BigDecimal#hash should return the same value for logically equivalent BigDecimal instances.

Author: nirvdrum

CHANGELOG.md

@@ -5,6 +5,7 @@ Bug fixes:
 * Fixed `BigDecimal#{clone,dup}` so it now just returns the receiver, per Ruby 2.5+ semantics (#1680).
 * Fixed creating `BigDecimal` instances from non-finite `Float` values (#1685).
 * Fixed `BigDecimal#inspect` output for non-finite values (e.g, NaN or -Infinity) (#1683).
+* Fixed `BigDecimal#hash` to return the same value for two `BigDecimal` objects that are equal (#1656).
 * Added missing `BigDecimal` constant definitions (#1684).
 * Implemented `rb_eval_string_protect`.
 * Fixed `rb_get_kwargs` to correctly handle optional and rest arguments.

src/main/java/org/truffleruby/stdlib/bigdecimal/BigDecimalNodes.java

@@ -1545,4 +1545,30 @@ public int toISpecial(DynamicObject value) {
         }
     }
 
+    @CoreMethod(names = "hash")
+    public abstract static class HashNode extends BigDecimalCoreMethodArrayArgumentsNode {
+
+        private static final int CLASS_SALT = 1468180038; // random number, stops hashes for similar values but different classes being the same, static because we want deterministic hashes.
+
+        @TruffleBoundary
+        @Specialization(guards = "isNormal(value)")
+        public Object hashNormal(DynamicObject value) {
+            // Ruby treats trailing zeroes as insignificant for hash calculation. Java's BigDecimal, however,
+            // may return different hash values for two numerically equivalent values with a different number
+            // of trailing zeroes. Stripping them away avoids the issue.
+            final BigDecimal bigDecimalValue = Layouts.BIG_DECIMAL.getValue(value).stripTrailingZeros();
+
+            return getContext().getHashing(this).hash(CLASS_SALT, bigDecimalValue.hashCode());
+        }
+
+        @TruffleBoundary
+        @Specialization(guards = "!isNormal(value)")
+        public Object hashSpecial(DynamicObject value) {
+            final BigDecimalType type = Layouts.BIG_DECIMAL.getType(value);
+
+            return getContext().getHashing(this).hash(CLASS_SALT, System.identityHashCode(type));
+        }
+
+    }
+
 }