//  Money

//

//  by Philip, January 16, 2021  (extremely slightly editted by Ben Hanson)

//

//  Simulate how money moves around between people in an economy, 

//  with the goal of seeing how different economic policies affect 

//  wealth distribution.  

//  

//  The size (area) of a person is proportional to their wealth.

//  Collisions are an opportunity for trade.

//  The Gini Index is constantly computed and displayed.  

//

​

let atoms = [];

​

let num = 200;

let v_min = 10;

let v_max = 50;

let r_min = 5;

​

let enable_income = false;     //  Press 'i' in the window to toggle basic

                               //  income and decay (wealth tax) on/off.

​

let balance_initial = 10.0;    //  How much money does everyone start with?

let fraction_to_risk = 0.5;    //  At most, what % of balance to pay?

​

let basic_income = 0.05;       //  Basic Income per time step. 

let decay_rate = 0.999;        //  Wealth tax (per time step)

​

​

​

let adjust_radius = 0.05;      //  How big are the people?

let min_radius = 2;

​

let repeat = false; 

let collisions = true; 

​

let moneySupply = balance_initial * num;

let averageBalance = moneySupply / num;

let numRich = 0;

let numPoor = 0;

let giniIndex = 0;

​

let frame = 0;

let transactions = 0;

​

let blue = "";

let red = "";

let green = "";

​

function setup() {

  createCanvas(800, 600);

  noStroke();

  blue = color(0, 0, 255);

  red = color(255, 0, 0);

  green = color(0, 255, 0);

  for (let i = 0; i < num; i++) {

    let r = r_min;

    let c = blue;

    let v = createVector(v_min + random(-1.0, 1.0) * (v_max - v_min), 

                         v_min + random(-1.0, 1.0) * (v_max - v_min)); 

    let p = createVector(random(width), random(height));

​

    let unequal = false

    let b = 1

    if (unequal) {

      if (i == 0) {

        b = moneySupply - (num - 1);

      }

    } else {

      b = balance_initial;

    }

    atoms[i] = new atom(p, v, r, c, b, atoms, i);                     

  }

}

​

​

function draw() {

  frame++;

  background(128);

  for (let i = 0; i < num; i++) {

    atoms[i].update();

    atoms[i].display();

  }

  noStroke();

  strokeWeight(1);

  updateStats(frame);

}

​

function computeGiniIndex() {

  //

  //  Gini index given as summation |xi - xj|

  // https://en.wikipedia.org/wiki/Gini_coefficient#Calculation

  //

  averageBalance = 0;

  for (let i = 0; i < num; i++) {

    averageBalance += atoms[i].balance;

  }

  averageBalance /= num;

  let sumOfDifferences = 0;

  for (let i = 0; i < num; i++) {

    for (let j = 0; j < num; j++) {

      sumOfDifferences += abs(atoms[i].balance - atoms[j].balance);

    }

  }

  return sumOfDifferences/(2 * num * num * averageBalance);

}

​

function updateStats(frame) {

  numRich = 0;

  numPoor = 0;

  moneySupply = 0;

  for (let i = 0; i < num; i++) {

    if (atoms[i].balance > averageBalance * 2) numRich++;

    else if (atoms[i].balance < averageBalance / 2) numPoor++;

      moneySupply += atoms[i].balance;

  }

  if (frame % 100 == 0) {

    // every once in a while, re-compute the giniIndex

    giniIndex = computeGiniIndex();

  }

  noStroke();

  fill(128,128,128,220);

  rect(0, height - 20, width, 20);

  fill(0);

  noStroke();

  fill(green);

  text(round(numRich/num * 100)+"%", 50, height - 5);

  fill(red);

  text(round(numPoor/num * 100)+"%", 80, height - 5);

  fill(blue);

  text(round((num - numPoor - numRich)/num * 100)+"%", 110, height - 5);

  fill (0);

  text("Gini: " + round(giniIndex * 100,0), 150, height - 5);

  text("kXs: " + round(transactions / 1000,0), width - 60, height - 5);

  text("MS: " + round(moneySupply), width - 150, height - 5);

  if (enable_income) {

    fill(red);

    text("Income ON", width / 2, height - 5);

  } else {

    fill(0);

    text("Income OFF", width / 2, height - 5);

