A Convex Hull Algorithm Running Time Analysis

1. Convex Hull Algorithm Analysis
Rex Yuan
March 29, 2015
Brute Force Approach
The most naive brute force approach to the convex hull problem iterates over
all points three time. First two iterations yield all possible lines formed by the
points, and the last iteration checks all points against all lines. If a pair of point
(p1, p2) satisfies the condition that all other points are in the same side from
p1p2, we confirm that (p1, p2) are two of the most outer points, and thus are of
the convex hull.
Pseudo Code
Algorithm 1 Convex Hull Brute Force Algorithm
procedure ConvexHullBF(Points)
results = []
for all p1 in Points do
for all p2 in Points do
for all p3 in Points do
Currentside ← side point3 is on respecting p1p2
if side not defined then
side ← Currentside
else if Currentside = side then
side ← False
break
end if
end for
if side = False then
append p1 and p2 to results
end if
end for
end for
end procedure
1

2. Time Complexity
The worst total operations of naive brute force algorithm takes three iterations
over n point, which is approximately 3n3
which is O(n3
).
Divide and Conquer Approach
The divide and conquer approach takes a collection of points, Points, and a pair
of two points (Rp, Lp) which are the rightmost and leftmost points in Points,
regarding x axis. Next, find the furthest point Furthestp from RpLp, and
divide Points into four parts with LpFurp and RpFurp. Then, recurse left
with (Lp, Furp) and the left/down portion of Points and right with (Furp, Rp)
and the right/up portion of Points.
Pseudo Code for Main Algorithm
Algorithm 2 Convex Hull Divide and Conquer Algorithm
results = []
function DC(Rp, Lp, Points)
if Points is empty then base case
return Θ(1)
end if
Furp ← Furp(RpLp, Points) Θ(n)
Lps, Rps ← LR(RpLp, FurpLp, RpFurp, Points) Θ(n)
append Furp to results Θ(1)
DC(Lp, Furp, Lps)
DC(Furp, Rp, Rps)
end function
procedure ConvexHullDC(results)
Rightp, Leftp ← rightmost and leftmost points in input, regarding x axis
Ups ← points in the upper sections of line RightpLeftp
Los ← points in the Lower sections of line RightpLeftp
DC(Rightp, Leftp, Ups)
DC(Rightp, Leftp, Los)
end procedure
The algorithm(Furp) to calculate furthest point takes a line(p1p2) and a col-
lection of point(ps), and then iterates through ps to find the point with furthest
distance from p1p2 in ps and return that point(result). The algorithm(LR)
to find left/right sections takes three lines(p1p2, p1p3, p2p3) and a collection
of points(ps) and use the slope(m) to find the appropriate left/right sections.
It then iterates over ps to find two collections of points(leftps, rightps), corre-
sponding to left/right sections of ps and return leftps and rightps
2

3. Pseudo Code for Helper Functions
Algorithm 3 Furthest Point in ps from p1p2
function Furp(p1p2, ps)
result = None, furd = 0
for all p in ps do
if Dist(p, p1p2) > furd then
furd ← Dist(p, p1p2)
result ← p
end if
end for
end function
Algorithm 4 Left and Right Sections of ps
function LR(p1p2, p1p3, p2p3, ps)
leftps = []
rightps = []
left ← check the slope(m) of p1p2, p1p3, p2p3 to find out how is ps
divided, and then decide which is the left side
right ← check the slope(m) of p1p2, p1p3, p2p3 to find out how is ps
divided, and then decide which is the right side
for all p in ps do
if p in left then
append p to leftps
else if p in right then
append p to rightps
end if
end for
return leftps, rightps
end function
Divide and Conquer Approach Analysis
The recurrence of the worst case of the divide and conquer approach is
T(n) = 2T(
n
2
) + O(n)
because every recursion call takes about 2n+1 operations which is O(n), and for
the two subsequent recursion calls it makes, the worst case would be no points
were eliminated in that call, so the sum of two subsequent inputs is still n. An
example of this of this would be when input points form a circle, making the
two subsequent input n
2 .
According to Master Method, this is Case 1(a = bd
). Thus, O(nlogn).
3

4. Input points form a hollow circle:no points will be eliminated in all calls.
Data Structure
In the actual implementation, I used the dictionary type in Python. With keys
representing index number of a point and its content a tuple of the coordinates
of that point.
{index : (x coordinate, y coordinate)}
Run Time Comparison
I created the following table using UNIX POSIX time function and round the
mean time of 10 trials to five digits after decimal point to calculate the time
past for either brute force and divide and conquer method.
Table 1: Multi-column table
Run Time Comparison(s)
Sample Brute Force Divide and Conquer
1 0.18544 0.00115
2 1.14565 0.00237
3 > 300 0.22048
4 > 300 24.31725
4