Add HashingUtils addToHash(Object[]) overloads

internal-api/src/main/java/datadog/trace/util/HashingUtils.java

@@ -1,5 +1,8 @@
 package datadog.trace.util;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 /**
  * This class is intended to be a drop-in replacement for the hashing portions of java.util.Objects.
  * This class provides more convenience methods for hashing primitives and includes overrides for
@@ -8,7 +11,7 @@
 public final class HashingUtils {
   private HashingUtils() {}
 
-  public static final int hashCode(Object obj) {
+  public static final int hashCode(@Nullable Object obj) {
     return obj != null ? obj.hashCode() : 0;
   }
 
@@ -102,7 +105,7 @@ public static final int addToHash(int hash, int value) {
     return 31 * hash + value;
   }
 
-  public static final int addToHash(int hash, Object obj) {
+  public static final int addToHash(int hash, @Nullable Object obj) {
     return addToHash(hash, hashCode(obj));
   }
 
@@ -134,6 +137,18 @@ public static final int addToHash(int hash, double value) {
     return addToHash(hash, Double.hashCode(value));
   }
 
+  /** Folds {@code arr[0..len)}; the array must be non-null, but its elements may be null. */
+  public static final int addToHash(int hash, @Nonnull Object[] arr, int len) {
+    for (int i = 0; i < len; i++) {
+      hash = addToHash(hash, arr[i]);
+    }
+    return hash;
+  }
+
+  public static final int addToHash(int hash, @Nonnull Object[] arr) {
+    return addToHash(hash, arr, arr.length);
+  }
+
   public static final int hash(Iterable<?> objs) {
     int result = 0;
     for (Object obj : objs) {

internal-api/src/main/java/datadog/trace/util/LongHashingUtils.java

@@ -1,5 +1,8 @@
 package datadog.trace.util;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 /**
  * This class is intended to be a drop-in replacement for the hashing portions of java.util.Objects.
  * This class provides more convenience methods for hashing primitives and includes overrides for
@@ -52,7 +55,7 @@ static final long hash(int hash0, int hash1) {
     return 31L * hash0 + hash1;
   }
 
-  private static final int intHash(Object obj) {
+  private static final int intHash(@Nullable Object obj) {
     return obj == null ? 0 : obj.hashCode();
   }
 
@@ -106,7 +109,7 @@ public static final long addToHash(long hash, int value) {
     return 31L * hash + value;
   }
 
-  public static final long addToHash(long hash, Object obj) {
+  public static final long addToHash(long hash, @Nullable Object obj) {
     return addToHash(hash, intHash(obj));
   }
 
@@ -138,14 +141,15 @@ public static final long addToHash(long hash, double value) {
     return addToHash(hash, Double.hashCode(value));
   }
 
-  public static final long addToHash(long hash, Object[] arr, int len) {
+  /** Folds {@code arr[0..len)}; the array must be non-null, but its elements may be null. */
+  public static final long addToHash(long hash, @Nonnull Object[] arr, int len) {
     for (int i = 0; i < len; i++) {
       hash = addToHash(hash, arr[i]);
     }
     return hash;
   }
 
-  public static final long addToHash(long hash, Object[] arr) {
+  public static final long addToHash(long hash, @Nonnull Object[] arr) {
     return addToHash(hash, arr, arr.length);
   }
 

internal-api/src/test/java/datadog/trace/util/HashingUtilsTest.java

@@ -143,6 +143,30 @@ public void hashArrayAndIterable() {
     assertEquals(hashArray, hashIterable);
   }
 
+  @Test
+  public void addToHashArrayFoldsFromSeedLikeChainedAddToHash() {
+    Object[] array = new Object[] {"foo", "bar", "quux"};
+
+    // Full-array overload folds every element onto the seed; from zero it matches hash(Object[]).
+    assertEquals(HashingUtils.hash(array), HashingUtils.addToHash(0, array));
+
+    // A non-zero seed carries through, so the result differs from the zero-seed fold.
+    assertNotEquals(HashingUtils.addToHash(0, array), HashingUtils.addToHash(1, array));
+  }
+
+  @Test
+  public void addToHashArrayRespectsLen() {
+    Object[] array = new Object[] {"foo", "bar", "quux"};
+
+    // The len override folds only the first len elements.
+    assertEquals(
+        HashingUtils.hash(new Object[] {"foo", "bar"}), HashingUtils.addToHash(0, array, 2));
+    assertNotEquals(HashingUtils.addToHash(0, array), HashingUtils.addToHash(0, array, 2));
+
+    // len==0 never enters the loop and returns the seed unchanged.
+    assertEquals(42, HashingUtils.addToHash(42, array, 0));
+  }
+
   @ParameterizedTest
   @ValueSource(booleans = {false, true})
   public void booleans(boolean value) {