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https://issues.apache.org/jira/browse/GEOMETRY-162?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17807452#comment-17807452
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Ivan Shuba commented on GEOMETRY-162:
-------------------------------------
Hi, [~erans]
I apologize for the unformatted code blocks, it's been a long time since I've
used Jira. Already changed that.
I've created two pull requests.
One contains the unit tests illustrating the issue -
[226|https://github.com/apache/commons-geometry/pull/226].
The other one contains the proposed fix for the issue -
[227|https://github.com/apache/commons-geometry/pull/227].
> Hash collision for vectors symmetrical about coordinate hyperplanes
> -------------------------------------------------------------------
>
> Key: GEOMETRY-162
> URL: https://issues.apache.org/jira/browse/GEOMETRY-162
> Project: Commons Geometry
> Issue Type: Bug
> Components: euclidean2D, euclidean3D
> Reporter: Ivan Shuba
> Priority: Minor
> Labels: Bug
>
> h1. Overview
> This issue is related to that the same value of hash code is returned where
> it is expected to be different for non-identical instances of `Vector2D` and
> `Vector3D` classes.
> In particular, for 2D case, if we generate a pair of vectors which are
> symmetrical about the X, or Y coordinate axes, then both such vectors will
> return same hash code value. For 3D case, the hash code seems to be equal
> only if the vectors are symmetrical about YZ plane.
> h2. Examples
> The following examples illustrate the problem and can be easily reproduced.
> h4. The Vector2D case, symmetrical about Y axis
> For example, if we have a following pair of collinear `Vector2D` instances:
> {code:java}
> Vector2D a = Vector2D.of(-10.0, 0); //400556032
> Vector2D b = Vector2D.of(+10.0, 0); //400556032 {code}
> Then they both will return a value of 400556032 if the `hashCode()` method is
> called.
> The same will happen if we make non-collinear pair of symmetrical vectors (
> with non-zero value of Y coordinate ):
> {code:java}
> Vector2D a = Vector2D.of(-20.0, 10.0); //-326631424
> Vector2D b = Vector2D.of(+20.0, 10.0); //-326631424 {code}
> h4. The Vector2D case, symmetrical about X axis
> The same holds if we consider a pair of vectors symmetrical about X axis
> {code:java}
> Vector2D a = Vector2D.of(10.0, -30.0); //-1251213312
> Vector2D b = Vector2D.of(10.0, +30.0); //-1251213312 {code}
> Even if we generate arbitrary random number to test the issue, the vectors
> will still return the same hash code value.
> {code:java}
> final double ANY = Math.random();
> int hashA = Vector2D.of(10.0, ANY);
> int hashB = Vector2D.of(10.0, ANY);
> Assertions.assertNotEquals(hashA, hashB); // fails {code}
> h4. The Vector3D case
> For Vector3D class, the issue seems to arise only if the instances are
> symmetrical about YZ plane.
> {code:java}
> final double ANY = Math.random();
> final double NEG = -ANY;
> final double POS = +ANY;
> int negX = Vector3D.of(NEG, 0.0, 0.0);
> int posX = Vector3D.of(POS, 0.0, 0.0);
> int negY = Vector3D.of(0.0, NEG, 0.0);int posY = Vector3D.of(0.0, POS, 0.0);
> int negZ = Vector3D.of(0.0, 0.0, NEG);
> int posZ = Vector3D.of(0.0, 0.0, POS);
> Assertions.assertNotEquals(negX, posX); // fails
> Assertions.assertNotEquals(negY, posY); // passes
> Assertions.assertNotEquals(negZ, posZ); // passes {code}
> h2. Possible solution
> To avoid clashes one could use modified version of the `hashCode` method.
> Here's one for Vector3D class:
>
> {code:java}
> @Override
> public int hashCode() {
> int result;
> long temp;
> temp = Double.doubleToLongBits;
> result = (int) (temp ^ (temp >>> 32));
> temp = Double.doubleToLongBits;
> result = 31 * result + (int) (temp ^ (temp >>> 32));
> temp = Double.doubleToLongBits(z);
> result = 31 * result + (int) (temp ^ (temp >>> 32));
> return result;
> } {code}
>
> P.S. I've prepared a couple of simple unit tests in `Vector2DTest` and
> `Vector3DTest` classes illustrating the issue.
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