#include <math.h></math.h>

#include <stdlib.h></stdlib.h>

#include <helianthus.h></helianthus.h>

#define MAX_BALLS   1000

#define MAX_LINKS   10000

#define BALL_RADIUS 1.0

#define BALL_FADE   0.8

#define LINK_K      0.05

typedef struct {

  double x, y;

  double vx, vy;

} Ball;

typedef struct {

  double dx, dy;

  Ball *a, *b;

} Link;

Ball balls[MAX_BALLS];

Link links[MAX_LINKS];

int ballsCount = 0;

int linksCount = 0;

double remainDt;

Ball *mball;

double mdx, mdy;

double mx, my;

void ballUpdate(Ball *ball, double dt) {

  double fade = pow(BALL_FADE, dt);

  ball->x += ball->vx*dt;

  ball->y += ball->vy*dt;

  ball->vx *= fade;

  ball->vy *= fade;

}

void linkUpdate(Link *link, double dt) {

  double dx = link->b->x - link->a->x;

  double dy = link->b->y - link->a->y;

  double dvx = (dx - link->dx)*0.5*LINK_K;

  double dvy = (dy - link->dy)*0.5*LINK_K;

  link->a->vx += dvx; link->b->vx -= dvx;

  link->a->vy += dvy; link->b->vy -= dvy;

}

void update(double dt) {

  remainDt += dt;

  while(remainDt > 0.001) {

    remainDt -= 0.001;

    for(int i = 0; i < linksCount; ++i)

      linkUpdate(&links[i], dt);

    for(int i = 0; i < ballsCount; ++i)

      ballUpdate(&balls[i], dt);

  }

}

void init() {

  int cnt = 10;

  double dist = 3*BALL_RADIUS;

  for(int i = 0; i < cnt; ++i) {

    for(int j = 0; j < cnt; ++j) {

      Ball *ball = &balls[ballsCount++];

      ball->x = (i - 0.5*(cnt - 1))*dist;

      ball->y = (j - 0.5*(cnt - 1))*dist;

      if (i > 0) {

        Link *link = &links[linksCount++];

        link->a = ball - cnt;

        link->b = ball;

        link->dx = dist;

      }

      if (j > 0) {

        Link *link = &links[linksCount++];

        link->a = ball - 1;

        link->b = ball;

        link->dy = dist;

      }

    }

  }

}

void draw() {

  double dt = windowGetFrameTime();

  double w = windowGetWidth();

  double h = windowGetHeight();

  saveState();

  translate(w/2, h/2);

  zoom(20);

  mx = mouseTransformedX();

  my = mouseTransformedY();

  if (mouseDown("left")) {

    if (!mball) {

      for(int i = ballsCount-1; i >= 0; --i) {

        Ball *ball = &balls[i];

        double dx = mx - ball->x;

        double dy = my - ball->y;

        if (dx*dx + dy*dy <= BALL_RADIUS*BALL_RADIUS) {

          mball = ball;

          mdx = dx;

          mdy = dy;

          break;

        }

      }

    }

  } else {

    mball = NULL;

  }

  update(dt);

  if (mball) {

    mball->x = mx - mdx;

    mball->y = my - mdy;

    mball->vx = 0;

    mball->vy = 0;

  }

  stroke(0xffe666ff);

  strokeWidth(BALL_RADIUS*0.1);

  noFill();

  for(int i = 0; i < linksCount; ++i) {

    Link *link = &links[i];

    line(link->a->x, link->a->y, link->b->x, link->b->y);

  }

  noStroke();

  fill(0xffe666ff);

  for(int i = 0; i < ballsCount; ++i) {

    Ball *ball = &balls[i];

    circle(ball->x, ball->y, BALL_RADIUS);

  }

  restoreState();

}

int main() {

  windowSetResizable(TRUE);

  windowSetVariableFrameRate();

  windowSetInit(&init);

  windowSetDraw(&draw);

  windowRun();

}