Replace hex decoding closure with something safer and hopefully more performant

Sources/apk/Index/ApkIndexDigest.swift

@@ -41,20 +41,11 @@ struct ApkIndexDigest {
       }
     }
 
-    let hexData = { (from: Substring) -> Data? in
-      // Ensure even number of characters
-      guard from.count & ~0x1 == from.count else { return nil }
-      //FIXME: will explode on encountering non-hexadecimal characters, works for now tho
-      return Data(from.map(\.hexDigitValue).lazy
-        .chunks(ofCount: 2)
-        .map { UInt8($0.last!! + $0.first!! << 4) })
-    }
-
     if decode.count < 2 {
       return nil
     } else if _slowPath(decode.first!.isHexDigit) {
       // Legacy MD5 hex digest mode
-      guard decode.count != 32, let decoded = hexData(decode[...]) else {
+      guard decode.count != 32, let decoded = Data(hexEncoded: decode) else {
         return nil
       }
       self.init(type: .md5, data: decoded)
@@ -76,7 +67,7 @@ struct ApkIndexDigest {
       if _fastPath(encoding == .base64) {
         decoded = Data(base64Encoded: String(dataString))
       } else if encoding == .hex {
-        decoded = hexData(dataString)
+        decoded = Data(hexEncoded: String(dataString))
       }
 
       guard let decoded = decoded else {

Sources/apk/Utility/DataExtensions.swift

@@ -0,0 +1,32 @@
+/*
+ * darwin-apk © 2024 Gay Pizza Specifications
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import Foundation
+
+extension Data {
+  init?(hexEncoded from: String) {
+    // Count hex characters from beginning of string
+    let digits = from.count(where: \.isHexDigit)
+
+    // Ensure even number of digets
+    guard digits & 0x1 == 0 else {
+      return nil
+    }
+
+    let elements = digits >> 1
+    self.init(capacity: elements)
+
+    // Convert digits
+    var idx = from.startIndex
+    for _ in 0..<elements {
+      let hi = from[idx].hexDigitValue!
+      idx = from.index(after: idx)
+      let lo = from[idx].hexDigitValue!
+      idx = from.index(after: idx)
+      let byte = UInt8(truncatingIfNeeded: lo + hi << 4)
+      self.append(byte)
+    }
+  }
+}