package com.espressif.iot.esptouch.util;

import java.util.zip.Checksum;

public class CRC8 implements Checksum {

	private final short init;

	private static final short[] crcTable = new short[256];

	private short value;

	private static final short CRC_POLYNOM = 0x8c;

	private static final short CRC_INITIAL = 0x00;

	static {
		for (int dividend = 0; dividend < 256; dividend++) {
			int remainder = dividend;// << 8;
			for (int bit = 0; bit < 8; ++bit)
				if ((remainder & 0x01) != 0)
					remainder = (remainder >>> 1) ^ CRC_POLYNOM;
				else
					remainder >>>= 1;
			crcTable[dividend] = (short) remainder;
		}
	}

	public CRC8() {
		this.value = this.init = CRC_INITIAL;
	}

	@Override
	public void update(byte[] buffer, int offset, int len) {
		for (int i = 0; i < len; i++) {
			int data = buffer[offset + i] ^ value;
			value = (short) (crcTable[data & 0xff] ^ (value << 8));
		}
	}

	/**
	 * Updates the current checksum with the specified array of bytes.
	 * Equivalent to calling <code>update(buffer, 0, buffer.length)</code>.
	 * 
	 * @param buffer
	 *            the byte array to update the checksum with
	 */
	public void update(byte[] buffer) {
		update(buffer, 0, buffer.length);
	}

	@Override
	public void update(int b) {
		update(new byte[] { (byte) b }, 0, 1);
	}

	@Override
	public long getValue() {
		return value & 0xff;
	}

	@Override
	public void reset() {
		value = init;
	}

}