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Dahua ICC Open API SDK for Java
There is a newer version: 1.0.13.7
package com.dahuatech.hutool.core.codec;

import com.dahuatech.hutool.core.util.ArrayUtil;

import java.io.ByteArrayOutputStream;
import java.io.Serializable;

/**
 * Base62编码解码实现，常用于短URL

 * From https://github.com/seruco/base62
 *
 * @author Looly, Sebastian Ruhleder, [email protected]
 * @since 4.5.9
 */
public class Base62Codec implements Serializable {
  private static final long serialVersionUID = 1L;

  private static final int STANDARD_BASE = 256;
  private static final int TARGET_BASE = 62;

  /** GMP风格 */
  private static final byte[] GMP = { //
    '0', '1', '2', '3', '4', '5', '6', '7', //
    '8', '9', 'A', 'B', 'C', 'D', 'E', 'F', //
    'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', //
    'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', //
    'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', //
    'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', //
    'm', 'n', 'o', 'p', 'q', 'r', 's', 't', //
    'u', 'v', 'w', 'x', 'y', 'z' //
  };

  /** 反转风格，即将GMP风格中的大小写做转换 */
  private static final byte[] INVERTED = { //
    '0', '1', '2', '3', '4', '5', '6', '7', //
    '8', '9', 'a', 'b', 'c', 'd', 'e', 'f', //
    'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', //
    'o', 'p', 'q', 'r', 's', 't', 'u', 'v', //
    'w', 'x', 'y', 'z', 'A', 'B', 'C', 'D', //
    'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', //
    'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', //
    'U', 'V', 'W', 'X', 'Y', 'Z' //
  };
  private final byte[] alphabet;
  private final byte[] lookup;

  /**
   * 构造
   *
   * @param alphabet 自定义字母表
   */
  public Base62Codec(byte[] alphabet) {
    this.alphabet = alphabet;
    lookup = new byte[256];
    for (int i = 0; i < alphabet.length; i++) {
      lookup[alphabet[i]] = (byte) (i & 0xFF);
    }
  }

  /**
   * 创建GMP风格的Base62编码解码器对象
   *
   * @return {@link Base62Codec}
   */
  public static Base62Codec createGmp() {
    return new Base62Codec(GMP);
  }

  /**
   * 创建Inverted风格的Base62编码解码器对象
   *
   * @return {@link Base62Codec}
   */
  public static Base62Codec createInverted() {
    return new Base62Codec(INVERTED);
  }

  /**
   * 编码指定消息bytes为Base62格式的bytes
   *
   * @param message 被编码的消息
   * @return Base62内容
   */
  public byte[] encode(byte[] message) {
    final byte[] indices = convert(message, STANDARD_BASE, TARGET_BASE);
    return translate(indices, alphabet);
  }

  /**
   * 解码Base62消息
   *
   * @param encoded Base62内容
   * @return 消息
   */
  public byte[] decode(byte[] encoded) {
    final byte[] prepared = translate(encoded, lookup);
    return convert(prepared, TARGET_BASE, STANDARD_BASE);
  }

  // --------------------------------------------------------------------------------------------------------------- Private method start
  /**
   * 按照字典转换bytes
   *
   * @param indices 内容
   * @param dictionary 字典
   * @return 转换值
   */
  private byte[] translate(byte[] indices, byte[] dictionary) {
    final byte[] translation = new byte[indices.length];

    for (int i = 0; i < indices.length; i++) {
      translation[i] = dictionary[indices[i]];
    }

    return translation;
  }

  /**
   * 使用定义的字母表从源基准到目标基准
   *
   * @param message 消息bytes
   * @param sourceBase 源基准长度
   * @param targetBase 目标基准长度
   * @return 计算结果
   */
  private byte[] convert(byte[] message, int sourceBase, int targetBase) {
    // 计算结果长度，算法来自：http://codegolf.stackexchange.com/a/21672
    final int estimatedLength = estimateOutputLength(message.length, sourceBase, targetBase);

    final ByteArrayOutputStream out = new ByteArrayOutputStream(estimatedLength);

    byte[] source = message;

    while (source.length > 0) {
      final ByteArrayOutputStream quotient = new ByteArrayOutputStream(source.length);

      int remainder = 0;

      for (int i = 0; i < source.length; i++) {
        final int accumulator = (source[i] & 0xFF) + remainder * sourceBase;
        final int digit = (accumulator - (accumulator % targetBase)) / targetBase;

        remainder = accumulator % targetBase;

        if (quotient.size() > 0 || digit > 0) {
          quotient.write(digit);
        }
      }

      out.write(remainder);

      source = quotient.toByteArray();
    }

    // pad output with zeroes corresponding to the number of leading zeroes in the message
    for (int i = 0; i < message.length - 1 && message[i] == 0; i++) {
      out.write(0);
    }

    return ArrayUtil.reverse(out.toByteArray());
  }

  /**
   * 估算结果长度
   *
   * @param inputLength 输入长度
   * @param sourceBase 源基准长度
   * @param targetBase 目标基准长度
   * @return 估算长度
   */
  private int estimateOutputLength(int inputLength, int sourceBase, int targetBase) {
    return (int) Math.ceil((Math.log(sourceBase) / Math.log(targetBase)) * inputLength);
  }
  // --------------------------------------------------------------------------------------------------------------- Private method end
}