From http://en.wikipedia.org/wiki/K-d_tree :

The k-d tree is a binary tree in which every node is a k-dimensional point. Every non-leaf node can be thought of as implicitly generating a splitting hyperplane that divides the space into two parts, known as subspaces. Points to the left of this hyperplane represent the left sub-tree of that node and points right of the hyperplane are represented by the right sub-tree. The hyperplane direction is chosen in the following way: every node in the tree is associated with one of the k-dimensions, with the hyperplane perpendicular to that dimension's axis. So, for example, if for a particular split the "x" axis is chosen, all points in the subtree with a smaller "x" value than the node will appear in the left subtree and all points with larger "x" value will be in the right sub tree. In such a case, the hyperplane would be set by the x-value of the point, and its normal would be the unit x-axis.[1]

The algorithm was inspired by the Wikipedia entry and by Overmars computational geometry book.

Implementation: multidim.ml spaces.ml kd_tree.ml

Usage:

• Compile with omake.

• Run './repl -input_file usa_cities.csv'

• In the repl, enter lat and long separated by comma, e.g.

  enter (lat,long) >>> -121.46,38.52
  {name=Parkway-South Sacramento; population=40797; state=California; latlong=(-121.45,38.51)}

Todo:

• Fix range search
• Implement algorithm in Scala, Clojure, and Haskell