fixed race condition in InMemoryCache

gbcs-common/src/main/kotlin/net/woggioni/gbcs/common/GBCS.kt

@@ -1,7 +1,9 @@
 package net.woggioni.gbcs.common
 
+import net.woggioni.jwo.JWO
 import java.net.URI
 import java.net.URL
+import java.security.MessageDigest
 
 object GBCS {
     fun String.toUrl() : URL = URL.of(URI(this), null)
@@ -9,4 +11,19 @@ object GBCS {
     const val GBCS_NAMESPACE_URI: String = "urn:net.woggioni.gbcs.server"
     const val GBCS_PREFIX: String = "gbcs"
     const val XML_SCHEMA_NAMESPACE_URI = "http://www.w3.org/2001/XMLSchema-instance"
+
+    fun digest(
+        data: ByteArray,
+        md: MessageDigest = MessageDigest.getInstance("MD5")
+    ): ByteArray {
+        md.update(data)
+        return md.digest()
+    }
+
+    fun digestString(
+        data: ByteArray,
+        md: MessageDigest = MessageDigest.getInstance("MD5")
+    ): String {
+        return JWO.bytesToHex(digest(data, md))
+    }
 }