Initial push

src/main/kotlin/Checksum.kt

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+import java.io.File
+import java.io.FileInputStream
+import java.io.IOException
+import java.io.InputStream
+import java.net.URL
+import java.security.MessageDigest
+import java.security.NoSuchAlgorithmException
+import kotlin.experimental.and
+
+object Checksum {
+    val localChecksum: String?
+        get() {
+            val local: File = File(Settings.SAVE_DIR + Settings.SAVE_NAME)
+            try {
+                FileInputStream(local).use { fis -> return calculateMd5(fis) }
+            } catch (e: Exception) {
+                e.printStackTrace()
+                MainWindow.loadingLabel.text = e.message
+                MainWindow.loadingLabel.repaint()
+            }
+            return null
+        }
+
+    fun getLocalChecksum(file: String?): String {
+        val local = File(file!!)
+        if(!local.exists()) return ""
+        else FileInputStream(local).use { fis -> return calculateMd5(fis) }
+    }
+
+    val remoteChecksum: String?
+        get() {
+            try {
+                URL(Settings.DOWNLOAD_URL).openStream().use { stream -> return calculateMd5(stream) }
+            } catch (e: Exception) {
+                e.printStackTrace()
+                MainWindow.loadingLabel.text = e.message
+                MainWindow.loadingLabel.repaint()
+                return null
+            }
+        }
+
+    fun getRemoteChecksum(url: String?): String? {
+        try {
+            URL(url).openStream().use { stream -> return calculateMd5(stream) }
+        } catch (e: Exception) {
+            e.printStackTrace()
+            MainWindow.loadingLabel.text = e.message
+            MainWindow.loadingLabel.repaint()
+            return null
+        }
+    }
+
+    fun calculateMd5(instream: InputStream): String {
+        return calculateDigest(instream, "MD5")
+    }
+
+    private fun calculateDigest(instream: InputStream, algorithm: String): String {
+        val buffer = ByteArray(4096)
+        val messageDigest = getMessageDigest(algorithm)
+        messageDigest!!.reset()
+        var bytesRead: Int
+        try {
+            while (instream.read(buffer).also { bytesRead = it } != -1) {
+                messageDigest.update(buffer, 0, bytesRead)
+            }
+        } catch (e: IOException) {
+            System.err.println("Error making a '$algorithm' digest on the inputstream")
+        }
+        return toHex(messageDigest.digest())
+    }
+
+    fun toHex(ba: ByteArray): String {
+        val baLen = ba.size
+        val hexchars = CharArray(baLen * 2)
+        var cIdx = 0
+        for (i in 0 until baLen) {
+            hexchars[cIdx++] = hexdigit[(ba[i].toInt() shr 4) and 0x0F]
+            hexchars[cIdx++] = hexdigit[(ba[i] and 0x0F).toInt()]
+        }
+        return String(hexchars)
+    }
+
+    fun getMessageDigest(algorithm: String): MessageDigest? {
+        var messageDigest: MessageDigest? = null
+        try {
+            messageDigest = MessageDigest.getInstance(algorithm)
+        } catch (e: NoSuchAlgorithmException) {
+            System.err.println("The '$algorithm' algorithm is not available")
+        }
+        return messageDigest
+    }
+
+    private val hexdigit = charArrayOf(
+        '0', '1', '2', '3', '4', '5',
+        '6', '7', '8', '9', 'a', 'b',
+        'c', 'd', 'e', 'f'
+    )
+}