I/O-Efficient Batched Union-Find and Its Applications to Terrain Analysis

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I/O-Efficient Batched Union-Find and Its
Applications to Terrain Analysis

• Pankaj K. Agarwal, Lars Arge, Ke Yi
• Duke University
• University of Aarhus

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The Union-Find Problem

• A universe of N elements x1, x2, , xN
• Initially N singleton sets x1, x2 , , xN
• Each set has a representative
• Maintain the partition under
• Union(xi, xj) Joins the sets containing xi and
  xj
• Find(xi) Returns the representative of the set
  containing xi

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The Solution
representatives
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Union(d, h)
Find(n)
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link-by-rank
path compression
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Complexity

• O(N a(N)) for a sequence of N union and find
  operations Tarjan 75
• a() Inverse Ackermann function (very slow!)
• Optimal in the worst case Tarjan79, Fredman and
  Saks 89
• Batched (Off-line) version
• Entire sequence known in advance
• Can be improved to linear on RAM Gabow and
  Tarjan 85
• Not possible on a pointer machine Tarjan79

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Simple and Good, as long as

• The entire data structure fits in memory

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The I/O Model
Main memory of size M
One I/O transfers B items between memory and disk
Disk of infinite size
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Sources of Non-Locality

• Two operands in a union
• Nodes on a leaf-to-root path
• Operands in consecutive operations
• Cannot remove for the on-line case
• Need to eliminate all of them in order to get
  less than one I/O per operation!

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Our Results

• An I/O-efficient algorithm for the batched
  union-find problem using O(sort(N)) O(N/B
  logM/B(N/B)) I/Os
• Same as sorting
• optimal in the worst case
• A practical algorithm using O(sort(N) log(N/M))
  I/Os
• Implemented
• Applications to terrain analysis
• Topological persistence O(sort(N)) I/Os
• Implemented
• Contour trees O(sort(N)) I/Os

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I/O-Efficient Batched Union-Find

• Assumption No redundant unions
• Each union must join two different sets
• Will remove later
• Two-stage algorithm
• Convert to interval union-find
• Compute an order on the elements s.t. each union
  joins two adjacent sets
• Solve batched interval union-find

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Union Tree
1 Union(d, g) 2 Union(a, c) 3 Union(r, b) 4
Union(a, e) 5 Union(e, i) 6 Union(r, a) 7
Union(a, d) g 8 Union(d, h)
r 9 Union(b, f)
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Equivalent union trees
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Transforming the Union Tree
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Weights along root-to-leaf path decrease
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Formulating as a Batched Problem
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For each edge, find the lowest ancestor edgewith
a higher weight
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Cast in a Geometry Setting
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Euler Tour
x weight y positions in the tour
In O(sort(N)) I/Os Chiang et al. 95
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Cast in a Geometry Setting
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For each edge, find the lowest ancestor edgewith
a higher weight
For each segment, find the shortest segment above
and containing it
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Distribution Sweeping
M/B vertical slabs
checkedrecursively
Total cost O(sort(N))
checked here
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In-Order Traversal
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• At u, with child u1,, uk (in increasing order
  of weight)
• Recursively visit subtree at u1
• Return u
• For i2 ,, kRecursively visit subtree at ui

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Claim this traversalproduces the right order
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Solving Interval Union-Find
Union x two operands y time stamp Find x
operand y time stamp
Four instances of batched ray shooting O(sort(N))
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Handling Redundant Unions

• Union tree becomes a graph
• Compute the minimum spanning tree
• O(sort(N)) I/Os (randomized) Chiang et al. 95
  O(sort(N) loglog B) I/Os (deterministic) Arge et
  al. 04
• Deterministic O(sort(N)) I/Os if graph is planar
• Only MST edges are non-redundant

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A Practical Algorithm

• Previous algorithm too complicated
• 2 Euler tours
• 4 instances of batched ray shooting
• MST
• A simple and practical algorithm
• Divide-and-conquer
• O(sort(N) log(N/M)) I/Os
• Implemented

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Applications

1. Topological Persistence
2. Contour Trees

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Topological Persistence
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Formulated as Batched Union-Find

• Represented as a triangulated mesh
• Consider minimum-saddle pairs
• When reach
• A minimum or maximum do nothing
• A regular poin u Issue union(u,v) for a lower
  neighbor v
• A saddle u let v and w be nodes from us two
  connected pieces in its lower link Issue
  find(v), find(w), union(u,v), union(u,w)

lower link
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Contour Trees
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Previous Results

• Directly maintain contours
• O(N log N) time van Kreveld et al. 97
• Needs union-split-find for circular lists
• Do not extend to higher dimensions
• Two sweeps by maintaining components, then merge
• O(N log N) time Carr et al. 03
• Extend to arbitrary dimensions

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Join Tree and Split Tree
Qualified nodes
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Join tree
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Join tree
Split tree
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Final Contour Tree
Hard to BATCH!
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Join tree
Split tree
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Another Characterization
Let w be the highest node that is a descendant of
v in join tree and ancestor of u in split tree,
(u, w) is a contour tree edge
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Now can BATCH!
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Join tree
Split tree
Contour tree
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Experiment 1Random Union-Find
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Experiment 2 Topological Persistence on Terrain
Data
Neuse River Basin of NC
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Experiment 2 Topological Persistence on Terrain
Data
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Summary

• An I/O-efficient algorithm for the batched
  union-find problem using O(sort(N)) O(N/B
  logM/B(N/B)) I/Os
• optimal in the worst case
• A practical algorithm using O(sort(N) log(N/M))
  I/Os
• Applications to terrain analysis
• Topological persistence O(sort(N)) I/Os
• Contour trees O(sort(N)) I/Os
• Open Question On-line case
• Can we get below O(N a(N)) I/Os?

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Thank you!