Range trees

Report
Orthogonal Range Searching I
Range Trees
Range Searching
• S = set of geometric objects
• Q = query object
• Report/Count objects in S that intersect Q
Query Q
Report/Count answers
Single-shot Vs Repeatitive
• Query may be:
• Single-shot (one-time). No need to
preprocess
• Repeatitive-Mode. Many queries are
expected. Preprocess S into a Data
Structure so that queries can be answered
fast
Efficiency Measures
• Repetitive-Mode
– Preprocessing Time: P(n)
– Space occupied by Data Structure: S(n)
– Query Time: Q(n)
– Dynamic Case: Update Time U(n)
• Single-Shot
– Space S(n) and Time T(n)
Orthogonal Range Searching in 1D
• S: Set of points on real line.
• Q= Query Interval [a,b]
a
b
Which query points lie inside the interval [a,b]?
Orthogonal Range Searching in 2D
• S = Set of points in the plane
• Q = Query Rectangle
Binary search trees
A binary search tree (BST) is a binary tree which has the
following properties:
• Each node has a value.
• A total order is defined on these values.
• The left subtree of a node contains only
values less than the node's value.
• The right subtree of a node contains only
values greater than or equal to the node's value.
The major advantage of binary search trees is that the related sorting
algorithms and search algorithms such as in-order traversal can be very
efficient.
1D Range Query
Build a balanced search tree where all data points are stored in
the leaves .
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2
4 5
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7 8
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15
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7
query: O(log n+k)
space: O(n)
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12
2
2
5
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5
8
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8
15
12
15
19
Querying Strategy
• Given interval [a,b], search for a and b
• Find where the paths split, look at subtrees inbetween
Paths split
a
b
Problem: linking leaves do not extends to higher dimensions.
Idea: if parents knew all descendants, wouldn’t need to link leaves.
Efficiency
• Preprocessing Time: O(n log n)
• Space: O(n)
• Query Time: O(log n + k)
• k = number of points reported
• Output-sensitive query time
• Binary search tree can be kept balanced in O(log n) time
per update in dynamic case
1D Range Counting
• S = Set of points on real line
• Q= Query Interval [a,b]
• Count points in [a,b]
Solution: At each node, store count of number of points in
the subtree rooted at the node.
Query: Similar to reporting but add up counts instead of
reporting points.
• Query Time: O(log n)
2D Range queries
• How do you efficiently find points that are inside of a
rectangle?
– Orthogonal range query ([x1, x2], [y1,y2]): find all
points (x, y) such that x1<x<x2 and y1<y<y2
y
y2
y1
x1
x2
x
Range trees
– Canonical subset P(v) of a node v in a BST is a set of points
(leaves) stored in a subtree rooted at v

Range tree is a multi-level data
structure:
• The main tree is a BST T on the
x-coordinate of points
• Any node v of T stores a pointer
to a BST Ty(v) (associated
structure of v), which stores
canonical subset P(v) organized
on the y-coordinate
• 2D points are stored in all leaves!
BST on y-coords
Ty(v)
T
v
P(v)
BST on x-coords
P(v)
Range trees
For each internal node vTx let P(v) be set of points stored in leaves of
subtree rooted at v.

Set P(v) is stored with v as another balanced binary search tree Ty(v)
(descendants by y) on y-coordinate. (have pointer from v to Ty(v))
p4
Tx
Ty(v)
p5
p1
p2
p3
p6
Ty(v)
T4
p7
v
p7 p5
v
P(v)
p1
p2
p3 p4
p 5 p6
p6
p7
P(v)
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Range trees
The diagram below shows what is stored at one node. Show what is
stored at EVERY node. Note that data is only stored at the leaves.
x
y
p1 1 2.5
p2 2 1
p3 3 0
p4 4 4
p5 4.5 3
p6 5.5 3.5
p7 6.5 2
p4
Tx
Ty(v)
p5
p1
p2
p3
p6
Ty(v)
T4
p7
v
p7 p5
v
P(v)
p1
p2
p3 p4
p 5 p6
p6
p7
P(v)
15
Range trees
The query time:
Querying a 1D-tree requires O(log n+k) time. How many 1D trees
(associated structures) do we need to query?
Query: [x,x’]
At most 2  height of T = 2 log n
Each 1D query requires O(log n+k’) time.

