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From "Neil Hooey (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (LUCENE-3814) Manhattan distance function is incorrect, not absolute distance between coordinates
Date Tue, 21 Feb 2012 17:36:35 GMT
Manhattan distance function is incorrect, not absolute distance between coordinates
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                 Key: LUCENE-3814
                 URL: https://issues.apache.org/jira/browse/LUCENE-3814
             Project: Lucene - Java
          Issue Type: Bug
          Components: modules/spatial
    Affects Versions: 4.0
            Reporter: Neil Hooey


The Lucene vectorDistance() function's Manhattan distance function is incorrect.

Wikipedia says: http://en.wikipedia.org/wiki/Manhattan_distance

"Taxicab geometry, blahblahblah, is a form of geometry in which the usual distance function
or metric of Euclidean geometry is replaced by a new metric in which the distance between
two points is the sum of the absolute differences of their coordinates."

I don't have a patch, but the offending code is here:

// lucene/contrib/spatial/src/java/org/apache/lucene/spatial/DistanceUtils.java

    } else if (power == 1.0) { 
      for (int i = 0; i < vec1.length; i++) { 
        result += vec1[i] - vec2[i]; 
    }

It just needs to use Math.abs() when subtracting the coordinates.

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