Patterns

Step through events/time/state transitions in order, updating world state per step

When to useGame-of-life-style ticks, robot movement on a grid, parsing scripted commands, processing timeline of timestamped events

iterative

The FIFO Model

queue_fifo_model

Real-World Queue Examples

real_world_queue_examples

Josephus Problem

josephus_problem

Simulation Coding Problems: Introduction and Strategies

simulation_intro

Turnstile | Hackerank SHL

amazon_oa_turnstile

Robot Bounded in Circle

amazon_oa_robot_in_circle

Authentication Tokens

twitter_oa_auth_token

INITIALIZE

world state (position, direction, scores, queues)

const queue = []; let head = 0; const results = [];

—

const queue = Array.from({ length: n }, (_, i) => i);

const dx = [0, 1, 0, -1]; const dy = [1, 0, -1, 0]; let dir = 0; let x = 0; let y = 0;

const indexed = arrTime.map((t, i) => [t, i]).sort(...);
const enterQueue = [], exitQueue = [];
let time = 0, i = 0, lastUsed = -1;
const result = new Array(n);

let x = 0, y = 0, dir = 0;
const dx = [0, 1, 0, -1];
const dy = [1, 0, -1, 0];

const employees = [], queue = [], phoneToEmployee = new Map(), output = [];

const slots = new Array(n).fill(null);
const output = [];

const inventory = new Map(); const borrowed = []; const output = [];

const products = new Map(); let balance = 0; const output = [];

const tokens = new Map();
const durations = new Map();
const output = [];

TRAVERSE

for each event/tick/step

for (const op of operations) {

—

while (queue.length > 1) {

for (const ch of commands) {

while (i < n || enterQueue.length > 0 || exitQueue.length > 0) {

for (const ch of instructions) {

for (const op of instructions) {

for (const op of instructions) {

for (const op of instructions) {

for (const op of instructions) {

for (const op of instructions) {

[STEP]

apply event's effect to world state

  if (op[0] === 'enqueue') {
    queue.push(op[1]);
  } else if (op[0] === 'dequeue') {
    if (head >= queue.length) results.push(null);
    else { results.push(queue[head]); head++; }
  }

const [arrivalTime, name] = arrivals[i];

  for (let i = 0; i < k - 1; i++) { queue.push(queue.shift()); }
  queue.shift();

  x += dx[dir]; y += dy[dir];  // or  dir = (dir±1+4) % 4

  // pick exit vs enter by priority rule; dequeue winner; record result[idx] = time; lastUsed = ...; time++;

  if (ch === 'G') { x += dx[dir]; y += dy[dir]; }
  else if (ch === 'L') { dir = (dir + 3) % 4; }
  else if (ch === 'R') { dir = (dir + 1) % 4; }

  if (op[0] === 'hire') { /* push employee */ }
  else if (op[0] === 'dispatch') { /* connect or enqueue */ }
  else if (op[0] === 'end') { /* free employee, drain queue */ }

  if (op[0] === 'park') { /* scan from start, place at first null */ }
  else if (op[0] === 'remove') { slots[spot] = null; }
  else if (op[0] === 'print') { output.push(...); }
  else if (op[0] === 'print_free_spots') { output.push(...); }

  // update inventory / borrowed / output per op type

  // update products / balance; push to output on purchase/checkout

  if (op[0] === 'generate') { ... }
  else if (op[0] === 'renew') { ... }
  else if (op[0] === 'count_unexpired') { ... }

track

record max/snapshot/state if needed

—

  // no snapshot needed — position/dir accumulates

—

  output.push(...) on connect/end

  // print/print_free_spots append to output inside [STEP]

  // output.push(...) on print_borrowed

  // output.push('true'/'false') or output.push(String(balance))

  output.push(count);

RETURN

return final state or aggregated result

return results;

return completed;

return queue[0];

return [x, y];

return result;

return (x === 0 && y === 0) || dir !== 0;

return output;

return output;

return output;

return output;

return output;