ykxmz/android/src/com/lanremote/watch/B64.java

package com.lanremote.watch;

/**
 * 自带的 Base64 实现。
 *
 * 不用 android.util.Base64（桌面 JVM 上没有），也不用 java.util.Base64
 * （Android 8.0 / API 26 才提供，手表可能更老）。
 * 这样整个协议层保持纯 Java，可在桌面直接联调。
 */
public final class B64 {

    private static final char[] ALPHABET =
            "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/".toCharArray();

    private static final int[] LOOKUP = new int[128];

    static {
        for (int i = 0; i < LOOKUP.length; i++) {
            LOOKUP[i] = -1;
        }
        for (int i = 0; i < ALPHABET.length; i++) {
            LOOKUP[ALPHABET[i]] = i;
        }
    }

    private B64() {
    }

    public static String encode(byte[] data) {
        if (data == null) {
            return "";
        }
        StringBuilder sb = new StringBuilder(((data.length + 2) / 3) * 4);
        int i = 0;
        while (i + 2 < data.length) {
            int triple = ((data[i] & 0xFF) << 16) | ((data[i + 1] & 0xFF) << 8) | (data[i + 2] & 0xFF);
            sb.append(ALPHABET[(triple >>> 18) & 0x3F]);
            sb.append(ALPHABET[(triple >>> 12) & 0x3F]);
            sb.append(ALPHABET[(triple >>> 6) & 0x3F]);
            sb.append(ALPHABET[triple & 0x3F]);
            i += 3;
        }
        int remaining = data.length - i;
        if (remaining == 1) {
            int chunk = (data[i] & 0xFF) << 16;
            sb.append(ALPHABET[(chunk >>> 18) & 0x3F]);
            sb.append(ALPHABET[(chunk >>> 12) & 0x3F]);
            sb.append("==");
        } else if (remaining == 2) {
            int chunk = ((data[i] & 0xFF) << 16) | ((data[i + 1] & 0xFF) << 8);
            sb.append(ALPHABET[(chunk >>> 18) & 0x3F]);
            sb.append(ALPHABET[(chunk >>> 12) & 0x3F]);
            sb.append(ALPHABET[(chunk >>> 6) & 0x3F]);
            sb.append('=');
        }
        return sb.toString();
    }

    public static byte[] decode(String text) {
        if (text == null || text.isEmpty()) {
            return new byte[0];
        }
        int length = text.length();
        while (length > 0 && text.charAt(length - 1) == '=') {
            length--;
        }
        int outLength = (length * 3) / 4;
        byte[] out = new byte[outLength];
        int buffer = 0;
        int bits = 0;
        int index = 0;
        for (int i = 0; i < length; i++) {
            char c = text.charAt(i);
            if (c >= 128) {
                continue;
            }
            int value = LOOKUP[c];
            if (value < 0) {
                continue;                 // 跳过换行/空白等
            }
            buffer = (buffer << 6) | value;
            bits += 6;
            if (bits >= 8) {
                bits -= 8;
                if (index < outLength) {
                    out[index++] = (byte) ((buffer >>> bits) & 0xFF);
                }
            }
        }
        if (index == outLength) {
            return out;
        }
        byte[] trimmed = new byte[index];
        System.arraycopy(out, 0, trimmed, 0, index);
        return trimmed;
    }
}