com.vividsolutions.jts.index.chain

Class MonotoneChain


  • public class MonotoneChain
extends Object
    Monotone Chains are a way of partitioning the segments of a linestring to allow for fast searching of intersections. They have the following properties:
    1. the segments within a monotone chain never intersect each other
    2. the envelope of any contiguous subset of the segments in a monotone chain is equal to the envelope of the endpoints of the subset.
    Property 1 means that there is no need to test pairs of segments from within the same monotone chain for intersection.

    Property 2 allows an efficient binary search to be used to find the intersection points of two monotone chains. For many types of real-world data, these properties eliminate a large number of segment comparisons, producing substantial speed gains.

    One of the goals of this implementation of MonotoneChains is to be as space and time efficient as possible. One design choice that aids this is that a MonotoneChain is based on a subarray of a list of points. This means that new arrays of points (potentially very large) do not have to be allocated.

    MonotoneChains support the following kinds of queries:

    • Envelope select: determine all the segments in the chain which intersect a given envelope
    • Overlap: determine all the pairs of segments in two chains whose envelopes overlap
    This implementation of MonotoneChains uses the concept of internal iterators ( MonotoneChainSelectAction and MonotoneChainOverlapAction) to return the results for queries. This has time and space advantages, since it is not necessary to build lists of instantiated objects to represent the segments returned by the query. Queries made in this manner are thread-safe.

    • Constructor Detail

      • MonotoneChain

        public MonotoneChain(Coordinate[] pts,
                     int start,
                     int end,
                     Object context)

    • Method Detail

      • setId

        public void setId(int id)

      • getId

        public int getId()

      • getContext

        public Object getContext()

      • getEnvelope

        public Envelope getEnvelope()

      • getStartIndex

        public int getStartIndex()

      • getEndIndex

        public int getEndIndex()

      • getLineSegment

        public void getLineSegment(int index,
                           LineSegment ls)
        Gets the line segment starting at index
        Parameters:
        index - index of segment
        ls - line segment to extract into

      • getCoordinates

        public Coordinate[] getCoordinates()
        Return the subsequence of coordinates forming this chain. Allocates a new array to hold the Coordinates

      • select

        public void select(Envelope searchEnv,
                   MonotoneChainSelectAction mcs)
        Determine all the line segments in the chain whose envelopes overlap the searchEnvelope, and process them.

        The monotone chain search algorithm attempts to optimize performance by not calling the select action on chain segments which it can determine are not in the search envelope. However, it *may* call the select action on segments which do not intersect the search envelope. This saves on the overhead of checking envelope intersection each time, since clients may be able to do this more efficiently.

        Parameters:
        searchEnv - the search envelope
        mcs - the select action to execute on selected segments

      • computeOverlaps

        public void computeOverlaps(MonotoneChain mc,
                            MonotoneChainOverlapAction mco)
        Determine all the line segments in two chains which may overlap, and process them.

        The monotone chain search algorithm attempts to optimize performance by not calling the overlap action on chain segments which it can determine do not overlap. However, it *may* call the overlap action on segments which do not actually interact. This saves on the overhead of checking intersection each time, since clients may be able to do this more efficiently.

        Parameters:
        searchEnv - the search envelope
        mco - the overlap action to execute on selected segments