August 20, 2002

1
Convex Hull for Dynamic Data
Jorge L. Vittes

• August 20, 2002
• (joint work with Umut Acar, and Guy Blelloch)

2
Motivation

• Application data is dynamic
• word processors slowly changing text
• graphics render similar images
• mobile phone networks continuously moving hosts
• Important to handle dynamic data efficiently

3
Dynamic Algorithms Changing Data
a
a
b
b
c
c
e
d
d
a
b
c
e
d
4
Kinetic Algorithms Moving Data
a
a
b
b
c
c
d
Time 1
d
Time 0
b
a
c
Time 1e ... 1
d
5
How to invent Dynamic/Kinetic Algorithms

• Just like any other algorithm. Think, ponder,
  divide, conquer...
• Or, use adaptivity...

6
Adaptivity

• Makes a standard algorithm dynamic or kinetic
• Requires little change to the standard algorithm
• Can be done semi-automatically
• Not all algorithms yield efficient adaptive
  algorithms
• Will talk about this more

7
How does Adaptivity Work?

• Represent a computation with a dynamic dependency
  graph
• nodes data, edges dependencies
• Sources input, sinks output,
• The user can
• change the input,
• update the output
• Update
• Take a changed node,
• Update all its children (the children are now
  changed)
• Repeat until no more changed nodes

8
Adaptivity Example
1
3
2
b
c
fun f (a,b,c) let u ab in if (u gt
0) then g(u) else g(bc) end
a
ab
1b
u
3

if ugt0 then g(u) else g(bc)
g(u)

r
0.33
9
Adaptivity Example Change
3
2
-2
b
c
a
fun f (a,b,c) let u ab in if (u gt
0) then g(u) else g(bc) end
ab
1b
u
3

if ugt0 then g(u) else g(bc)
g(u)

r
0.33
10
Update
-2
3
2
b
c
a
fun f (a,b,c) let u ab in if (u gt
0) then g(u) else g(bc) end
ab
-2b
u
v
0
5

if ugt0 then g(u) else g(bc)
g(u)
g(v)

r
0.5
11
Update
-2
3
2
b
c
a
fun f (a,b,c) let u ab in if (u gt
0) then g(u) else g(bc) end
ab
-2b
bc
2c
u
v
0
5
g'(v)

if ugt0 then g(u) else g(bc)
g(v)

r
0.5
12
Adaptivity and Stability

• Adaptivity updates the result by rerunning the
  parts of the computation affected by the input
  change
• It is efficient when the computation is
  stable,I.e., computations on similar inputs
  are similar
• We apply the adaptivity technique to convex hulls
• Result Efficient Dynamic and Kinetic convex
  hulls

13
1-D Convex Hull Max and Min

• Just consider upper hull Finding the maximum
• Consider two algorithms
• The March March through the list
• The Tournament pair up the elements and take the
  max of each pair

14
Kinetic Maximum

• Numbers increase/decrease continuously in time
  ni(t) ni ci t

ni
n1(t) 2t
n2(t) 5
n3(t) 6-t
time
tgreen 1
tblue 3
tred 0
15
Sampling
n1(t) 2t
ni
n2(t) 5
n3(t) 6-t
time
tgreen 1
tblue 3
tred 0
16
Internal and External Events
External event Final result changes
Internal event Final result does not change
but a test fails
17
Proof Simulation via Certificates

• A set of comparisons that prove the current
  maximum
• Associate a certificate with each comparison
• certificate comparison result failure time
• Consider the times that a certificate fails
• We need an algorithm that updates the result as
  well as the certificates
• Use adaptivity to obtain to do the update
  efficiently

18
Kinetic March
0.66
0.5
4
Time0
0.66
Time0.5e
Time0.66e
19
Kinetic March Performance

• O(n) Because an item in the beginning of the
  list could become the maximum and it will be
  compared to the rest of the list
• Not acceptable because computing from scratch
  takes O(n)

20
The Kinetic Tournament
Time 0
f 2
21
The Kinetic Tournament
Time2e
7
f6
1t
5-t
22
The Kinetic Tournament
f7
8
Time6e
1t
1t
5-t
23
The Kinetic Tournament
f7
1t
Time7e
1t
1t
5-t
24
Performance of Kinetic Tournament

• Worst case log n time per event
• This kinetic algorithm is an adaptive version of
  the standard tournament algorithm for finding
  maximum

25
2-D Convex Hull

• Many algorithms Quick Hull, Graham Scan,
  Incremental, Merge Hull, Ultimate, Improved
  Ultimate...
• We will focus on the Quick Hull algorithm
• Input A list of points P
• Output The boundary points on the hull of P
• Example Input a,b,c,d Output
  a,b,d

a
b
c
d
26
Quick Hull Example
J
O
K
P
A
L
N
A B C D E F G H I J K L M N O P
27
Quick Hull Example - Filter
J
O
K
P
A
L
N
A B D F G H J K M O P
28
Quick Hull Example - Maximum
J
O
K
P
A
L
N
A B D F G H J K M O P
29
Quick Hull Example - Filter
J
O
K
P
A
L
N
A B F J J O P
30
Quick Hull Example - Maximum
J
O
B
K
P
A
L
N
A B F J J O P
31
Quick Hull Example - Base Case
J
O
B
K
P
A
L
N
A B B J J O P
32
Quick Hull Example - Done
J
O
B
K
P
A
L
N
A B B J J O O P
33
Kinetic Quick Hull

• Two kinds of tests Line-side and distance
  comparisons
• Filtering gt Line Side
• Finding the furthest point gt Distance
  comparisons
• Have certificates for these two events that is all

34
Line Side Test Fails
J
O
K
P
A
L
I
N
A B D F G H J K M O P
35
I is inserted in the middle of the list
J
O
K
P
I
A
L
N
A B D F G I H J K M O P
36
Recompute Maximum
J
O
K
P
I
A
L
N
A B D F G I H J K M O P
37
Dynamic Tournament - Random Trees
38
Dynamic Tournament - Random Trees
6
9
39
Dynamic Tournament - Random Trees
9
9
9
9
6
9
40
Distance Comparison Fails - Case 1
J
O
K
P
A
L
N
A B D F G H J K M O P
41
Distance Comparison Fails - Case 2
J
O
K
P
A
L
N
A B D F G H J K M O P
42
B is the new maximum
B
J
O
K
P
A
L
N
A B D F G H J K M O P
43
New recursive calls
B
J
O
K
P
A
L
N
A B J M O
44
Experiments
45
Experiments
Events per External Event
25
20
events/external event
5logn
5logn2
15
event/external
10
5
0
0
20
40
60
80
100
120
Points
46
Experiments
47
Summary of my Work

• Kinetic Algorithms for convex hulls using
  adaptivity
• Bounce events Can maintain convex hull of
  points in a box - the points bounce off of the
  walls of the box
• Streamlined library for kinetic convex hulls in
  the SML language
• A standard algorithm can be made kinetic in a few
  hours of work

48
Work In Progress

• Timothy Chans O(h log n) algorithm Improved
  Ultimate Convex Hull Have a working version
• Graham Scan Algorithm Have a working version
• Merge Hull
• Incremental Convex Hull
• Graham Scan, Merge and Incremental Hull
  algorithms require an extension to adaptivity
• Performance analysis of various convex hull
  algorithms

49
Conclusion and Future Work

• Used adaptivity to solve the kinetic convex hull
  problem.
• Encouraging results.
• Adaptivity makes writing dynamic/kinetic
  algorithms a simple edition on the standard
  algorithm
• The quickhull algorithm updates based on events
  efficiently in the expected case.