {
		if (n == 8) {
			int nbits = 0xFF;

			int i = buffer.get() & 0xff;

			if (i == nbits) {
				int m = 1;
				int b;
				do {
					b = 0xff & buffer.get();
					i = i + (b & 127) * m;
					m = m * 128;
				} while ((b & 128) == 128);
			}
			return i;
		}

		int nbits = 0xFF >>> (8 - n);

		int i = buffer.get(buffer.position() - 1) & nbits;

		if (i == nbits) {
			int m = 1;
			int b;
			do {
				b = 0xff & buffer.get();
				i = i + (b & 127) * m;
				m = m * 128;
			} while ((b & 128) == 128);
		}
		return i;
	}
}