Nerd Paradise : Coding Interview 13: Byte Image

Nerd Paradise : Coding Interview 13: Byte Image

You have a monochrome image (black and white only). You are to write a function that will take in this image, an x-coordinate, a y-coordinate, a width, and a height, and draw a black rectangle that has its top left corner position at the x/y coordinate and with the given width and height.

Also, the image is stored as a two-dimensional array of bytes where each byte represents 8 horizontally consecutive pixels. (1 = white, 0 = black).

This isn't a particularly complex question. The correct description would be something more along the lines of a "compound" question. This is more or less a test to see how well you can think through lots of small moving parts.

The answer isn't particularly profound, but do try to work through it on a whiteboard first before looking at the answer.

void Draw(byte[,] pixels, int left, int top, int width, int height)
{

    int right = left + width - 1;
    int left_x = left;
    int right_x = right;
    int left_column = left_x / 8;
    int right_column = right_x / 8;

    byte left_mask = (byte)(~((1 << (8 - (left % 8))) - 1));
    byte right_mask = (byte)((1 << (7 - (right % 8))) - 1);

    if (left_column == right_column)
    {
        left_mask = left_mask | right_mask;
        right_mask = 0xff;
    }

    for (int y = top; y < top + height; ++y)
    {
        pixels[left_column,y] &= left_mask;
        pixels[right_column,y] &= right_mask;
        for (int x = left_column + 1; x < right_column; ++x)
        {
            byte[x,y] = 0;
        }
    }
}

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