Solution to Polygon-Concavity-Index by codility

18 Oct

October 18, 2015 Sheng 6

Question: https://codility.com/demo/take-sample-test/polygon_concavity_index/

Question Name: Polygon-Concavity-Index or PolygonConcavityIndex

Thanks to Robert Sedgewick, the Youtube video (Algorithms, Part I – Convex Hull) tells everything about this problem.

def _IsClockwise(point_A, point_B, point_C):

''' Return the direction from points A -> B -> C.

'''

result = (point_B.x - point_A.x) * (point_C.y - point_A.y) -

(point_B.y - point_A.y) * (point_C.x - point_A.x)

# The direction of a->b->c is:

if result > 0: return 1 # counter-clockwise

elif result < 0: return -1 # clockwise

else: return 0 # a staight line

def solution(A):

''' The solution refers to:

https://www.youtube.com/watch?v=0HZaRu5IupM

'''

# Find the lowest point(s) in y-axis.

lowest_y = A[0].y

lowest_y_index = []

for i in xrange(len(A)):

if A[i].y < lowest_y:

lowest_y = A[i].y

lowest_y_index = [i]

elif A[i].y == lowest_y:

lowest_y_index.append(i)

else:

continue

# Find a point, which is not the lowest in y-axis and immediately

# after a lowest-in-y-axis point.

start_point = lowest_y_index[0]

lowest_y_array = [False] * len(A)

for i in lowest_y_index: lowest_y_array[i] = True

while lowest_y_array[start_point] == True:

start_point = (start_point + 1) % len(A)

start_point = (start_point - 1 + len(A)) % len(A)

# Re-organize the points so that, it is easier to check every three

# consecutive points in one loop (without module operation %).

rotated_A = A[start_point : ] + A[ : start_point]

# We find the start point such that, the direction is non-zero.

direction = _IsClockwise(rotated_A[-1], rotated_A[0], rotated_A[1])

extened_A = rotated_A + rotated_A[:2]

for i in xrange(len(A)):

temp = _IsClockwise(extened_A[i], extened_A[i+1], extened_A[i+2])

if temp * direction < 0:

# Compute the original index and return

return (i + 1 + start_point)%len(A)

# Every point is on the convex hull.

return -1

6 Replies to “Solution to Polygon-Concavity-Index by codility”

Levi says:
July 17, 2017 at 11:25 am
Hi! I do not understand how exactly to do it in languages that can have arithmetic overflow in functions like _IsClockwise. Levi
Reply
- Sheng says:
  July 18, 2017 at 10:25 pm
  hmm~ Good catch! Python does not have the overflow issue. And Java has BigInteger/BigDecimal. For C/C++, you can have a similar big number class or use more if-else in overflow detection.
  Reply

Is following code really required in above solution?

lowest_y_array = [False] * len(A)

for i in lowest_y_index: lowest_y_array[i] = True

while lowest_y_array[start_point] == True:

start_point = (start_point + 1) % len(A)

start_point = (start_point - 1 + len(A)) % len(A)

Sheng says:
September 3, 2020 at 6:23 pm
It seems the reference video is unavailable now. You can check the algorithm here: https://www.coursera.org/learn/algorithms-part1. The course is free and the best.
Reply

This is my C++ solution:
https://app.codility.com/demo/results/trainingFPR47Z-YXZ/

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#include <iostream>

#include <vector>

using namespace std;

struct Point2D {

int x;

int y;

};

int dir(const Point2D & point, const Point2D & previous, const Point2D & next) {

double v_x = point.x - previous.x;

double v_y = point.y - previous.y;

double w_x = next.x - previous.x;

double w_y = next.y - previous.y;

double determinant = v_x * w_y - v_y * w_x;

int result = 0;

if (determinant < 0) {

result = -1;

} else if (determinant > 0) {

result = 1;

}

return result;

}

int solution(vector<Point2D> &A) {

int N = A.size();

if (N < 4) return -1;

// step 1 - finding a point in the border of the the convex hull

int start_index = 0;

int lowest = A[0].x;

for (int i = 1; i < N; ++i) {

if (A[i].x < lowest) {

lowest = A[i].x;

start_index = i;

}

// step 2 - checking for direction changing

// discarding direction == 0

// if previous direction != current direction then it is not convex!

