[GR-16086] BigDecimal objects with the same value should have the same hash code.

PullRequest: truffleruby/866

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.

spec/ruby/library/bigdecimal/hash_spec.rb

@@ -0,0 +1,30 @@
+require_relative '../../spec_helper'
+require 'bigdecimal'
+
+describe "BidDecimal#hash" do
+  describe "two BigDecimal objects with the same value" do
+    it "should have the same hash for ordinary values" do
+      BigDecimal('1.2920').hash.should == BigDecimal('1.2920').hash
+    end
+
+    it "should have the same hash for infinite values" do
+      BigDecimal("+Infinity").hash.should == BigDecimal("+Infinity").hash
+      BigDecimal("-Infinity").hash.should == BigDecimal("-Infinity").hash
+    end
+
+    it "should have the same hash for NaNs" do
+      BigDecimal("NaN").hash.should == BigDecimal("NaN").hash
+    end
+
+    it "should have the same hash for zero values" do
+      BigDecimal("+0").hash.should == BigDecimal("+0").hash
+      BigDecimal("-0").hash.should == BigDecimal("-0").hash
+    end
+  end
+
+  describe "two BigDecimal objects with numerically equal values" do
+    it "should have the same hash value" do
+      BigDecimal("1.2920").hash.should == BigDecimal("1.2920000").hash
+    end
+  end
+end

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, type.hashCode());
+        }
+
+    }
+
 }