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# Box2D

Box2D is an open source C++ engine to simulate rigid bodies in 2D, it is also, AngryBirds' motor engine. You can check wikipedia Box2D explanation.

Box2D library is stored at erincatto/box2d, which is generated from this github repo.

## Bounces of a circle falling

In this example you will calculate whenever a circle falls from a certain height and bounces at a defined lower limit in the created world.

### Creating a new project

Create a new project and a main.cpp file:

```\$ bii init box2d_example -L
\$ cd box2d_example
\$ # Create main.cpp```

Now place the following code inside main.cpp:

main.cpp

```#include "Box2D/Box2D.h"
#include <iostream>

using namespace std;

int main(int argc, char** argv)
{
B2_NOT_USED(argc);
B2_NOT_USED(argv);

//**************************************************//
//                 Creating a World                        //
//**************************************************//

// Define the gravity vector.
b2Vec2 gravity(0.0f,-10.0f);

// Construct a world object, which will hold and simulate the rigid bodies.
b2World world(gravity);

//**************************************************//
//                 Creating a Ground Box               //
//**************************************************//

// Define the ground body.
b2BodyDef groundBodyDef;
groundBodyDef.position.Set(0.0f,-10.0f);

// Call the body factory which allocates memory for the ground body
// from a pool and creates the ground box shape (also from a pool).
// The body is also added to the world.
b2Body* groundBody = world.CreateBody(&groundBodyDef);

// Define the ground box shape.
b2PolygonShape groundBox;

// The extents are the half-widths of the box.
groundBox.SetAsBox(50.0f, 10.0f);

// Add the ground fixture to the ground body.
groundBody->CreateFixture(&groundBox, 0.0f);

//**************************************************//
//            Creating a Circle Shape                   //
//**************************************************//

b2BodyDef BodyDef;
BodyDef.type = b2_dynamicBody;
BodyDef.position = b2Vec2(0.0f, 4.0f);
BodyDef.userData = (void *) "Circle";
b2Body* body = world.CreateBody(&BodyDef);

b2CircleShape circle;

b2FixtureDef fixtureDef;
fixtureDef.density = 1.0f;
fixtureDef.friction = 2.0f;
fixtureDef.restitution = 0.5f;

fixtureDef.shape = &circle;

body->CreateFixture(&fixtureDef);

//**************************************************//
//            Simulating the World (of Box2D)     //
//**************************************************//

float32 timeStep = 1.0f / 60.0f;
int32 velocityIterations = 6;
int32 positionIterations = 2;

// This is our little game loop.
for (int32 i = 0; i < 100; ++i)
{
// Instruct the world to perform a single step of simulation.
// It is generally best to keep the time step and iterations fixed.
world.Step(timeStep, velocityIterations, positionIterations);

// Now print the position and angle of the body.
b2Vec2 position = body->GetPosition();
float32 angle = body->GetAngle();

if (position.y - 1.00 <= 0.001)
cout<< "Ball hits the ground!!" << endl;
else
cout<<"X = " << position.x << " Y = " << position.y << endl;
}
}```

Check the dependencies of the project with bii deps:

```\$ bii deps
your_user/box2d_example depends on:
system:
iostream
unresolved:
Box2D/Box2D.h```

Now, edit the biicode.conf file generated in the project folder. Add your `[requirements]` depending on the version you want and map your `[includes]`:

```[requirements]
erincatto/box2d: 10

[includes]
Box2D/Box2d.h: erincatto/box2d
```

Check again with bii deps and now all dependencies are resolved.

### Build the project

Next, the only thing left is building the project:

`\$ bii build`

Execute the binary placed in bin directory and this is how output looks like:

```~/box2d\$ bin/myuser_box2d_example_main
X = 0 Y = 3.99722
X = 0 Y = 3.99167
X = 0 Y = 3.98333
X = 0 Y = 3.97222
X = 0 Y = 3.95833
X = 0 Y = 3.94167
X = 0 Y = 3.92222
Ball hits the ground!!```

That's it!

## Open and build

This example is already in biicode: examples/box2d.

To give it a try, create a new project and open the block:

```\$ bii init box2d_example
\$ cd box2d_example
\$ bii open examples/box2d```

Build the example and execute it:

```\$ bii build
\$ cd bin
\$ # Execute it
...
X = 0 Y = 3.95833
X = 0 Y = 3.94167
X = 0 Y = 3.92222
Ball hits the ground!!```