2D Platformer Collision Detection/Resolution

Question

I am working on a 2D tile based platformer and was hoping to get someone to review my collision detection. I've only included the X collision check since the Y is essentially the same. Any suggestions/criticisms are appreciated.

    // Corrects X movement if a collision occurs.
    private void checkXCollision()
    {
        // Find the range of tiles the player intersects
        Point topLeft = new Point(((int)(xtemp + dx) - collisionWidth/2) / map.getTileSize(), ((int)ytemp - collisionHeight/2) / map.getTileSize());
        Point bottomRight = new Point(((int)(xtemp + dx) + collisionWidth/2) / map.getTileSize(), ((int)ytemp + collisionHeight/2) / map.getTileSize());

        // Left
        if(dx < 0)
        {
            boolean collision = false;
            for(int row = topLeft.y; row <= bottomRight.y; row++)
            {
                if(map.getType(row, topLeft.x) == Tile.SOLID) 
                { 
                    collision = true; 
                    break;
                }
            }

            if(collision)
            {
                dx = 0;

                // Move player to edge of solid tile
                int col = ((int)(xtemp + dx) - collisionWidth/2) / map.getTileSize();
                xtemp = col * map.getTileSize() + collisionWidth / 2;
            }
            else
            {
                xtemp += dx;
            }
        }

        // Right
        if(dx > 0)
        {
            boolean collision = false;
            for(int row = topLeft.y; row <= bottomRight.y; row++)
            {
                if(map.getType(row, bottomRight.x) == Tile.SOLID) 
                { 
                    collision = true;
                    break;
                }
            }

            if(collision)
            {
                dx = 0;

                // Move player to edge of solid tile
                int col = ((int)(xtemp + dx) + collisionWidth/2) / map.getTileSize();
                xtemp = (col + 1) * map.getTileSize() - collisionWidth / 2 - 1;
            }
            else
            {
                xtemp += dx;
            }
        }
    }

Answer 1

• You have some code duplication there

  for(int row = topLeft.y; row <= bottomRight.y; row++)
  {
      if(map.getType(row, topLeft.x) == Tile.SOLID) 
      { 
          collision = true; 
          break;
      }
  }
  

  which should be extracted to a method like

  private boolean hasCollision(int start, int stop, int x){
  
      for(int row = start; row <= stop; row++)
      {
          if(map.getType(row, x) == Tile.SOLID) 
          { 
              return true;
          }
      }
      return false;
   }  
  

• If a collision is found you have this

  dx = 0;
  
  // Move player to edge of solid tile
  int col = ((int)(xtemp + dx) - collisionWidth/2) / map.getTileSize();
  

  Do you on purpose set dx = 0 and calling then (xtemp + dx) ?

• You are also doing a lot of collisionWidth/2 which should be extracted to a variable.

• you should extract the left and right checking to separate methods.

• in Java the convention is to place the opening brace { on the same line.

• using a guard clause for dx == 0 well remove the creation of the Point's

• comments should describe why something is done. **What is done should be described by the code itself by using meaningful names.

Implementing most the points above leads to

// Corrects X movement if a collision occurs.
private void checkXCollision()
{
    if (dx == 0) {
        return;
    }

    Point topLeft = new Point(((int)(xtemp + dx) - collisionWidth/2) / map.getTileSize(), ((int)ytemp - collisionHeight/2) / map.getTileSize());
    Point bottomRight = new Point(((int)(xtemp + dx) + collisionWidth/2) / map.getTileSize(), ((int)ytemp + collisionHeight/2) / map.getTileSize());

    if(dx < 0){

        checkLeftXCollision(topLeft, bottomRight);

    } else {

        checkRightXCollision(topLeft, bottomRight);

    }
}

private void checkLeftXCollision(Point topLeft, Point bottomRight) {

    if(hasCollision(topLeft.y, bottomRight.y, topLeft.x)) {
        dx = 0;

        int col = ((int)(xtemp + dx) - collisionWidth/2) / map.getTileSize();
        xtemp = col * map.getTileSize() + collisionWidth / 2;
    } else {
        xtemp += dx;
    }
}

private void checkRightXCollision(Point topLeft, Point bottomRight) {

    if(hasCollision(topLeft.y, bottomRight.y, bottomRight.x)) {
        dx = 0;

        int col = ((int)(xtemp + dx) + collisionWidth/2) / map.getTileSize();
        xtemp = (col + 1) * map.getTileSize() - collisionWidth / 2 - 1;
    } else {
        xtemp += dx;
    }
}    

private boolean hasCollision(int start, int stop, int x){

    for(int row = start; row <= stop; row++)
    {
        if(map.getType(row, x) == Tile.SOLID) 
        { 
            return true;
        }
    }
    return false;
}