Ruiqi Dong created MATH-1690:
--------------------------------
Summary: SparseGradient.equals() uses ULP-based comparison but
hashCode() uses exact doubles
Key: MATH-1690
URL: https://issues.apache.org/jira/browse/MATH-1690
Project: Commons Math
Issue Type: Bug
Components: legacy
Reporter: Ruiqi Dong
*Summary*
SparseGradient.equals(Object) treats values within 1 ULP as equal, but
hashCode() hashes the exact double values. This breaks the Java contract that
equal objects must have equal hash codes.
*Affected code*
File:
commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/analysis/differentiation/SparseGradient.java
{code:java}
if (other instanceof SparseGradient) {
final SparseGradient rhs = (SparseGradient)other;
if (!Precision.equals(value, rhs.value, 1)) {
return false;
}
...
if (!Precision.equals(entry.getValue(),
rhs.derivatives.get(entry.getKey()), 1)) {
return false;
}
return true;
}
@Override
public int hashCode() {
return 743 + 809 * Double.hashCode(value) + 167 * derivatives.hashCode();
} {code}
*Reproducer*
Add the following test to
commons-math-legacy/src/test/java/org/apache/commons/math4/legacy/analysis/differentiation/SparseGradientTest.java:
{code:java}
@Test
public void testHashCodeMatchesEqualsForUlpAdjacentValues() {
final SparseGradient constantA = SparseGradient.createConstant(1.0);
final SparseGradient constantB =
SparseGradient.createConstant(JdkMath.nextUp(1.0));
Assert.assertEquals(constantA, constantB);
Assert.assertEquals(constantA.hashCode(), constantB.hashCode());
final SparseGradient derivativeA = SparseGradient.createVariable(0,
0.0).multiply(1.0);
final SparseGradient derivativeB =
SparseGradient.createVariable(0, 0.0).multiply(JdkMath.nextUp(1.0));
Assert.assertEquals(derivativeA, derivativeB);
Assert.assertEquals(derivativeA.hashCode(), derivativeB.hashCode());
} {code}
Run:
{code:java}
mvn -q -pl commons-math-legacy
-Dtest=org.apache.commons.math4.legacy.analysis.differentiation.SparseGradientTest
test {code}
Observed behavior:
{code:java}
SparseGradientTest.testHashCodeMatchesEqualsForUlpAdjacentValues:60
expected:<225444583> but was:<225445392> {code}
Expected behavior:
If two SparseGradient instances are equal according to equals(), they need to
return the same hash code.
The current implementation mixes approximate equality with exact hashing, which
is a direct equals/hashCode contract violation.
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