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JVM AOT compiler currently generating JavaScript, C++, Haxe, with initial focus on Kotlin and games.
There is a newer version: 0.6.8
package libcore.io;

import libcore.util.EmptyArray;

import java.nio.charset.StandardCharsets;

/**
 * Base64 encoder/decoder.
 * In violation of the RFC, this encoder doesn't wrap lines at 76 columns.
 */
public final class Base64 {
	private Base64() {
	}

	public static byte[] decode(byte[] in) {
		return decode(in, in.length);
	}

	public static byte[] decode(byte[] in, int len) {
		// approximate output length
		int length = len / 4 * 3;
		// return an empty array on empty or short input without padding
		if (length == 0) {
			return EmptyArray.BYTE;
		}
		// temporary array
		byte[] out = new byte[length];
		// number of padding characters ('=')
		int pad = 0;
		byte chr;
		// compute the number of the padding characters
		// and adjust the length of the input
		for (;;len--) {
			chr = in[len-1];
			// skip the neutral characters
			if ((chr == '\n') || (chr == '\r') ||
				(chr == ' ') || (chr == '\t')) {
				continue;
			}
			if (chr == '=') {
				pad++;
			} else {
				break;
			}
		}
		// index in the output array
		int outIndex = 0;
		// index in the input array
		int inIndex = 0;
		// holds the value of the input character
		int bits = 0;
		// holds the value of the input quantum
		int quantum = 0;
		for (int i=0; i= 'A') && (chr <= 'Z')) {
				// char ASCII value
				//  A    65    0
				//  Z    90    25 (ASCII - 65)
				bits = chr - 65;
			} else if ((chr >= 'a') && (chr <= 'z')) {
				// char ASCII value
				//  a    97    26
				//  z    122   51 (ASCII - 71)
				bits = chr - 71;
			} else if ((chr >= '0') && (chr <= '9')) {
				// char ASCII value
				//  0    48    52
				//  9    57    61 (ASCII + 4)
				bits = chr + 4;
			} else if (chr == '+') {
				bits = 62;
			} else if (chr == '/') {
				bits = 63;
			} else {
				return null;
			}
			// append the value to the quantum
			quantum = (quantum << 6) | (byte) bits;
			if (inIndex%4 == 3) {
				// 4 characters were read, so make the output:
				out[outIndex++] = (byte) (quantum >> 16);
				out[outIndex++] = (byte) (quantum >> 8);
				out[outIndex++] = (byte) quantum;
			}
			inIndex++;
		}
		if (pad > 0) {
			// adjust the quantum value according to the padding
			quantum = quantum << (6*pad);
			// make output
			out[outIndex++] = (byte) (quantum >> 16);
			if (pad == 1) {
				out[outIndex++] = (byte) (quantum >> 8);
			}
		}
		// create the resulting array
		byte[] result = new byte[outIndex];
		System.arraycopy(out, 0, result, 0, outIndex);
		return result;
	}

	private static final byte[] map = new byte[]
		{'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', '0', '1', '2', '3',
			'4', '5', '6', '7', '8', '9', '+', '/'};

	public static String encode(byte[] in) {
		int length = (in.length + 2) * 4 / 3;
		byte[] out = new byte[length];
		int index = 0, end = in.length - in.length % 3;
		for (int i = 0; i < end; i += 3) {
			out[index++] = map[(in[i] & 0xff) >> 2];
			out[index++] = map[((in[i] & 0x03) << 4) | ((in[i+1] & 0xff) >> 4)];
			out[index++] = map[((in[i+1] & 0x0f) << 2) | ((in[i+2] & 0xff) >> 6)];
			out[index++] = map[(in[i+2] & 0x3f)];
		}
		switch (in.length % 3) {
			case 1:
				out[index++] = map[(in[end] & 0xff) >> 2];
				out[index++] = map[(in[end] & 0x03) << 4];
				out[index++] = '=';
				out[index++] = '=';
				break;
			case 2:
				out[index++] = map[(in[end] & 0xff) >> 2];
				out[index++] = map[((in[end] & 0x03) << 4) | ((in[end+1] & 0xff) >> 4)];
				out[index++] = map[((in[end+1] & 0x0f) << 2)];
				out[index++] = '=';
				break;
		}
		return new String(out, 0, index, StandardCharsets.US_ASCII);
	}
}