Java Base64

Base64.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Base64;
import java.util.List;

public class Main {

    private final static String base64chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

    public static void main(String[] args) {
        byte[] a = "あああああ".getBytes();
        final String e = encode(a);

        System.out.println(e);
        System.out.println(Base64.getEncoder().encodeToString(a));

        System.out.println(new String(decode(e)));
        System.out.println(new String(Base64.getDecoder().decode(e)));

    }

    static int paddingCount(byte[] a) {
        if (a.length % 3 == 1) {
            return 2;
        } else if (a.length % 3 == 2) {
            return 1;
        }
        return 0;
    }

    static String encode(byte[] value) {
        int padding = paddingCount(value);
        byte[] b = Arrays.copyOf(value, value.length + padding);
        StringBuilder sb = new StringBuilder();

        for (int i = 0; i < b.length; i += 3) {
            // 3byte(24bit) to 6bit * 4
            final int a24 = (b[i] & 0xFF) << 16 | (b[i + 1] & 0xFF) << 8 | (b[i + 2] & 0xFF); // 3byte to 24bit int
            final int a1 = a24 >> 18 & 0b111111; // 1st 6bit
            final int a2 = a24 >> 12 & 0b111111; // 2nd 6bit
            final int a3 = a24 >> 6 & 0b111111; // 3rd 6bit
            final int a4 = a24 & 0b111111; // 4th 6bit

            sb.append(base64chars.charAt(a1));
            sb.append(base64chars.charAt(a2));

            if (i + 2 == b.length - 1) {
                if (padding <= 1) {
                    sb.append(base64chars.charAt(a3));
                } else {
                    sb.append('=');
                }

                if (padding == 0) {
                    sb.append(base64chars.charAt(a4));
                } else {
                    sb.append('=');
                }
            } else {
                sb.append(base64chars.charAt(a3));
                sb.append(base64chars.charAt(a4));
            }
        }
        return sb.toString();
    }

    private static byte[] decode(String encoded) {
        List<Byte> bytes = new ArrayList<>();
        // 4chars(6bit) to 24byte(111111222222333333444444)
        for (int i = 0; i < encoded.length(); i += 4) {
            int i1 = base64chars.indexOf(encoded.charAt(i));
            int i2 = base64chars.indexOf(encoded.charAt(i + 1));

            int b1 = i1 << 2 | i2 >> 4; // 11111122
            bytes.add((byte) b1);
            int i3 = 0;
            if (encoded.charAt(i + 2) != '=') {
                i3 = base64chars.indexOf(encoded.charAt(i + 2));
                int b2 = i2 << 4 & 0b11110000 | i3 >> 2; // 22223333
                bytes.add((byte) b2);
            }
            if (encoded.charAt(i + 3) != '=') {
                int i4 = base64chars.indexOf(encoded.charAt(i + 3));
                int b3 = i3 << 6 & 0b11000000 | i4; // 33444444
                bytes.add((byte) b3);
            }
        }

        byte[] result = new byte[bytes.size()];
        for (int i = 0; i < bytes.size(); i++) {
            result[i] = bytes.get(i);
        }
        return result;
    }

    static void print(int b) {
        String str = Integer.toBinaryString(b);
        System.out.println(String.format("%32s", str).replace(' ', '0'));
    }
}