lruCacheMap.js

/**
@func
Map object factory

@return {Map}
*/
export const createLruMap = () => new Map();


/**
@func curry
set a key/value pair to the back of the queue

@notes
curried to set the Map source and queue limit
enforces a max size of the queue

@param {Map} o
@param {number} limit
@return {(k: string, v: *) => void}
*/
export const writeMap = (o, limit) => (k, v) => {
  if (o.size >= limit) {
    for (let [k] of o) { //evict (delete) the oldest key/value pair entry
      o.delete(k);
      break;
    }
  }
  o.set(k, v);
};


/**
@func curry
get a value mapping to the supplied key

@notes
curried to set the map source

@param {Map} o Map object
@return {(k: string) => *} returns the value of a key/value pair
*/
export const readMap = o => k => {
  const v = o.get(k); // move the k to the front, to indicate it's the most recently read item
  o.delete(k);
  o.set(k, v);
  return o.get(k);
}

//@tests
//timeStart();
//const iter = 1e6;
//for (let i = 0; i < iter; i++) {
  const lruMap = createLruMap();
  const w3 = writeMap(lruMap, 3);
  const r3 = readMap(lruMap);
  w3("a", "Apple");
  w3("b", "Banana");
  w3("c", "Carrot");
  r3("a"); //Apple - change {a,b,c} {oldest...newest} -> {b,c,a}
  w3("d", "Donut"); //should evict b
  r3("b"); //should be null
  r3("c"); //Carrot
//}
//timeLog(); //1.9 for 1e6