import std.array, bitmap;

void floodFill(Color)(Image!Color img, in uint x, in uint y,
                      in Color color)
/*pure*/ nothrow in {
    assert (y < img.ny && x < img.nx);
} body {
    immutable target = img[x, y];
    static struct Pos { uint x, y; }
    auto stack = [Pos(x, y)];

    while (!stack.empty) {
        immutable p = stack.back;
        stack.popBack;
        if (p.y < img.ny && p.x < img.nx && img[p.x, p.y] == target) {
            img[p.x, p.y] = color;
            stack.assumeSafeAppend;
            stack ~= [Pos(p.x,     p.y + 1), Pos(p.x,     p.y - 1),
                      Pos(p.x + 1, p.y),     Pos(p.x - 1, p.y)];
        }
    }
}

void main() {
    Image!RGB img;
    loadPPM6(img, "unfilled_circ.ppm");
    img.floodFill(200, 200, RGB(127, 0, 0));
    img.savePPM6("unfilled_circ_flooded.ppm");
}