//  Random Transactions

//

//  by Philip, February 21, 2021  

//

//  People wander around and make transactions when they run into each other.   

//  

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

//

//  The Gini Index is constantly computed and displayed.  

//   

​

let people = 500;                //  How many people in the simulation?

let balance_initial = 3400.0;    //  How much money does everyone start with?

                                 //  3400 is the median cash balance for USA

​

let transactions_per_day = 2;    //  How many transactions correspond to an average person's day?

​

let adjust_radius = 0.0;  //0.05; //  How big are the people relative to their wealth?

let min_radius = 10;

​

​

let days = 0;                    //  How many days have gone by?

let frame = 0;

let days_last_frame = 0;

let transactions = 0;

​

let v_min = 10;

let v_max = 50;

let r_min = 5;

let atoms = [];

​

let repeat = false; 

let collisions = true; 

​

let moneySupply = balance_initial * people;

let averageBalance = moneySupply / people;

let numRich = 0;

let numPoor = 0;

let giniIndex = 0;

​

let blue = "";

let red = "";

let green = "";

​

​

function maybeTrade(i, j) {

  //

  //  Two people meet, and maybe they make a transaction 

  //

  let richer = j;

  let poorer = i;

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

    richer = i;

    poorer = j;

  }

  //  Randomly choose whether richer person buys or sells:

  let buy = (random() > 0.5);

  let buyer = (buy) ? richer : poorer;

  let seller = (buy) ? poorer : richer;               

​

  let amount = 0;

  let method = 0;

​

  if (method == 1) {

    // 

    //  Method 1:  Transaction amount is fixed fraction of buyer's balance

    //

    amount = atoms[buyer].balance * 0.05;

  }else if (method == 2) {

    //

    //  Method 2:  Transaction amount is fixed fraction of poorer person's balance 

    // 

    amount = atoms[poorer].balance * 0.05;

  } else if (method == 3) {

    //

    //  Method 3:  Transaction is a fixed amount, and only happens if buyer can afford it.

    //

    amount = 0.01 * averageBalance; 

  } else if (method == 4) {

    //

    //  random transfers

    // 

    amount = atoms[buyer].balance * random(1.0);

  }

  if (atoms[buyer].balance >= amount) {

    atoms[buyer].balance -= amount ;

    atoms[seller].balance += amount;

    transactions++;

  }

}

​

​

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 < people; 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));

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

  }

}

​

function draw() {

  frame++;

  background(128);

  let last_transactions = transactions;

  for (let i = 0; i < people; i++) {

    atoms[i].update();

    atoms[i].display();

  }

  days_last_frame = (transactions - last_transactions) / people / transactions_per_day;

  days += days_last_frame;

  noStroke();

  strokeWeight(1);

  updateStats(frame);

}

​

function computeGiniIndex() {

  //

  //  Uses discrete formula found at:

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

  //

  averageBalance = 0;

  for (let i = 0; i < people; i++) {

    averageBalance += atoms[i].balance;

    if (atoms[i].balance < 0.0) {

      fill(0);

      noStroke();

      text("Error: negative balance!", width/2, height/2);

    }

  }

  averageBalance /= people;

  let sumOfDifferences = 0;

  for (let i = 0; i < people; i++) {

    for (let j = 0; j < people; j++) {

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

    }

  }

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

}

​

function updateStats(frame) {

  numRich = 0;

  numPoor = 0;

  moneySupply = 0;

  for (let i = 0; i < people; 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/people * 100)+"%", 10, height - 5);

  fill(red);

  text(round(numPoor/people * 100)+"%", 40, height - 5);

  fill(blue);

  text(round((people - numPoor - numRich)/people * 100)+"%", 70, height - 5);

  fill (0);

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

  let stats = "Money: " + round(moneySupply) + 

       "   kXs: " + round(transactions / 1000,0) + 

       "   Days: " + round(days);

  text(stats, width - (stats.length * 6) , height - 5);

}

​

​

// atom class

function atom(p, v, r, c, _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 * random(1.0);

​

  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

            maybeTrade(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);

​

    let average_balance = moneySupply / people;

    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() {

}

​

function mousePressed() {

  enable_dividend = !enable_dividend;

}