Add Kotlin version

Kotlin/ShortURL.kt

@@ -0,0 +1,46 @@
+/*
+ * ShortURL (https://github.com/delight-im/ShortURL)
+ * Copyright (c) delight.im (https://www.delight.im/)
+ * Licensed under the MIT License (https://opensource.org/licenses/MIT)
+ */
+
+/**
+ * ShortURL: Bijective conversion between natural numbers (IDs) and short strings
+ *
+ * ShortURL.encode() takes an ID and turns it into a short string
+ * ShortURL.decode() takes a short string and turns it into an ID
+ *
+ * Features:
+ * + large alphabet (51 chars) and thus very short resulting strings
+ * + proof against offensive words (removed 'a', 'e', 'i', 'o' and 'u')
+ * + unambiguous (removed 'I', 'l', '1', 'O' and '0')
+ *
+ * Example output:
+ * 123456789 <=> pgK8p
+ */
+object ShortURL {
+
+	private val ALPHABET = "23456789bcdfghjkmnpqrstvwxyzBCDFGHJKLMNPQRSTVWXYZ-_"
+	private val BASE = ALPHABET.length
+
+	fun encode(num: Int): String {
+		var number = num
+		val stringBuilder = StringBuilder()
+		while (number > 0) {
+			stringBuilder.insert(0, ALPHABET[number % BASE])
+			number /= BASE
+		}
+
+		return stringBuilder.toString()
+	}
+
+	fun decode(string: String): Int {
+		var number = 0
+		for (i in 0 until string.length) {
+			number = number * BASE + ALPHABET.indexOf(string[i])
+		}
+
+		return number
+	}
+
+}