feat: bv<-->av 增加java实现 (#1021)

docs/misc/bvid_desc.md

@@ -248,6 +248,87 @@ print(av2bv(avid: 111298867365120))
 print(bv2av(bvid: "BV1L9Uoa9EUx"))
 ```
 
+
+### Java
+
+```
+import java.math.BigInteger;
+
+/**
+ * @author cctyl
+ */
+public class AVBVConverter {
+
+    private static final BigInteger XOR_CODE = BigInteger.valueOf(23442827791579L);
+    private static final BigInteger MASK_CODE = BigInteger.valueOf(2251799813685247L);
+    private static final BigInteger MAX_AID = BigInteger.ONE.shiftLeft(51);
+    private static final int BASE = 58;
+
+    private static final String DATA = "FcwAPNKTMug3GV5Lj7EJnHpWsx4tb8haYeviqBz6rkCy12mUSDQX9RdoZf";
+
+    public static String av2bv(int aidParam) {
+
+        BigInteger aid = BigInteger.valueOf(aidParam);
+        char[] bytes = {'B', 'V', '1', '0', '0', '0', '0', '0', '0', '0', '0', '0'};
+        int bvIndex = bytes.length - 1;
+        BigInteger tmp = MAX_AID.or(aid).xor(XOR_CODE);
+        while (tmp.compareTo(BigInteger.ZERO) > 0) {
+            bytes[bvIndex] = DATA.charAt(tmp.mod(BigInteger.valueOf(BASE)).intValue());
+            tmp = tmp.divide(BigInteger.valueOf(BASE));
+            bvIndex -= 1;
+        }
+        swap(bytes, 3, 9);
+        swap(bytes, 4, 7);
+        StringBuilder sb = new StringBuilder(bytes.length);
+        for (Character ch : bytes) {
+            sb.append(ch);
+        }
+        return sb.toString();
+    }
+
+    public static int bv2av(String bvid) {
+        char[] bvidArr = bvid.toCharArray();
+        swap(bvidArr, 3, 9);
+        swap(bvidArr, 4, 7);
+        String adjustedBvid = new String(bvidArr, 3, bvidArr.length - 3);
+        BigInteger tmp = BigInteger.ZERO;
+        for (char c : adjustedBvid.toCharArray()) {
+            tmp = tmp.multiply(BigInteger.valueOf(BASE)).add(BigInteger.valueOf(DATA.indexOf(c)));
+        }
+        BigInteger xor = tmp.and(MASK_CODE).xor(XOR_CODE);
+        return xor.intValue();
+    }
+
+
+    private static void swap(char[] array, int i, int j) {
+        char temp = array[i];
+        array[i] = array[j];
+        array[j] = temp;
+    }
+
+    public static void main(String[] args) {
+
+        final int aid1 = 643755790;
+        final String bv1 = "BV1bY4y1j7RA";
+
+        final int aid2 = 305988942;
+        final String bv2 = "BV1aP411K7it";
+
+
+        //av ==> bv
+        assert av2bv(aid1).equals(bv1);
+        assert av2bv(aid2).equals(bv2);
+
+
+        //bv ==>av
+        assert bv2av(bv1) == aid1;
+        assert bv2av(bv2) == aid2;
+    }
+}
+
+```
+
+
 ## 老版算法存档
 
 **以下算法已失效**，编解码函数值域有限，不推荐使用，在此仅作为存档