memoize-puzzle.js

"use strict";

function cache_store(callback, key, value) {
  callback('store ' + value + ' to cache' );
};

function slow_provider_function(callback, key) {
  const providerResult = 'provider result';
  setTimeout(function() {
    console.log('slow result fetched');
    callback(providerResult);
  }, Math.floor((Math.random()*1000)+1));
};


function cache_retrieve(callback, key) {
  const cacheResult = 'cache result';
  setTimeout(function() {
    console.log('fast result fetched');
    callback(cacheResult);
  }, Math.floor((Math.random()*500)+1));
}


function memoize(slow_function, key) {

  let callback_called_once = false;

  let execute_callback = function(result) {
    // Check if callback was already called
    if (callback_called_once === false) {
      callback_called_once = true;
      console.log(result);
    }
  }

  return function() {
    cache_retrieve(function(result) {
      execute_callback(result);
    }, key);

    slow_function(function(result) {
      execute_callback(result);
      cache_store(console.log, key, 'slow provider value');
    }, key);
  }
}


const faster_function = memoize(slow_provider_function, 'key');

faster_function();

Answer for bonus question:

Short Answer: 74 seconds long should be the TTL of the cache to obtain 95% accuracy.

Explanation:

I assumed an exponential decay and used the following equations:

N0 is the initial quantity
N(t) is the quantity that has not decayed after a time t
T is the mean lifetime of the decaying quantity

Since you gave me the half-life of the cache I could calculate the mean lifetime with this equation:

The mean lifetime of a cache entry is around 1442.695 seconds

With that I had everything together for the equation above:

N0: 100 (cache entries)
N(t): 95 (since we want that 95 times out of 100 (N0) cache entries can be retrieved at any time)
T: 1442.695 seconds

Now I had to rearrange the equation to be able to calculate t which would be the time after which at least 95% of all cache entries would be still valid.

After rearranging the equation, I got this: t = - (ln 0.95 * 1442.695)

Which is 74 seconds