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A Java implementation of Concise Binary Object Representation (CBOR), a general-purpose binary data format defined in RFC 7049.
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package com.upokecenter.cbor;
/*
Written by Peter O. in 2014.
Any copyright is dedicated to the Public Domain.
http://creativecommons.org/publicdomain/zero/1.0/
If you like this, you should donate to Peter O.
at: http://peteroupc.github.io/
 */

  final class Base64 {
private Base64() {
}
    private static final String Base64URL =
      "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";

    private static final String Base64Classic =
      "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

    public static void WriteBase64(
      StringOutput writer,
      byte[] data,
      int offset,
      int count,
      boolean padding) throws java.io.IOException {
      WriteBase64(writer, data, offset, count, true, padding);
    }

    public static void WriteBase64URL(
      StringOutput writer,
      byte[] data,
      int offset,
      int count,
      boolean padding) throws java.io.IOException {
      WriteBase64(writer, data, offset, count, false, padding);
    }

    private static void WriteBase64(
      StringOutput writer,
      byte[] data,
      int offset,
      int count,
      boolean classic,
      boolean padding) throws java.io.IOException {
      if (writer == null) {
        throw new NullPointerException("writer");
      }
      if (offset < 0) {
        throw new IllegalArgumentException("offset (" + offset + ") is less than " +
                    "0 ");
      }
      if (offset > data.length) {
        throw new IllegalArgumentException("offset (" + offset + ") is more than " +
                    data.length);
      }
      if (count < 0) {
        throw new IllegalArgumentException("count (" + count + ") is less than " +
                    "0 ");
      }
      if (count > data.length) {
        throw new IllegalArgumentException("count (" + count + ") is more than " +
                    data.length);
      }
      if (data.length - offset < count) {
        throw new IllegalArgumentException("data's length minus " + offset + " (" +
                (data.length - offset) + ") is less than " + count);
      }
      String alphabet = classic ? Base64Classic : Base64URL;
      int length = offset + count;
      int i = offset;
      char[] buffer = new char[4];
      for (i = offset; i < (length - 2); i += 3) {
        buffer[0] = (char)alphabet.charAt((data[i] >> 2) & 63);
        buffer[1] = (char)alphabet.charAt(((data[i] & 3) << 4) +
                ((data[i + 1] >> 4) & 15));
        buffer[2] = (char)alphabet.charAt(((data[i + 1] & 15) << 2) + ((data[i +
                2] >> 6) & 3));
        buffer[3] = (char)alphabet.charAt(data[i + 2] & 63);
        writer.WriteCodePoint((int)buffer[0]);
        writer.WriteCodePoint((int)buffer[1]);
        writer.WriteCodePoint((int)buffer[2]);
        writer.WriteCodePoint((int)buffer[3]);
      }
      int lenmod3 = count % 3;
      if (lenmod3 != 0) {
        i = length - lenmod3;
        buffer[0] = (char)alphabet.charAt((data[i] >> 2) & 63);
        if (lenmod3 == 2) {
          buffer[1] = (char)alphabet.charAt(((data[i] & 3) << 4) + ((data[i + 1] >>
                4) & 15));
          buffer[2] = (char)alphabet.charAt((data[i + 1] & 15) << 2);
          writer.WriteCodePoint((int)buffer[0]);
          writer.WriteCodePoint((int)buffer[1]);
          writer.WriteCodePoint((int)buffer[2]);
          if (padding) {
            writer.WriteCodePoint((int)'=');
          }
        } else {
          buffer[1] = (char)alphabet.charAt((data[i] & 3) << 4);
          writer.WriteCodePoint((int)buffer[0]);
          writer.WriteCodePoint((int)buffer[1]);
          if (padding) {
            writer.WriteCodePoint((int)'=');
            writer.WriteCodePoint((int)'=');
          }
        }
      }
    }
  }