Move EnhancedDoubleHasher.mod() to a public BitMap API (#396)

* Made mod() public

* Moved mod() method to BitMap class.

* Add Javadoc since tag

* Added mod() tests and updated documentation.

* fixed formatting issues

---------

Co-authored-by: Gary Gregory <garydgregory@users.noreply.github.com>

src/main/java/org/apache/commons/collections4/bloomfilter/BitMap.java

@@ -113,4 +113,24 @@ public class BitMap {
         // this will identify an incorrect bit.
         return 1L << bitIndex;
     }
+
+    /**
+     * Performs a modulus calculation on an unsigned long and an positive integer divisor.
+     *
+     * <p><em>If the divisor is negative the behavior is not defined.</em></p>
+     *
+     * @param dividend a unsigned long value to calculate the modulus of.
+     * @param divisor the divisor for the modulus calculation, must be positive.
+     * @return the remainder or modulus value.
+     * @since 4.5
+     */
+    public static int mod(final long dividend, final int divisor) {
+        // See Hacker's Delight (2nd ed), section 9.3.
+        // Assume divisor is positive.
+        // Divide half the unsigned number and then double the quotient result.
+        final long quotient = (dividend >>> 1) / divisor << 1;
+        final long remainder = dividend - quotient * divisor;
+        // remainder in [0, 2 * divisor)
+        return (int) (remainder >= divisor ? remainder - divisor : remainder);
+    }
 }

src/main/java/org/apache/commons/collections4/bloomfilter/EnhancedDoubleHasher.java

@@ -129,22 +129,6 @@ public class EnhancedDoubleHasher implements Hasher {
         return increment;
     }
 
-    /**
-     * Performs a modulus calculation on an unsigned long and an integer divisor.
-     * @param dividend a unsigned long value to calculate the modulus of.
-     * @param divisor the divisor for the modulus calculation (must be strictly positive).
-     * @return the remainder or modulus value.
-     */
-    static int mod(final long dividend, final int divisor) {
-        // See Hacker's Delight (2nd ed), section 9.3.
-        // Assume divisor is positive.
-        // Divide half the unsigned number and then double the quotient result.
-        final long quotient = (dividend >>> 1) / divisor << 1;
-        final long remainder = dividend - quotient * divisor;
-        // remainder in [0, 2 * divisor)
-        return (int) (remainder >= divisor ? remainder - divisor : remainder);
-    }
-
     @Override
     public IndexProducer indices(final Shape shape) {
         Objects.requireNonNull(shape, "shape");
@@ -168,8 +152,8 @@ public class EnhancedDoubleHasher implements Hasher {
                 // The final hash is:
                 // hash[i] = ( h1(x) - i*h2(x) - (i*i*i - i)/6 ) wrapped in [0, bits)
 
-                int index = mod(initial, bits);
+                int index = BitMap.mod(initial, bits);
-                int inc = mod(increment, bits);
+                int inc = BitMap.mod(increment, bits);
 
                 final int k = shape.getNumberOfHashFunctions();
                 if (k > bits) {

src/test/java/org/apache/commons/collections4/bloomfilter/BitMapTest.java

@@ -16,6 +16,7 @@
  */
 package org.apache.commons.collections4.bloomfilter;
 
+import static org.junit.Assert.assertNotEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertThrows;
@@ -102,4 +103,19 @@ public class BitMapTest {
         ary[1] = 1;
         assertTrue(BitMap.contains(ary, 64));
     }
+
+    private void assertMod(long l, int i) {
+        assertEquals(Math.floorMod(l, i), BitMap.mod(l, i));
+    }
+
+    @Test
+    public final void testMod() {
+        assertMod(Long.MAX_VALUE, Integer.MAX_VALUE);
+        assertMod(Long.MAX_VALUE, Integer.MAX_VALUE-1);
+        assertThrows(ArithmeticException.class, () -> BitMap.mod(Long.MAX_VALUE, 0));
+        assertMod(Long.MAX_VALUE-1, Integer.MAX_VALUE);
+        assertMod(Long.MAX_VALUE-1, Integer.MAX_VALUE-1);
+        assertMod(0, Integer.MAX_VALUE);
+        assertNotEquals(Math.floorMod(5, -1), BitMap.mod(5, -1));
+    }
 }

src/test/java/org/apache/commons/collections4/bloomfilter/EnhancedDoubleHasherTest.java

@@ -99,7 +99,7 @@ public class EnhancedDoubleHasherTest extends AbstractHasherTest {
         for (final long dividend : new long[] {-1, -2, -3, -6378683, -23567468136887892L, Long.MIN_VALUE, 345, 678686,
             67868768686878924L, Long.MAX_VALUE}) {
             for (final int divisor : new int[] {1, 2, 3, 5, 13, Integer.MAX_VALUE}) {
-                assertEquals((int) Long.remainderUnsigned(dividend, divisor), EnhancedDoubleHasher.mod(dividend, divisor),
+                assertEquals((int) Long.remainderUnsigned(dividend, divisor), BitMap.mod(dividend, divisor),
                         () -> String.format("failure with dividend=%s and divisor=%s.", dividend, divisor));
             }
         }

src/test/java/org/apache/commons/collections4/bloomfilter/IncrementingHasher.java

@@ -66,8 +66,8 @@ final class IncrementingHasher implements Hasher {
                 // This avoids any modulus operation inside the while loop. It uses a long index
                 // to avoid overflow.
 
-                long index = EnhancedDoubleHasher.mod(initial, bits);
+                long index = BitMap.mod(initial, bits);
-                final int inc = EnhancedDoubleHasher.mod(increment, bits);
+                final int inc = BitMap.mod(increment, bits);
 
                 for (int functionalCount = 0; functionalCount < shape.getNumberOfHashFunctions(); functionalCount++) {
                     if (!consumer.test((int) index)) {