update lcg

Author: benwtrent

server/src/main/java/org/elasticsearch/index/codec/vectors/SampleReader.java

@@ -100,12 +100,12 @@ static class RandomLinearCongruentialMapper {
         private final int randomLinearShift;
 
         RandomLinearCongruentialMapper(long smaller, long larger, Random random) {
-            assert smaller > 0 && larger > 0;
+            assert smaller > 0 && larger > 0 : "smaller and larger must be positive; received: " + smaller + ", " + larger;
             assert smaller < larger;
             this.n = smaller;
             this.m = larger;
             this.multiplier = findLargeOddCoprime(n);
-            this.randomLinearShift = random.nextInt(0, 1024 * 1024);
+            this.randomLinearShift = random.nextInt(1024, 1024 * 1024) | 1;
         }
 
         // need to ensure positive modulus only
@@ -123,9 +123,9 @@ long map(long i) {
         }
 
         private static long findLargeOddCoprime(long n) {
-            long candidate = n | 1; // make sure it's odd
+            long candidate = Math.max(n | 1, 5); // make sure it's odd
             while (MathUtil.gcd(candidate, n) != 1) {
-                candidate += 2;
+                candidate += 4;
             }
             return candidate;
         }

server/src/test/java/org/elasticsearch/index/codec/vectors/SampleReaderTests.java

@@ -31,4 +31,25 @@ public void testRandomSampling() {
         }
     }
 
+    public void testRandomSamplingEdges() {
+        for (int base = 1; base < 24; base++) {
+            int smaller = 1 << base;
+            for (int upperBase = base + 1; upperBase <= 24; upperBase++) {
+                int larger = 1 << upperBase;
+                SampleReader.RandomLinearCongruentialMapper mapper = new SampleReader.RandomLinearCongruentialMapper(
+                    smaller,
+                    larger,
+                    random()
+                );
+                FixedBitSet valueSeen = new FixedBitSet(larger + 1);
+                for (int i = 0; i < smaller; i++) {
+                    long mapped = mapper.map(i);
+                    assertTrue(mapped >= 0);
+                    assertTrue(mapped <= larger);
+                    assertFalse(valueSeen.getAndSet((int) mapped));
+                }
+            }
+        }
+    }
+
 }