Fibonacci Generators

fibonacci_abridged.js

////// Fun with ES2015 Generators & Fibonacci Sequence //////
//
// Generators are awesome for lazy calculations, but they're less 
// than ideal for certain use-cases (give me the result of the
// 1000th iteration).
//
// This is an experiment where I try to have my cake and eat it too, 
// by wrapping a generator with a factory function.
//
//// Note: This is the comment-free version!
//// The fully-annotated, unabridged version is available here:
//// https://gist.github.com/joshwcomeau/c26f64232095322a9b3ce1b36b83107b


function* fibonacciGenerator() {
  let previous = 0, current = 1;
  
  while (true) {
    [previous, current] = [current, previous + current];
    
    yield(current);
  }
}

function fibonacciFactory(n = 1) {
  const generator = fibonacciGenerator();
  const sequence = [null];

  return (n = 1) => {
    while (typeof sequence[n] === 'undefined') {
      sequence.push(generator.next().value);
    }
    
    return sequence[n]
  }
}


////////////////// Test! //////////////////

const t1Start = performance.now();
const t1Generator = fibonacciGenerator();

for (let i = 0; i < 25; i++) {
  let sequenceNum = 0;
  while (sequenceNum <= 1000) {
    t1Generator.next();
    sequenceNum++;
  }
}

const t1Time = performance.now() - t1Start;


const t2Start = performance.now();
const t2Factory = fibonacciFactory();

for (let i = 0; i < 25; i++) {
  fibonacciFactory(1000);
}

const t2Time = performance.now() - t2Start;

console.log(`Generator took ${t1Time}ms`); // ~10-20ms
console.log(`Factory took ${t2Time}ms`);   // 0.01 - 0.2ms