---
id: rect-geo
title: "Rectangle Geometry"
author: Darren Yao, Michael Cao, Benjamin Qi
description: "Geometry problems on USACO Bronze are usually quite simple and limited to intersections and unions of squares or rectangles."
frequency: 2
---
import { Problem } from "../models";

export const metadata = {
  problems: {
    blocked: [
        new Problem("Bronze", "Blocked Billboard", "759", "Easy", false, ["rect"]),
    ],
    general: [
		new Problem("Bronze", "Square Pasture", "663", "Intro", false, ["rect"]),
		new Problem("Bronze", "Blocked Billboard II", "783", "Easy", false, ["rect"]),
		new Problem("CF", "Div. 3 C - White Sheet", "contest/1216/problem/C", "Normal", false, ["rect"],"See this code (TODO; codeforces is down) for a nice implementation using the Java Rectangle class."),
    ]
  }
};

Most only include two or three squares or rectangles, in which case you can simply draw out cases on paper. This should logically lead to a solution. 

## Example: Blocked Billboard

<problems-list problems={metadata.problems.blocked} />

### Naive Solution

Since all coordinates are in the range $[-1000,1000]$, we can simply go through each of the $2000^2$ possible visible squares and check which ones are visible using nested for loops.

<spoiler title="Nested Loops">

```cpp
#include <bits/stdc++.h>
using namespace std;

bool ok[2000][2000];

int main() {
	freopen("billboard.in","r",stdin);
	freopen("billboard.out","w",stdout);
	for (int i = 0; i < 3; ++i) {
		int x1, y1, x2, y2; cin >> x1 >> y1 >> x2 >> y2;
		x1 += 1000, y1 += 1000, x2 += 1000, y2 += 1000;
		for (int x = x1; x < x2; ++x) 
			for (int y = y1; y < y2; ++y) {
				if (i < 2) ok[x][y] = 1;
				else ok[x][y] = 0;
			}
	}
	int ans = 0;
	for (int x = 0; x < 2000; ++x) 
		for (int y = 0; y < 2000; ++y) 
			ans += ok[x][y];
	cout << ans << "\n";
}
```
</spoiler>

Of course, this wouldn't suffice if the coordinates were up to $10^9$.

### Rectangle Class (Java)

A useful class in `Java` for dealing with rectangle geometry problems (though probably overkill) is the built-in [`Rectangle`](https://docs.oracle.com/javase/8/docs/api/java/awt/Rectangle.html) class. To create a new rectangle, use the following constructor:

```java
// creates a rectangle with upper-left corner at (x,y) with a specified width and height
Rectangle newRect = new Rectangle(x, y, width, height); 
```

The `Rectangle` class supports numerous useful methods. 

  - `firstRect.intersects(secondRect)` checks if two rectangles intersect.
  - `firstRect.union(secondRect)` returns a rectangle representing the union of two rectangles.
  - `firstRect.contains(x, y)` checks whether the integer point $(x,y)$ exists in firstRect.
  - `firstRect.intersection(secondRect)` returns a rectangle representing the intersection of two rectangles.
    - what happens when intersection is empty?

This class can often lessen the implementation needed in a lot of bronze problems and CodeForces problems.

For example, here is a nice implementation of the problem ([editorial](http://www.usaco.org/current/data/sol_billboard_bronze_dec17.html)).