int last_dir = 0;

for (int i = start_index, counter = 0; counter < N; ++counter) {

int previous = (i + N - 1) % N;

int next = (i + 1) % N;

int direction = dir(A[i], A[previous], A[next]);

if (direction != 0) {

if (last_dir != 0) {

if (last_dir != direction) {

return i;

}

} else {

last_dir = direction;

}

i = (i + 1) % N;

}

// no direction changing found. Indicate convex hull by returning -1

return -1;

}

void check(int number, vector<Point2D> &A, int expected) {

int actual = solution(A);

cout << number << ") " << actual << " should be " << expected;

if (actual != expected) {

cout << " (WRONG) ";

}

cout << "\n";

}

void check1(int number) {

vector<Point2D> A(5);

A[0].x = -1; A[0].y = 3;

A[1].x = 1; A[1].y = 2;

A[2].x = 3; A[2].y = 1;

A[3].x = 0; A[3].y = -1;

A[4].x = -2; A[4].y = 1;

check(number, A, -1);

}

void check2(int number) {

vector<Point2D> A(7);

A[0].x = -1; A[0].y = 3;

A[1].x = 1; A[1].y = 2;

A[2].x = 1; A[2].y = 1;

A[3].x = 3; A[3].y = 1;

A[4].x = 0; A[4].y = -1;

A[5].x = -2; A[5].y = 1;

A[6].x = -1; A[6].y = 2;

check(number, A, 2);

}

void check3(int number) {

vector<Point2D> A(5);

A[0].x = -1; A[0].y = 3;

A[1].x = 1; A[1].y = 2;

A[2].x = 3; A[2].y = 1;

A[3].x = 0; A[3].y = -1;

A[4].x = 0; A[4].y = 1;

check(number, A, 4);

}

void check4(int number) {

//triangle

vector<Point2D> A(3);

A[0].x = 0; A[0].y = 0;

A[1].x = 1; A[1].y = 0;

A[2].x = 1; A[2].y = 1;

check(number, A, -1);

}

void check5(int number) {

// square

vector<Point2D> A(4);

A[0].x = 0; A[0].y = 1;

A[1].x = 1; A[1].y = 1;

A[2].x = 1; A[2].y = 0;

A[3].x = 0; A[3].y = 0;

check(number, A, -1);

}

void check6(int number) {

// boomerang

vector<Point2D> A(4);

A[0].x = 0; A[0].y = 10;

A[1].x = 1; A[1].y = 1;

A[2].x = 2; A[2].y = 10;

A[3].x = 1; A[3].y = 0;

check(number, A, 1);

}

void check7(int number) {

// losangle

vector<Point2D> A(4);

A[0].x = 0; A[0].y = 0;

A[1].x = 1; A[1].y = 10;

A[2].x = 2; A[2].y = 0;

A[3].x = 1; A[3].y = -1;

check(number, A, -1);

}

void check8(int number) {

// boomerang

vector<Point2D> A(4);

A[0].x = 0; A[0].y = 0;

A[1].x = 1; A[1].y = 10;

A[2].x = 2; A[2].y = 0;

A[3].x = 1; A[3].y = 1;

check(number, A, 3);

}

void check9(int number) {

// square counter clockwise

vector<Point2D> A(4);

A[0].x = 0; A[0].y = 0;

A[1].x = 1; A[1].y = 0;

A[2].x = 1; A[2].y = 1;

A[3].x = 0; A[3].y = 1;

check(number, A, -1);

}

void check10(int number) {

// straight-line

vector<Point2D> A(4);

A[0].x = 0; A[0].y = 0;

A[1].x = 1; A[1].y = 2;

A[2].x = 3; A[2].y = 6;

A[3].x = 5; A[3].y = 8;

check(number, A, -1);

}

int main() {

int caseNumber = 1;

check1(caseNumber++);

check2(caseNumber++);

check3(caseNumber++);

check4(caseNumber++);

check5(caseNumber++);

check6(caseNumber++);

check7(caseNumber++);

check8(caseNumber++);

check9(caseNumber++);

return 0;

}

sol says:
February 15, 2025 at 6:28 am
Your solution is wrong.
you don’t sort the points. For the following input it will return 0:
(-1, 3), (-2, 1), (1, 2), (3, 1), (0, -1)
Reply

6 Replies to “Solution to Polygon-Concavity-Index by codility”

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