box-car-2d/GLEssentials/Path.cpp

//
//  Path.cpp
//  OSXGLEssentials
//
//  Created by Joshua Moerman on 10/03/14.
//
//

#include "Path.h"

#include <cmath>
#include <random>

// A default thingy which makes implementing it easier. A subclass only have to
// provide new points, which will be connected (So this is not suitable for
// e.g. jumps or circles)
struct DefaultBuilder : PathBuilder {
    // number of vertices in one chunk
    static constexpr auto chainLength = 8;
    
    DefaultBuilder(b2World & world_, b2Vec2 startingPoint)
    : world(world_)
    , previous(startingPoint)
    , current(startingPoint)
    {
        fixture.friction = 2.0;
    }
    
    void buildUpTo(float x) override {
        b2BodyDef groundBodyDef;
        groundBodyDef.position = {0.0f, -10.0f};
        
        while(current.x < x){
            auto oldPreviousPoint = previous;
            
            b2Vec2 chainPositions[chainLength];
            chainPositions[0] = current;
            for(int i = 1; i < chainLength; ++i){
                chainPositions[i] = next;
                previous = current;
                current = next;
                // we always generate one more, for the b2 ghost vertices
                generate_next();
            }

            b2ChainShape chain;
            chain.CreateChain(chainPositions, chainLength);
            chain.SetPrevVertex(oldPreviousPoint);
            chain.SetNextVertex(next);
            
            fixture.shape = &chain;
            auto groundBody = world.CreateBody(&groundBodyDef);
            groundBody->CreateFixture(&fixture);
        }
    }
    
    virtual void generate_next() = 0;
    
protected:
    std::mt19937 generator{0};
    std::uniform_real_distribution<float> distribution{-1.0, 1.0};
    
    b2World & world;
    
    b2Vec2 previous;
    b2Vec2 current;
    b2Vec2 next;
    
    b2FixtureDef fixture;
};

// Creates random terrain, roughness comes with time
struct RandomBuilder : DefaultBuilder {
    RandomBuilder(b2World & world_, b2Vec2 startingPoint)
    : DefaultBuilder(world_, startingPoint)
    {
        generate_next();
    }

    virtual void generate_next() override {
        auto t = current.x;
        
        float m = std::sqrt(0.001f*t + 1) - 1;
        if(t <= 0) m = 0;
        float slope = m * distribution(generator);
        
        auto rot = b2Rot(slope);
        next = b2Mul(rot, {10, 0}) + current;
    }
};

// A flat road
struct FlatBuilder : DefaultBuilder {
    FlatBuilder(b2World & world_, b2Vec2 startingPoint)
    : DefaultBuilder(world_, startingPoint)
    {
        generate_next();
    }
    
    virtual void generate_next() override {
        b2Vec2 newPiece = {10, 0};
        next = newPiece + current;
    }
};

Path::Path(b2World & world_)
: world(world_)
, impl(new RandomBuilder(world, {-20, 0}))
{}

void Path::buildUpTo(float x){
    impl->buildUpTo(x);
}