key_value.js

/*
Implement a key-value store that presents the following minimal interface:

  `put(k, v)`
  `get(k, timestamp=None)`
 
When a timestamp is specified for the `get` operation, the `get` should return
the value that was associated with the key at that point in time. (Semantically, 
this should be equivalent to getting in a time machine, going back to that point 
in time, and then asking the key-value store for the current value.)
*/

var key_values = []

function put (key, value) {

  if (!key_values[key]) {
    key_values[key] = []
  }

  var value_to_insert = {
    value: value,
    time: new Date()
  }

  if (key_values[key]) {
    key_values[key].push(value_to_insert)
  }

  return value_to_insert;
}

function get (key, timestamp) {

  // return latest value if not timestamp provided
  if (!timestamp) {
    return key_values[key][key_values.length]
  }

  console.log('looking up value for time:', timestamp)

  var values = key_values[key]

  // if timestamp is greater than that of last value inserted, return it
  var last_item_added = values[values.length-1]
  if (timestamp >= last_item_added.time) {
    return last_item_added.value
  }

  for (var i=0; i<values.length; i++) {

    var time = values[i].time
    var next_time = values[i+1].time

    if (timestamp >= time) {
      if (timestamp < next_time) {
        return values[i].value
      }
    }
  }  
}

// save time for testing
var past_time = put('foo','value').time

// to ensure we have a different timestamp
setTimeout(function () {

  put('foo','value2')

  // should return first value inserted
  console.log( get('foo', past_time) )

  // should return last value inserted
  // console.log( get('foo', new Date()) )

}, 500)