fixed issue with negative integer seed for MT (https://issues.apache.org/jira/browse/MATH-1322)

Author: csoroiu

samples/mt-rand.php

@@ -0,0 +1,7 @@
+<?php
+echo mt_getrandmax(), "\n";
+mt_srand(1234567890, MT_RAND_MT19937);
+for ($i = 0; $i < 10; $i++) {
+    echo mt_rand(0, 16), "\n";
+}
+?>

samples/odities.txt

@@ -3,8 +3,15 @@ Odities
 2. .Net nextInt(int) allows 0 as a bound(maxValue).
 
 3. FreePascal's nextInt(int) generates value between bound(exclusive) and 0 (exclusive) if bound is negative.
+4. FreePascal does implicit downcast for int64 to integer, thus, when using a 64 bit seed, the high 32 bits are lost.
 
-4. Delphi's considers bound as unsigned even if signed and generates value
+5. Python, for negative int seed drops the sign, while in java implementation we consider that
+   seed as unsigned and work with it as is. This is similar to using the unsigned version of the seed in
+   python. For instance value 0xbe991a15, as signed it is -1097262571 and as unsigned 3197704725.
+   Java generates for -1097262571 same values python generates for 3197704725. But Python, for value
+   -1097262571 drops the sign and generates the same values as for seed 1097262571.
+
+6. Delphi's considers bound as unsigned even if signed and generates value
    between 0 and the received parameter, returning it as a signed it after that.
    (seems was done on purpose, to allow Math.RandomRange to function properly for a wide range).
    The java implementation of nextInt(int) throws exception if bound is negative.

src/main/java/ro/derbederos/untwist/ReverseRandomGenerator.java

@@ -128,8 +128,4 @@ public interface ReverseRandomGenerator extends RandomGenerator, Serializable {
      * {@code 0} (inclusive) and {@code bound} (exclusive).
      */
     long prevLong(long bound);
-
-//    long nextLong(long origin, long bound);
-//
-//    long prevLong(long origin, long bound);
 }

src/main/java/ro/derbederos/untwist/ReversibleMersenneTwister.java

@@ -155,7 +155,7 @@ public ReversibleMersenneTwister(long seed) {
      */
     private void initGenRand(int seed) {
         // we use a long masked by 0xFFFFFFFFL as a poor man unsigned int
-        long longMT = seed;
+        long longMT = Integer.toUnsignedLong(seed);
         // NB: unlike original C code, we are working with java longs, the cast below makes masking unnecessary
         mt[0] = (int) longMT;
         for (mti = 1; mti < N; ++mti) {

src/main/java/ro/derbederos/untwist/TurboPascalRandom.java

@@ -17,7 +17,6 @@
 package ro.derbederos.untwist;
 
 import static java.lang.Integer.toUnsignedLong;
-import static java.lang.Math.abs;
 
 /**
  * Java implementation of the random number generator from Turbo Pascal 7/Delphi.