Query time = O(log2 n + k)
x
x’
Answer to query = Union of answers to subqueries: k = ∑k’.
Size of the range tree
• Size of the range tree:
– At each level of the main tree associated structures store all the
data points once (with constant overhead): O(n).
– There are O(log n) levels.
– Thus, the total size is O(n log n).
Building the range tree
• Efficient building of the range tree:
– Sort the points on x and on y (two arrays: X,Y).
– Take the median v of X and create a root, build its associated
structure using Y.
– Split X into sorted XL and XR, split Y into sorted YL and YR (s.t.
for any pXL or pYL, p.x < v.x and for any pXR or pYR, p.x 
v.x).
– Build recursively the left child from XL and YL and the right child
from XR and YR.
• The running time is O(n log n).
Generalizing to higher dimensions
• d-dimensional Range Tree can be build recursively from (d1) dimensional range trees.
• Build a Binary Search Tree on coordinates for dimension d.
• Build Secondary Data Structures with (d-1) dimensional
Range Trees.
• Space O(n logd-1 n).
• Query Time O(logd n + k).
Fractional Cascading
• Search on a subset can be speeded up by
adding pointers
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55 66
68
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Layered Range Trees
• Node v and subtrees Left(v) and Right(v).
• Secondary Structures T(Left(v)) and T(Right(v)) are built
on subsets of points on which secondary structure T(v) is
built.
• Fractional Cascading can be used to speed up searches.
• d-dimensional (d >= 2) Range Tree with fractional
cascading:
– Space: O(n logd-1n)
– Query Time: O(logd-1 n + k)
Range trees: summary
• Range trees
– Building (preprocessing time): O(n log n)
– Size: O(n log n)
– Range queries: O(log2 n + k)
• Running time can be improved to O(log n + k) without
sacrificing the preprocessing time or size:
– Layered range trees (uses fractional cascading)
Application
• Given a set of rectangles, report all pairs of
rectangles (R1,R2) such that R1 encloses R2
d
y2
y1
c
a
x1
x2
b
Reduction to Range Search
• x1 >= a, x2 <= b, y1 >= c, y2 <= d.
• x1 ε [a, infty], x2 ε [-infty,b],
• y1 ε [c, infty], y2 ε [-infty,d].
• This is a 4D Range Search Problem:
– Map each rectangle to 4D point and create a data structure D.
– Map each rectangle to 4D interval and query D.
– O(n log3 n) space and O(log4 n + k) query time, but
O(log3 n + k) query time using fractional cascading.
Orthogonal Range Searching II
Interval Trees
1D point enclosure
S : set of n intervals on the real line.
q : query point.
Query: Which intervals contain Q?
Build a data structure with the intervals, so that queries can
be answered fast.
Subdivide the problem
• Consider all 2n endpoints.
• Xmid = Median of the endpoints.
• At most half the midpoints are to the left of the
median, at most half to the right.
• Construct a binary tree based on this idea with
Xmid as the “root discriminator”.
Subdivide set of intervals
What do we do with the segments that intersect xmid?
Imid
Ileft
xmid
Iright
Interval trees
Idea: Use an associated data structure!
xmid
xmid
Imid
Interval trees
Idea:
L
xmid
Store the segments twice; once for
the left endpoints and once for the
right endpoints.
L
Need sorted list of left and right
endpoints
Sorted list of left endpoints
Recursive subdivision
Left and right subtrees are interval trees.
The interval tree has O(log n) depth and uses O(n) storage
(each interval gets stored exactly at one node).
Querying
If q < Xmid
– consider left endpoints in Imid
– query left subtree recursively
Else
– consider right endpoints in Imid
– query right subtree recursively
Efficiency
• Preprocessing Time O(n log n).
• Space O(n).
• Query Time = O(log n + k) :
– we visit at most one node at any depth of the tree.
– we report kv intervals at node v.
– k = total number of intervals reported, k = ∑kv.
Application
S: set of horizontal line segments in the plane.
q: vertical line segment.
Query: find all horizontal segments intersecting q.
q
Solution
• Build Interval Tree on Horizontal Segments.
• Instead of storing left and right endpoints in an
ordered list we need a secondary data structure
at each node.
Secondary data structure = 2D range tree
Analysis
• Secondary data structure = 2D range tree:
– O(n log n) space.
– O(log n + k’) query time.
• Overall:
– Preprocessing Time: O(n log n).
– Space: O(n log n) (we have range trees instead of ordered lists).
– At each of the O(log n) nodes v on the search path we spend
O(log n + kv) time.
– Overall Query Time: O(log2 n + k).
k = total number of intervals reported, k = ∑kv.
Windowing Query
S: axes parallel segments.
q: query axes parallel rectangle.
Query: find all horizontal segments intersecting q.
Solution
•
Can be solved with range trees and interval trees.
•
Consider 2 subproblems:
– Intervals with some endpoint in query. Range trees:
•
•
O(log n + k’) query time
O(n log n) space and preprocessing
– Number of endpoints inside query. Interval trees:
•
•
•
O(log2 n + k’’) query time
O(n log n) space and preprocessing
Overall:
– Preprocessing Time: O(n log n).
– Query Time O(log2 n + k).
– Space O(n log n).

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