#include <graphics.h>
#include <math.h>
#include <time.h>
#define WIDTH        1024        // 屏幕宽
#define HEIGHT        576            // 屏幕高
#define NUM_MOVERS    600            // 小球数量
#define    FRICTION    0.96f        // 摩擦力


// 定义小球结构
struct Mover
{
    COLORREF    color;            // 颜色
    float        x,     y;            // 坐标
    float        vX,    vY;            // 速度
};


// 定义全局变量
Mover    movers[NUM_MOVERS];            // 小球数组
int        mouseX,        mouseY;            // 当前鼠标坐标
int        mouseVX,    mouseVY;        // 鼠标速度
int        prevMouseX,    prevMouseY;        // 上次鼠标坐标
bool    isMouseDown;                // 鼠标左键是否按下
DWORD*    pBuffer;                    // 显存指针


// 初始化
void init()
{
    // 初始化小球数组
    for (int i = 0; i < NUM_MOVERS; i++)
    {
        movers[i].color = RGB(rand() % 256, rand() % 256, rand() % 256);
        movers[i].x   = WIDTH * 0.5;
        movers[i].y   = HEIGHT * 0.5;
        movers[i].vX  = float(cos(float(i))) * (rand() % 34);
        movers[i].vY  = float(sin(float(i))) * (rand() % 34);
    }
   
    // 初始化鼠标变量
    mouseX = prevMouseX = WIDTH / 2;
    mouseY = prevMouseY = HEIGHT / 2;

    // 设置随机种子
    srand((unsigned int)time(NULL));

    // 获取显存指针
    pBuffer = GetImageBuffer(NULL);
}


// 全屏变暗 50%
void darken()
{
    for(int i = WIDTH * HEIGHT - 1; i >= 0; i--)
        if (pBuffer[i] != 0)
            pBuffer[i] = RGB(GetRValue(pBuffer[i]) >> 1, GetGValue(pBuffer[i]) >> 1, GetBValue(pBuffer[i]) >> 1);
}


// 绝对延时
void delay(DWORD ms)
{
    static DWORD oldtime = GetTickCount();

    while(GetTickCount() - oldtime < ms)
        Sleep(1);

    oldtime = GetTickCount();
}


// 绘制动画(一帧)
void run()
{
    darken();    // 全屏变暗

    mouseVX    = mouseX - prevMouseX;
    mouseVY    = mouseY - prevMouseY;
    prevMouseX = mouseX;
    prevMouseY = mouseY;
   
    float toDist   = WIDTH * 0.86f;
    float stirDist = WIDTH * 0.125f;
    float blowDist = WIDTH * 0.5f;
   
    for(int i = 0; i < NUM_MOVERS; i++)
    {
        float x  = movers[i].x;
        float y  = movers[i].y;
        float vX = movers[i].vX;
        float vY = movers[i].vY;
        
        float dX = x - mouseX;
        float dY = y - mouseY;
        float d  = (float)sqrt(dX * dX + dY * dY);
        dX = d ? dX / d : 0;
        dY = d ? dY / d : 0;
        
        if (isMouseDown && d < blowDist)
        {
            float blowAcc = (1 - (d / blowDist)) * 14;
            vX += dX * blowAcc + 0.5f - float(rand()) / RAND_MAX;
            vY += dY * blowAcc + 0.5f - float(rand()) / RAND_MAX;
        }
        
        if (d < toDist)
        {
            float toAcc = (1 - (d / toDist)) * WIDTH * 0.0014f;
            vX -= dX * toAcc;
            vY -= dY * toAcc;            
        }
        
        if (d < stirDist)
        {
            float mAcc = (1 - (d / stirDist)) * WIDTH * 0.00026f;
            vX += mouseVX * mAcc;
            vY += mouseVY * mAcc;            
        }
        
        vX *= FRICTION;
        vY *= FRICTION;
        
        float avgVX = (float)fabs(vX);
        float avgVY = (float)fabs(vY);
        float avgV  = (avgVX + avgVY) * 0.5f;
        
        if (avgVX < 0.1) vX *= float(rand()) / RAND_MAX * 3;
        if (avgVY < 0.1) vY *= float(rand()) / RAND_MAX * 3;
        
        float sc = avgV * 0.45f;
        sc = max(min(sc, 3.5f), 0.4f);
        
        float nextX = x + vX;
        float nextY = y + vY;
        
        if        (nextX > WIDTH)    { nextX = WIDTH;    vX *= -1; }
        else if (nextX < 0)        { nextX = 0;        vX *= -1; }
        if        (nextY > HEIGHT){ nextY = HEIGHT;    vY *= -1; }
        else if (nextY < 0)        { nextY = 0;        vY *= -1; }
        
        movers[i].vX = vX;
        movers[i].vY = vY;
        movers[i].x  = nextX;
        movers[i].y  = nextY;
        
        // 画小球
        setcolor(movers[i].color);
        setfillstyle(movers[i].color);
        fillcircle(int(nextX + 0.5), int(nextY + 0.5), int(sc + 0.5));
    }
}


// 主函数
void main()
{
    // 创建绘图窗口
    initgraph(WIDTH, HEIGHT);
    // 启用批绘图模式
    BeginBatchDraw();

    // 初始化
    init();

    // 鼠标消息变量
    MOUSEMSG m;

    while(true)
    {
        // 处理鼠标消息
        while(MouseHit())
        {
            m = GetMouseMsg();

            switch(m.uMsg)
            {
                case WM_MOUSEMOVE:        mouseX = m.x;    mouseY = m.y;    break;
                case WM_LBUTTONDOWN:    isMouseDown = true;                break;
                case WM_LBUTTONUP:        isMouseDown = false;            break;
            }
        }

        // 绘制动画
        run();

        // 显示缓存的绘制内容
        FlushBatchDraw();

        // 延时 20 毫秒
        delay(20);
    }

    // 关闭
    EndBatchDraw();
    closegraph();
}