Sweep Line algorithm [closed] - c++

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SWEEP LINE EXAMPLE WITH EXAMPLE: AFTER CORRECTION
The code Contais 3 classes:
Class Line to represent the Horizontal lines: Parameters:
(x1,y1)----------(x2,y2)
Class Point:
Class segment for the sweep line algorithm
I am trying to use the following code, to compute the intersections between horizontal lines, As you can from the coordinates that My lines are Horizontal:
Coordinates: x1,y1,x2,y2 I have x1 < x2, and y1 = y
Class Point:
#ifndef POINT_H
#define POINT_H
class Point
{
public:
Point(float, float);
float x;
float y;
};
#endif // POINT_H
#include "Point.h"
Point::Point(float xi, float yi):x(xi),y(yi)
{
}
Class Line
#ifndef LINE_H
#define LINE_H
class Line
{
public:
Line(int,int,int,int);
int x1, y1, x2, y2;
};
#endif // LINE_H
#include "Line.h"
Line::Line(int x_1, int y_1, int x_2,int y_2):x1(x_1),y1(y_1),x2(x_2),y2(y_2)
{
//ctor
}
Class Segment:
#ifndef SEGMENT_H
#define SEGMENT_H
#include "Point.h"
#include "Line.h"
#include <vector>
#include <set>
#include <algorithm>
using namespace std;
enum event_type { EVENT_END,EVENT_VERTICAL,EVENT_START};
struct event
{
event_type type;
int x;
int line; // index into list of lines
event() {}
event(event_type type, int x, int line) : type(type), x(x), line(line) {}
// sort by X then by type
bool operator <(const event &b) const
{
if (x != b.x) return x < b.x;
else return type < b.type;
}
};
class Segment
{
public:
Segment();
vector<Point> hv_intersections(const vector<Line> &);
};
#endif // SEGMENT_H
#include "Segment.h"
Segment::Segment()
{
//ctor
}
vector<Point> Segment::hv_intersections(const vector<Line> & lines)
{
int L = lines.size();
vector<event> events;
vector<Point> ans;
// Y coordinates of active lines
multiset<int> active;
// Convert lines into events
for (int i = 0; i < L; i++)
{
if (lines[i].y1 != lines[i].y2)
events.push_back(event(EVENT_VERTICAL, lines[i].x1, i));
else if (lines[i].x1 != lines[i].x2)
{
events.push_back(event(EVENT_START, lines[i].x1, i));
events.push_back(event(EVENT_END, lines[i].x2, i));
}
}
// Sort events by X
sort(events.begin(), events.end());
// Returns all intersections between lines. The lines must all be either
// horizontal and vertical, and only horizontal-vertical intersections are
// counted (i.e. not overlaps). Lines are considered to exclude their
// endpoints. Also, each line must have x1 <= x2 and y1 <= y2.
for (vector<event>::const_iterator e = events.begin(); e != events.end(); ++e)
{
switch (e->type)
{
case EVENT_START:
active.insert(lines[e->line].y1);
break;
case EVENT_END:
active.erase(active.find(lines[e->line].y1));
break;
case EVENT_VERTICAL:
{
// Iterate over all y values for intersecting horizontal lines
multiset<int>::const_iterator first, last, i;
first = active.upper_bound(lines[e->line].y1);
last = active.lower_bound(lines[e->line].y2);
for (i = first; i != last; ++i)
ans.push_back(Point(e->x, *i));
}
break;
}
}
return ans;
}
int main()
{
vector<Line> v;
v.push_back(Line(0, 5, 10, 5));
v.push_back(Line(5, 0, 5, 10));
Segment s;
vector<Point> p = s.hv_intersections(v);
cout << p.size() << endl;
return 0;
}

Answer 1

The code is correct (assuming you have proper #include and main() methods). The lines you provided have no intersections. If you add any vertical line like 800 100 800 2000 you will get a vector<pnt> of intersections.

Answer 2

Parallel lines (in this case horizontal lines) do not intersect. Never.
http://en.wikipedia.org/wiki/Parallel_(geometry)