make my own UUID

Author: thsc42

src/net/sharksystem/asap/ASAP.java

@@ -1,5 +1,7 @@
 package net.sharksystem.asap;
 
+import java.util.Random;
+
 public class ASAP {
     public static final int INITIAL_ERA = 0;
     public static final int MAX_ERA = Integer.MAX_VALUE;
@@ -17,4 +19,78 @@ public static int previousEra(int workingEra) {
         }
         return workingEra-1;
     }
+
+    /**
+     * produces a unique id - is made up by current time in millis added with some random digits.
+     * @return
+     */
+    public static String createUniqueID() {
+        long now = System.currentTimeMillis();
+
+        /* now is a 64 bit value. Actually, only 63 bits are relevant because it is positive value.
+        Moreover, we are already in the 21. century - could reduce bit even more. We ignore that obvious fact
+        at first.
+
+        Calcuations: 2^64 = 1,8.. * 10^19; we take a..z and A..Z and 0..9 -> 62 digits
+        62^11 = 5,2.. * 10^19 we need 11 digits
+         */
+
+        int randomDigits = 3;
+        int timeDigits = 11;
+        int digits = timeDigits + randomDigits;
+        int basis = 62;
+
+        /*
+        0..9 --> 0..9
+        10..35 -> a..z
+        36..61 -> A..Z
+         */
+
+        char[] idChars = new char[digits];
+        // init
+        for(int i = 0;  i < digits; i++) idChars[i] = '0';
+
+        // let's fill it
+        int i = 0;
+        long rest = 0;
+        while(now > 0) {
+            rest = now % basis; // rest is number [0,63]
+            now /= basis;
+
+            idChars[i++] = ASAP.int2charID((int) rest);
+        }
+
+        // set index
+        i = timeDigits;
+
+        // random digits
+        long rValue = now + rest;
+        Random random = new Random(rValue);
+        for(int j = 0; j < randomDigits; j++) {
+            int r = random.nextInt(basis);
+            idChars[i++] = ASAP.int2charID(r);
+            rValue = (rValue * r) % basis;
+            random.setSeed(rValue);
+        }
+
+        return new String(idChars);
+    }
+
+    private static char int2charID(int value) {
+        // convert value into valid values;
+        // 0..9 -> 0..9
+        if(value <= 9) {
+            return (char)((int)'0' + value);
+        } else {
+            value -= 10;
+            // a..z
+            if(value <= 25) {
+                return (char)((int)'a' + value);
+            } else {
+                // A..Z
+                value -= 26;
+                return (char)((int)'A' + value);
+            }
+        }
+    }
 }

test/net/sharksystem/asap/BasisMethodsTests.java

@@ -0,0 +1,14 @@
+package net.sharksystem.asap;
+
+import org.junit.Test;
+
+public class BasisMethodsTests {
+
+    @Test
+    public void asapID() throws InterruptedException {
+        for(int i = 0; i < 10; i++) {
+            System.out.println(ASAP.createUniqueID());
+            Thread.sleep(2);
+        }
+    }
+}