/*
 * Copyright (C) 2014 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 libcore.util;

/**
 * Hexadecimal encoding where each byte is represented by two hexadecimal digits.
 * @hide
 */
@libcore.api.CorePlatformApi(status = libcore.api.CorePlatformApi.Status.STABLE)
public class HexEncoding {

    private static final char[] LOWER_CASE_DIGITS = {
            '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'
    };

    private static final char[] UPPER_CASE_DIGITS = {
            '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'
    };

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

    /**
     * Encodes the provided byte as a two-digit hexadecimal String value.
     *
     * @param  b byte to encode
     * @param  upperCase {@code true} to use uppercase letters, {@code false}
     *         for lowercase
     * @return the encoded string
     */
    @libcore.api.CorePlatformApi(status = libcore.api.CorePlatformApi.Status.STABLE)
    public static String encodeToString(byte b, boolean upperCase) {
        char[] digits = upperCase ? UPPER_CASE_DIGITS : LOWER_CASE_DIGITS;
        char[] buf = new char[2]; // We always want two digits.
        buf[0] = digits[(b >> 4) & 0xf];
        buf[1] = digits[b & 0xf];
        return new String(buf, 0, 2);
    }

    /**
     * Encodes the provided data as a sequence of hexadecimal characters.
     *
     * @param  data byte array to encode
     * @return the encoded data, using uppercase letters
     */
    @libcore.api.CorePlatformApi(status = libcore.api.CorePlatformApi.Status.STABLE)
    public static char[] encode(byte[] data) {
        return encode(data, 0, data.length, true /* upperCase */);
    }

    /**
     * Encodes the provided data as a sequence of hexadecimal characters.
     *
     * @param  data byte array to encode
     * @param  upperCase {@code true} to use uppercase letters, {@code false}
     *         for lowercase
     * @return the encoded data
     */
    @libcore.api.CorePlatformApi(status = libcore.api.CorePlatformApi.Status.STABLE)
    public static char[] encode(byte[] data, boolean upperCase) {
        return encode(data, 0, data.length, upperCase);
    }

    /**
     * Encodes the provided data as a sequence of hexadecimal characters.
     *
     * @param  data byte array containing the data to encode
     * @param  offset offset of the data to encode in the {@code data} array
     * @param  len length of the data to encode in the {@code data} array
     * @return the encoded data, using uppercase letters
     */
    @libcore.api.CorePlatformApi(status = libcore.api.CorePlatformApi.Status.STABLE)
    public static char[] encode(byte[] data, int offset, int len) {
        return encode(data, offset, len, true /* upperCase */);
    }

    /**
     * Encodes the provided data as a sequence of hexadecimal characters.
     */
    private static char[] encode(byte[] data, int offset, int len, boolean upperCase) {
        char[] digits = upperCase ? UPPER_CASE_DIGITS : LOWER_CASE_DIGITS;
        char[] result = new char[len * 2];
        for (int i = 0; i < len; i++) {
            byte b = data[offset + i];
            int resultIndex = 2 * i;
            result[resultIndex] = (digits[(b >> 4) & 0x0f]);
            result[resultIndex + 1] = (digits[b & 0x0f]);
        }

        return result;
    }

    /**
     * Encodes the provided data as a sequence of hexadecimal characters.
     *
     * @param  data byte array to encode
     * @return the encoded data, using uppercase letters
     */
    @libcore.api.CorePlatformApi(status = libcore.api.CorePlatformApi.Status.STABLE)
    public static String encodeToString(byte[] data) {
        return encodeToString(data, true /* upperCase */);
    }

    /**
     * Encodes the provided data as a sequence of hexadecimal characters.
     *
     * @param  data byte array to encode.
     * @param  upperCase {@code true} to use uppercase letters, {@code false}
     *         for lowercase
     * @return the encoded data
     */
    @libcore.api.CorePlatformApi(status = libcore.api.CorePlatformApi.Status.STABLE)
    public static String encodeToString(byte[] data, boolean upperCase) {
        return new String(encode(data, upperCase));
    }

