public static void FloodFill(ref Color[] source, Point pt, int width, int height, Color targetColor, Color replacementColor)
    {
        Stack<Point> pixels = new Stack<Point>();

        targetColor = source[P(pt.x, pt.y, width, height)];
        pixels.Push(pt);

        while (pixels.Count > 0)
        {
            Point a = pixels.Pop();

            if (source[P(a.x, a.y, width, height)] == targetColor)
            {
                source[P(a.x, a.y, width, height)] = replacementColor;

                if (a.x > 0)
                    pixels.Push(new Point(a.x - 1, a.y));

                if (a.x < width)
                    pixels.Push(new Point(a.x + 1, a.y));

                if (a.y > 0)
                    pixels.Push(new Point(a.x, a.y - 1));

                if (a.y < height)
                    pixels.Push(new Point(a.x, a.y + 1));
            }
        }
    }

private IEnumerator MainGenerationDebug(Color[] source, Color32[] target, int width, int height)
    {
        // (1)
        for (int i = 0; i < source.Length; Interlocked.Increment(ref i))
        {
            // (2)
            Color c = source[i];

            // (3)
            int x = i % width,
                y = i / width;

            // (4) & (5)
            yield return IntMapIteration(source, target, width, c, i, exceptions, (s, t) =>
            {
                source = s;
                target = t;
            });

            // (6)
            if (c == Color.white)
            {
                F.FloodFill(ref source, new Point(x, y), width, height, Color.white, Color.red);
            }
        }
    }