  }

}

​

function maybeTrade(i, j) {

  //  Two people meet, maybe they exchange some money. 

  let richer = j;

  let poorer = i;

  if (atoms[i].balance >= atoms[j].balance) {

    richer = i;

    poorer = j;

  }

  let buy = (random() > 0.5);

  //let amount = (buy ? atoms[richer].balance : atoms[poorer].balance) * random(fraction_to_risk);

  let amount = atoms[poorer].balance * random(fraction_to_risk);

  atoms[richer].balance += (buy ? -1.0 : 1.0) * amount;

  atoms[poorer].balance += (buy ? 1.0 : -1.0) * amount;

  transactions++;

}

​

function maybeTradeProportionally(i, j) {

  //  Two people meet, maybe they exchange some money. 

  let richer = j;

  let poorer = i;

  if (atoms[i].balance >= atoms[j].balance) {

    richer = i;

    poorer = j;

  }

  let buy = (random() > 0.5);

  //let amount = (buy ? atoms[richer].balance : atoms[poorer].balance) * random(fraction_to_risk);

  let prop = random(fraction_to_risk)

  let poorAmount = atoms[poorer].balance * prop;

  let richAmount = atoms[richer].balance * prop;

  if (buy) {

    atoms[richer].balance += poorAmount;

    atoms[poorer].balance -= poorAmount;

  } else {

    atoms[richer].balance -= richAmount;

    atoms[poorer].balance += richAmount;

  }

//  atoms[richer].balance += (buy ? -1.0 : 1.0) * poorAmount;

//  atoms[poorer].balance += (buy ? 1.0 : -1.0) * richAmount;

  transactions++;

}

​

// atom class

function atom(p, v, r, c, b, _others, _id) {

  this.p = p.copy();

  this.v = v.copy();

  this.r = r;

  this.c = c;

  this.others = _others;

  this.id = _id;

  this.dt = 1.0/60.0;

//  this.balance = balance_initial;

  this.balance = b

  this.update = function() {

    if (collisions) {

      // Check for collisions and add restoring forces

      let collisionForces = createVector(0,0,0);

      for (let i = 0; i < this.others.length; i++) {

        if (i != this.id) {

          let d = p5.Vector.sub(this.p, this.others[i].p);

          let penetration = ((this.r + this.others[i].r)) - d.mag();

          if (penetration > 0) {

            d.normalize();

            // Elastic component - restitution (note: does not conserve energy)

            collisionForces.add(d.mult(10 * penetration));

            //  When in collision, after first few secs, there may be trade

            if (frame > 300)  maybeTradeProportionally(this.id, i);

          }

        }

      }

      this.v.add(collisionForces.mult(1.0));

    }

    if (this.v.mag() > v_max) this.v.mult(v_max / this.v.mag());

    this.p.add(p5.Vector.mult(this.v, this.dt));

    // Your radius corresponds to your fraction of the money supply

    this.r = max(sqrt(this.balance / moneySupply * width * height * adjust_radius), min_radius);

    if (enable_income) {

      this.balance += basic_income;

      this.balance *= decay_rate;      

    }

​

    let average_balance = moneySupply / num;

    if (this.balance < average_balance / 2) this.c = red;

    else if (this.balance > average_balance * 2) this.c = green;

    else this.c = blue;

    //  Edge conditions

    if (repeat) {

      if (this.p.x > width) this.p.x -= width;

      if (this.p.x < 0) this.p.x += width;

      if (this.p.y > height) this.p.y -= height;

      if (this.p.y < 0) this.p.y += height;

    } else {

      if (this.p.x >= width) {

        this.v.x *= -1.0;

        this.p.x = width - 1;

      }

      if (this.p.x < 0) {

        this.v.x *= -1.0;

        this.p.x = 0.0;

      }

      if (this.p.y >= height) {

        this.v.y *= -1.0;

        this.p.y = height - 1;

      }

      if (this.p.y < 0) {

        this.v.y *= -1.0;

        this.p.y = 0.0;

      }

    }

  }

  this.display = function() {

    stroke(0);

    //strokeWeight(1);

    noStroke();

    fill(this.c);        

    ellipse(this.p.x, this.p.y, this.r * 2, this.r * 2);

  }

​

};

​

function keyTyped() {

  if (key === 'i') {

    enable_income = !enable_income;

  }

}

​