为avid-bvid转换和Wbi签名添加Swift实现 (#890)

* Update bvid_desc.md
* Update wbi.md

docs/misc/bvid_desc.md

@@ -192,6 +192,65 @@ print(bv2av("BV1L9Uoa9EUx"))
 
 参考 <https://github.com/Colerar/abv/blob/main/src/lib.rs>
 
+### Swift
+
+```swift
+fileprivate let XOR_CODE: UInt64 = 23442827791579
+fileprivate let MASK_CODE: UInt64 = 2251799813685247
+fileprivate let MAX_AID: UInt64 = 1 << 51
+
+fileprivate let data: [UInt8] = [70, 99, 119, 65, 80, 78, 75, 84, 77, 117, 103, 51, 71, 86, 53, 76, 106, 55, 69, 74, 110, 72, 112, 87, 115, 120, 52, 116, 98, 56, 104, 97, 89, 101, 118, 105, 113, 66, 122, 54, 114, 107, 67, 121, 49, 50, 109, 85, 83, 68, 81, 88, 57, 82, 100, 111, 90, 102]
+
+fileprivate let BASE: UInt64 = 58
+fileprivate let BV_LEN: Int = 12
+fileprivate let PREFIX: String = "BV1"
+
+func av2bv(avid: UInt64) -> String {
+    var bytes: [UInt8] = [66, 86, 49, 48, 48, 48, 48, 48, 48, 48, 48, 48]
+    var bvIdx = BV_LEN - 1
+    var tmp = (MAX_AID | avid) ^ XOR_CODE
+    
+    while tmp != 0 {
+        bytes[bvIdx] = data[Int(tmp % BASE)]
+        tmp /= BASE
+        bvIdx -= 1
+    }
+    
+    bytes.swapAt(3, 9)
+    bytes.swapAt(4, 7)
+    
+    return String(decoding: bytes, as: UTF8.self)
+}
+
+func bv2av(bvid: String) -> UInt64 {
+    let fixedBvid: String
+    if bvid.hasPrefix("BV") {
+        fixedBvid = bvid
+    } else {
+        fixedBvid = "BV" + bvid
+    }
+    var bvidArray = Array(fixedBvid.utf8)
+    
+    bvidArray.swapAt(3, 9)
+    bvidArray.swapAt(4, 7)
+    
+    let trimmedBvid = String(decoding: bvidArray[3...], as: UTF8.self)
+    
+    var tmp: UInt64 = 0
+    
+    for char in trimmedBvid {
+        if let idx = data.firstIndex(of: char.utf8.first!) {
+            tmp = tmp * BASE + UInt64(idx)
+        }
+    }
+    
+    return (tmp & MASK_CODE) ^ XOR_CODE
+}
+
+print(av2bv(avid: 111298867365120))
+print(bv2av(bvid: "BV1L9Uoa9EUx"))
+```
+
 ## 老版算法存档
 
 算法参考自[【揭秘】av号转bv号的过程](https://www.bilibili.com/video/BV1N741127Tj)