collision

#include <SFML/Graphics.hpp>
#include <time.h>
using namespace sf;

const int M = 20;
const int N = 10;

int field[M][N] = {0}; // tetris grid

struct Point
{ int x,y; } a[4], b[4]; // current/next tile = 4 points

int figures[7][4] = // encode blocks as 4 squares on a 2x4 rectangular grid indexed 0,...,7
{
  1,3,5,7, // I
  2,4,5,7, // Z
  3,5,4,6, // S
  3,5,4,7, // T
  2,3,5,7, // L
  3,5,7,6, // J
  2,3,4,5, // 0
};

bool check() // collision detection
{
  for (int i = 0; i < 4; i++)
    if (a[i].x < 0 || a[i].x >= N || a[i].y >= M) return 0;
    else if (field[a[i].y][a[i].x]) return 0;

  return 1;
};

int main()
{
  srand(time(0));
  RenderWindow window(VideoMode(320, 480), "The Game!"); // window config

  Texture t;
  t.loadFromFile("/Users/alleyuan/code-snippets/16_games/01 Tetris/images/tiles.png"); // load sprite. replace my absolute path

  Sprite s(t);
  s.setTextureRect(IntRect(0,0,18,18));

  int dx = 0; bool rotate = 0; int colorNum=1;
  float timer = 0, delay = 0.3; // tickrate

  Clock clock;

  while (window.isOpen()) // game loop
  {
    float time = clock.getElapsedTime().asSeconds();
    clock.restart();
    timer += time;

    Event e;
    while (window.pollEvent(e)) // close window
    {
      if (e.type == Event::Closed)
        window.close();

      if (e.type == Event::KeyPressed) //rotate/move tile
        if (e.key.code == Keyboard::Up) rotate = true;
        else if (e.key.code == Keyboard::Left) dx=-1;
        else if (e.key.code == Keyboard::Right) dx=1;
    }

    // MOVE

    for (int i =0; i < 4; i++) { b[i] = a[i]; a[i].x += dx; } //left/right movement
    if (!check()) for (int i = 0; i < 4; i++) a[i] = b[i]; // reset a on invalid move

    if (rotate) // rotate clockwise
    {
      Point p = a[1]; // center of rotation
      for (int i = 0; i < 4; i++) // shift each square
      {
        int x = a[i].y-p.y;
        int y = a[i].x-p.x;
        a[i].x = p.x - x;
        a[i].y = p.y + y;
      }
      if (!check()) for (int i = 0; i < 4; i++) a[i] = b[i];
    }

    if (timer > delay) // drop the block
    {
      for (int i = 0; i < 4; i++) { b[i] = a[i]; a[i].y += 1; }

      if (!check()) // hit bottom => paint block and load a random next block
      {
        for (int i = 0; i < 4; i++) field[b[i].y][b[i].x] = colorNum;

        colorNum = 1+rand()%7;
        int n = rand()%7;
        for (int i = 0; i < 4; i++)
        {
          a[i].x = figures[n][i] % 2;
          a[i].y = figures[n][i] / 2;
        }
      }
      timer = 0;
    }

    dx = 0; rotate = 0;

    // DRAW

    window.clear(Color::White); // draw tiles

    for (int i = 0; i < M; i++) // paint all blocks
      for (int j = 0; j < N; j++)
      {
        if (field[i][j] == 0) continue;
        s.setPosition(j*18,i*18);
        window.draw(s);
      }
    for (int i = 0; i < 4; i++) // paint current block
    {
      s.setPosition(a[i].x*18,a[i].y*18);
      window.draw(s);
    }
    window.display();
  }

  return 0;
}