What exactly does this platformer scrolling code do?

Question

I am reading the source code of an existing platformer game and I came accross this:

    /* assign the horizontal position of the TileMap on the screen to offsetX
     * this, center aligns the player. */
    int offsetX = width / 2 - Math.round(player.getX()) - TILE_SIZE;
    /* stop the map scrolling if the player reaches the two ends
     * of the map */
    offsetX = Math.min(offsetX, 0); // if offsetX < 0 , offsetX = 0
    offsetX = Math.min(offsetX, width-tileMap.getWidth()); //if offsetX > map width, offsetX = mapwidth

    int offsetY = height - toPixels(tileMap.getHeight()); // not really necessary, I think

    int firstTile = toTiles(-offsetX); // ???
    int lastTile = firstTile + toTiles(width) + 1; // why the +1

I have commented some parts I think I understand, and asked questions about others in the comments.

The things that bother me most are:

1- how offsetX is assigned (width/2 ... ?) I have understood that it assigns offsetX to some place on the map which center aligns the player, but I have no idea how.

2-In the 3rd line, why did the developer write width - tileMap.getWidth()?

Note: If it is too troublesome to explain the code line by line, please give me a rough idea, perhaps with a diagram? of what the developer is trying to do here. thanks.

Answer 1

1-i think this offset is for drawing the tiles not for centering the player, this function increases the offset with a negative number when the player moves, the farther to the right he moves, the higher player.getX() is, the higher the offset is.

example:

width=300
player.getX()=300
TILE_SIZE=10
tileMap[0]=(x,y)=(10,10)

offset=300/2-300-10
offset= -160

now this offset is used to draw the tiles with a negative offset, in other words, further left, in this case tileMap[0]=(10-160,10) which means tileMap[0] is out of the screen range(tile scrolled to the left, player to the right)

2-i think this should be offsetX = Math.max(offsetX, width-tileMap.getWidth()); in that case its an extra check to only scroll to the end of the map.

example:

    width=300
    player.getX()=900
    TILE_SIZE=10
    tileMap[last]=(1010,10)
    tileMap.getWidth()=1000

    offset=300/2-900-10
    offset= -760
    offsetX = Math.min(-760, 0);
    offsetX = -760
    offsetX = Math.max(-760, 300-1000);
    offsetX = -700