    /**
     * Decodes the provided hexadecimal sequence. Odd-length inputs are not
     * allowed.
     *
     * @param  encoded string of hexadecimal characters to decode. Letters
     *         can be either uppercase or lowercase.
     * @return the decoded data
     * @throws IllegalArgumentException if the input is malformed
     */
    @libcore.api.CorePlatformApi(status = libcore.api.CorePlatformApi.Status.STABLE)
    public static byte[] decode(String encoded) throws IllegalArgumentException {
        return decode(encoded.toCharArray());
    }

    /**
     * Decodes the provided hexadecimal sequence.
     *
     * @param  encoded string of hexadecimal characters to decode. Letters
     *         can be either uppercase or lowercase.
     * @param  allowSingleChar If {@code true} odd-length inputs are allowed and
     *         the first character is interpreted as the lower bits of the first
     *         result byte. If {@code false} odd-length inputs are not allowed.
     * @return the decoded data
     * @throws IllegalArgumentException if the input is malformed
     */
    @libcore.api.CorePlatformApi(status = libcore.api.CorePlatformApi.Status.STABLE)
    public static byte[] decode(String encoded, boolean allowSingleChar)
            throws IllegalArgumentException {
        return decode(encoded.toCharArray(), allowSingleChar);
    }

    /**
     * Decodes the provided hexadecimal sequence. Odd-length inputs are not
     * allowed.
     *
     * @param  encoded char array of hexadecimal characters to decode. Letters
     *         can be either uppercase or lowercase.
     * @return the decoded data
     * @throws IllegalArgumentException if the input is malformed
     */
    @libcore.api.CorePlatformApi(status = libcore.api.CorePlatformApi.Status.STABLE)
    public static byte[] decode(char[] encoded) throws IllegalArgumentException {
        return decode(encoded, false);
    }

    /**
     * Decodes the provided hexadecimal sequence.
     *
     * @param  encoded char array of hexadecimal characters to decode. Letters
     *         can be either uppercase or lowercase.
     * @param  allowSingleChar If {@code true} odd-length inputs are allowed and
     *         the first character is interpreted as the lower bits of the first
     *         result byte. If {@code false} odd-length inputs are not allowed.
     * @return the decoded data
     * @throws IllegalArgumentException if the input is malformed
     */
    @libcore.api.CorePlatformApi(status = libcore.api.CorePlatformApi.Status.STABLE)
    public static byte[] decode(char[] encoded, boolean allowSingleChar)
            throws IllegalArgumentException {
        int encodedLength = encoded.length;
        int resultLengthBytes = (encodedLength + 1) / 2;
        byte[] result = new byte[resultLengthBytes];

        int resultOffset = 0;
        int i = 0;
        if (allowSingleChar) {
            if ((encodedLength % 2) != 0) {
                // Odd number of digits -- the first digit is the lower 4 bits of the first result
                // byte.
                result[resultOffset++] = (byte) toDigit(encoded, i);
                i++;
            }
        } else {
            if ((encodedLength % 2) != 0) {
                throw new IllegalArgumentException("Invalid input length: " + encodedLength);
            }
        }

        for (; i < encodedLength; i += 2) {
            result[resultOffset++] = (byte) ((toDigit(encoded, i) << 4) | toDigit(encoded, i + 1));
        }

        return result;
    }

    private static int toDigit(char[] str, int offset) throws IllegalArgumentException {
        // NOTE: that this isn't really a code point in the traditional sense, since we're
        // just rejecting surrogate pairs outright.
        int pseudoCodePoint = str[offset];

        if ('0' <= pseudoCodePoint && pseudoCodePoint <= '9') {
            return pseudoCodePoint - '0';
        } else if ('a' <= pseudoCodePoint && pseudoCodePoint <= 'f') {
            return 10 + (pseudoCodePoint - 'a');
        } else if ('A' <= pseudoCodePoint && pseudoCodePoint <= 'F') {
            return 10 + (pseudoCodePoint - 'A');
        }

        throw new IllegalArgumentException("Illegal char: " + str[offset] + " at offset " + offset);
    }
}