<spoiler title="Java Solution With Built-in Rectangle Class">

```java
import java.awt.Rectangle; //needed to use Rectangle class
import java.io.*;
import java.util.*;

public class blockedBillboard{
	public static void main(String[] args) throws IOException{
		Scanner sc = new Scanner(new File("billboard.in"));
		PrintWriter pw = new PrintWriter(new FileWriter("billboard.out"));
		int x1, y1, x2, y2;

		//the top left point is (0,0), so you need to do -y2

		x1 = sc.nextInt(); y1 = sc.nextInt(); x2 = sc.nextInt(); y2 = sc.nextInt();
		Rectangle firstRect = new Rectangle(x1, -y2, x2-x1, y2-y1);

		x1 = sc.nextInt(); y1 = sc.nextInt(); x2 = sc.nextInt(); y2 = sc.nextInt();
		Rectangle secondRect = new Rectangle(x1, -y2, x2-x1, y2-y1);

		x1 = sc.nextInt(); y1 = sc.nextInt(); x2 = sc.nextInt(); y2 = sc.nextInt();
		Rectangle truck = new Rectangle(x1, -y2, x2-x1, y2-y1);

		long firstIntersect = getArea(firstRect.intersection(truck));
		long secondIntersect = getArea(secondRect.intersection(truck));

		pw.println(getArea(firstRect) + getArea(secondRect) 
				- firstIntersect - secondIntersect);
		pw.close();
	}
	public static long getArea(Rectangle r){
		if(r.getWidth() <= 0 || r.getHeight() <= 0){
			return 0;
		}
		return (long)r.getHeight() * (long)r.getWidth();
	}
}
```
</spoiler>

<spoiler title="Java Solution Without Built-in Rectangle Class">

```java
import java.io.*;
import java.util.*;

class blockedBillboard{
    public static void main(String[] args) throws IOException{
    	Scanner sc = new Scanner(new File("billboard.in"));
        PrintWriter pw = new PrintWriter(new FileWriter("billboard.out"));
        Rect a = new Rect();
        Rect b = new Rect();
        Rect t = new Rect();
        int x1, y1, x2, y2;
        a.x1 = sc.nextInt(); a.y1 = sc.nextInt(); a.x2 = sc.nextInt(); a.y2 = sc.nextInt();
        b.x1 = sc.nextInt(); b.y1 = sc.nextInt(); b.x2 = sc.nextInt(); b.y2 = sc.nextInt();
        t.x1 = sc.nextInt(); t.y1 = sc.nextInt(); t.x2 = sc.nextInt(); t.y2 = sc.nextInt();
        pw.println(area(a) + area(b) - intersect(a, t) - intersect(b, t));
        pw.close();
        sc.close();
    }
    static int area(Rect r){
        return (r.y2 - r.y1) * (r.x2 - r.x1);
    }
    static int intersect(Rect p, Rect q) {
        int xOverlap = Math.max(0, Math.min(p.x2, q.x2) - Math.max(p.x1, q.x1));
        int yOverlap = Math.max(0, Math.min(p.y2, q.y2) - Math.max(p.y1, q.y1));
        return xOverlap * yOverlap;
    }
}

class Rect{
    int x1, y1, x2, y2;
    Rect(){

    }
}
```
</spoiler>

### Rectangle Class (C++)

Unfortunately, C++ doesn't have a built in rectangle class, so you need to write the functions yourself. Here is the solution to Blocked Billboard written in C++ (thanks, Brian Dean!).

<spoiler title="C++ Solution">

```cpp
#include <iostream>
#include <fstream>
using namespace std;

struct Rect{
	int x1, y1, x2, y2;
};

int area(Rect r){
  	return (r.y2 - r.y1) * (r.x2 - r.x1);
}

int intersect(Rect p, Rect q){
  	int xOverlap = max(0, min(p.x2, q.x2) - max(p.x1, q.x1));
  	int yOverlap = max(0, min(p.y2, q.y2) - max(p.y1, q.y1));
  	return xOverlap * yOverlap;
}

int main(){
	ifstream cin ("billboard.in");
	ofstream cout ("billboard.out");
 	Rect a, b, t;  // billboards a, b, and the truck
  	cin >> a.x1 >> a.y1 >> a.x2 >> a.y2;
 	cin >> b.x1 >> b.y1 >> b.x2 >> b.y2;
  	cin >> t.x1 >> t.y1 >> t.x2 >> t.y2;

	cout << area(a) + area(b) - intersect(a, t) - intersect(b, t) << endl;
	return 0;
}

```
</spoiler>

## Problems

<problems-list problems={metadata.problems.general} />
