/*
 * Copyright (C) 2012 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.android.apksig.internal.util;

import java.nio.ByteBuffer;

/**
 * Hexadecimal encoding where each byte is represented by two hexadecimal digits.
 */
public class HexEncoding {

    /** Hidden constructor to prevent instantiation. */
    private HexEncoding() {}

    private static final char[] HEX_DIGITS = "0123456789abcdef".toCharArray();

    /**
     * Encodes the provided data as a hexadecimal string.
     */
    public static String encode(byte[] data) {
        return encode(data, 0, data.length);
    }

    /**
     * Encodes the provided data as a hexadecimal string.
     */
    public static String encode(byte[] data, int offset, int len) {
      StringBuilder result = new StringBuilder(len * 2);
      for (int i = 0; i < len; i++) {
          byte b = data[offset + i];
          result.append(HEX_DIGITS[(b >>> 4) & 0x0f]);
          result.append(HEX_DIGITS[b & 0x0f]);
      }
      return result.toString();
    }

    /**
     * Encodes the provided data as a hexadecimal string.
     */
    public static String encode(ByteBuffer buf) {
        return encode(buf.array(), buf.arrayOffset() + buf.position(), buf.remaining());
    }

    /**
     * Decodes the provided hexadecimal string into an array of bytes.
     */
    public static byte[] decode(String encoded) {
        // IMPLEMENTATION NOTE: Special care is taken to permit odd number of hexadecimal digits.
        int resultLengthBytes = (encoded.length() + 1) / 2;
        byte[] result = new byte[resultLengthBytes];
        int resultOffset = 0;
        int encodedCharOffset = 0;
        if ((encoded.length() % 2) != 0) {
            // Odd number of digits -- the first digit is the lower 4 bits of the first result byte.
            result[resultOffset++] =
                    (byte) getHexadecimalDigitValue(encoded.charAt(encodedCharOffset));
            encodedCharOffset++;
        }
        for (int len = encoded.length(); encodedCharOffset < len; encodedCharOffset += 2) {
          result[resultOffset++] = (byte)
                  ((getHexadecimalDigitValue(encoded.charAt(encodedCharOffset)) << 4)
                          | getHexadecimalDigitValue(encoded.charAt(encodedCharOffset + 1)));
        }
        return result;
    }

    private static int getHexadecimalDigitValue(char c) {
        if ((c >= 'a') && (c <= 'f')) {
            return (c - 'a') + 0x0a;
        } else if ((c >= 'A') && (c <= 'F')) {
            return (c - 'A') + 0x0a;
        } else if ((c >= '0') && (c <= '9')) {
            return c - '0';
        } else {
            throw new IllegalArgumentException(
                    "Invalid hexadecimal digit at position : '"
                            + c + "' (0x" + Integer.toHexString(c) + ")");
        }
    }
}