More Acceptance Tests

Box2d Testbed project 藏有許多漂亮的 demos，之前已經完成最簡單的 freefall 測試，這次玩複雜一點的 VaryingFriction 測試：準備五個箱子，摩擦係數皆不同。摩擦係數高 > 0，走的距離短，最後會卡在滑坡上；摩擦係數零的箱子會一直滑下去 (牛頓第一運動定律：動者恆動靜者恆靜)。

這次 test 採用 gtest 的 Test Fixtures，這樣就可以寫各式各樣類似的測試 (僅僅 time_step 會變)。

整個 code 很簡單：

#include <gtest/gtest.h>
#include "physics_simulation/box2d/box2d_service.h"
#include "physics_simulation/box2d/box2d_world.h"
#include "physics_simulation/box2d/box2d_body.h"

#include "physics_simulation/world_config.h"
#include "physics_simulation/body_config.h"
#include "physics_simulation/fixture_config.h"
#include "physics_simulation/polygon_shape_config.h"
#include <math.h>

namespace physics_simulation {
namespace box2d {

class VaryingFrictionTest : public testing::Test {
 protected:
  virtual void SetUp();
  virtual void TearDown();

  World* world_;
  Body* box_[5];
  Body* track_[3];
  Body* ground_;

 private:
  void SetUpWorld();
  void SetUpGround();
  void SetUpTopTrack();
  void SetUpMiddleTrack();
  void SetUpBottomTrack();
  void SetUpRightStopper();
  void SetUpLeftStopper();
  void SetUpFiveBoxes();

  Body* stopper_[2];  
};

void VaryingFrictionTest::SetUp() {
  SetUpWorld();
  SetUpGround();
  SetUpTopTrack();
  SetUpMiddleTrack();
  SetUpBottomTrack();
  SetUpRightStopper();
  SetUpLeftStopper();
  SetUpFiveBoxes();
}

void VaryingFrictionTest::TearDown() {
  delete world_;
}

void VaryingFrictionTest::SetUpWorld() {
  Service* service = new Box2dService();

  WorldConfig config;
  config.gravity.set(0.0f, -10.0f);
  this->world_ = service->createWorld(config);

  delete service;
}

void VaryingFrictionTest::SetUpGround() {
  BodyConfig config;
  this->ground_ = world_->createBody(config);

  PolygonShapeConfig shape;
  shape.setAsEdge(math::Vector2(-40.0f, 0.0f),
                  math::Vector2( 40.0f, 0.0f));
  FixtureConfig fixture;
  fixture.shape_config = &shape;
  fixture.density = 0.0f;
  this->ground_->createFixture(fixture);
}

void VaryingFrictionTest::SetUpTopTrack() {
  BodyConfig config;
  config.position.set(-4.0f, 22.0f);
  config.angle = -0.25f;
  this->track_[0] = world_->createBody(config);

  PolygonShapeConfig shape;
  shape.setAsBox(13.0f, 0.25f);
  FixtureConfig fixture;
  fixture.shape_config = &shape;
  fixture.density = 0.0f;
  this->track_[0]->createFixture(fixture);
}

void VaryingFrictionTest::SetUpMiddleTrack() {
  BodyConfig config;
  config.position.set(4.0f, 14.0f);
  config.angle = 0.25f;
  this->track_[1] = world_->createBody(config);

  PolygonShapeConfig shape;
  shape.setAsBox(13.0f, 0.25f);
  FixtureConfig fixture;
  fixture.shape_config = &shape;
  fixture.density = 0.0f;
  this->track_[1]->createFixture(fixture);
}

void VaryingFrictionTest::SetUpBottomTrack() {
  BodyConfig config;
  config.position.set(-4.0f, 6.0f);
  config.angle = -0.25f;
  this->track_[2] = world_->createBody(config);

  PolygonShapeConfig shape;
  shape.setAsBox(13.0f, 0.25f);
  FixtureConfig fixture;
  fixture.shape_config = &shape;
  fixture.density = 0.0f;
  this->track_[2]->createFixture(fixture);
}

void VaryingFrictionTest::SetUpRightStopper() {
  BodyConfig config;
  config.position.set(10.5f, 19.0f);
  this->stopper_[0] = world_->createBody(config);

  PolygonShapeConfig shape;
  shape.setAsBox(0.25f, 1.0f);
  FixtureConfig fixture;
  fixture.shape_config = &shape;
  fixture.density = 0.0f;
  this->stopper_[0]->createFixture(fixture);
}

void VaryingFrictionTest::SetUpLeftStopper() {
  BodyConfig config;
  config.position.set(-10.5f, 11.0f);
  this->stopper_[1] = world_->createBody(config);

  PolygonShapeConfig shape;
  shape.setAsBox(0.25f, 1.0f);
  FixtureConfig fixture;
  fixture.shape_config = &shape;
  fixture.density = 0.0f;
  this->stopper_[1]->createFixture(fixture);
}

void VaryingFrictionTest::SetUpFiveBoxes() {
  PolygonShapeConfig shape;
  shape.setAsBox(0.5f, 0.5f);

  FixtureConfig fixture;
  fixture.shape_config = &shape;
  fixture.density = 25.0f;

  const float friction[5] = { 0.75f, 0.5f, 0.35f, 0.1f, 0.0f };
  for (int i = 0; i < 5; ++i) {
    BodyConfig config;
    config.type = BodyConfig::Dynamic;
    config.position.set(-15.0f + 4.0f * i, 28.0f);
    box_[i] = world_->createBody(config);

    fixture.friction = friction[i];
    box_[i]->createFixture(fixture);
  }
}

namespace {

float dist(const Vector2& u, const Vector2& v) {
  return Vector2(u.x - v.x, u.y - v.y).length();
}

}  // namespace

TEST_F(VaryingFrictionTest, AfterOneMinute) {
  const int steps = 60 * 60; // One minute
  const float time_step = 1.0f/60.0f;
  for (int i = 0; i < steps; ++i) {
    world_->simulate(time_step);
  }

  // Box #0 and #1 are on the top track.
  for (int i = 0; i < 2; ++i) {
    EXPECT_LT(dist(box_[i]->position(), track_[0]->position()),
              dist(box_[i]->position(), track_[1]->position()));
    EXPECT_LT(dist(box_[i]->position(), track_[0]->position()),
              dist(box_[i]->position(), track_[2]->position()));
  }

  // Box #2 is on the bottom track
  EXPECT_LT(dist(box_[2]->position(), track_[2]->position()),
            dist(box_[2]->position(), track_[1]->position()));
  EXPECT_LT(dist(box_[2]->position(), track_[2]->position()),
            dist(box_[2]->position(), track_[0]->position()));

  // Box #3 is on the ground
  EXPECT_FLOAT_EQ(0.51503909f, box_[3]->position().y);

  // Box #4 is out of the screen
  EXPECT_TRUE(box_[4]->position().length() > 2000.0f);
}

}  // namespace box2d
}  // namespace physics_simulation

強力推薦大家使用 Google C++ Testing Framework. 接下來再寫幾個 acceptance tests，然後開始偷竊游戲物理設定。