Getting old selection indices using seedrandom, and hooking javascript functions

get_old_selection_idx.js

function get_old_selection_idx(seed)
{
 	Math.seedrandom(+seed);	
	return Math.floor(Math.random() * 4);
}

js-hook-func.md

javascript snippets

hooking javascript functions

Because javascript is prototypical, it's very easy to hook functions to run your own code. For example, you might want to count how many times a function is called (at least, this is why I needed it) or print it out when it's called.

All you have to do is just reassign it to a different variable, preferably global.

b = Math.random;         // we make a reference to a so we don't called a recursively in its body

a = function()
{
  console.log("hook!")  // the hook code goes here
  return b();           // return b() so we don't blow up the call stack
}

Math.random = a;        // and now Math.random looks at a, which calls b and "hook!"

and now, when you call Math.random(), you should see "hook!" in the console:

convert an array of 0-depth objects to a csv string

/**
 * Converts an array of arrays, i.e.
 * [ [...], [...], ... ]
 * to a csv string. 
 */
Array.toCSVString = (array) =>
{
  for(let elem of array)
  {
    //Run through each element of the array
    //..

    //If the iterated element is a string, then wrap this
    //whole string with quotes 
    for(let i = 0; i < elem.length; i++)
      if(typeof(elem[i]) == "string")
        elem[i] = elem[i].replace(/(.+\,.+)/, "\"$1\"");
  }

  //Then just join each column with "," and each row up with "\n".
  return array.map((x) => x.join(",")).join("\n");
}

/**
 * Converts an array of objects, i.e.
 * [ {...}, {...}, ...]
 * to a csv string. At the moment this can only
 * handle top-level object properties (at depth 0).
 */
Array.toCSV = (array) =>
{
  //Is this an array?
  assert(Array.isArray(array));

  //Is this array not empty?
  assert(array.length > 0);

  //Get the headers of the first element -- this will be the
  //same throughout
  let headers = Object.keys(array[0]);

  //Map all data to its actual values
  let data = array.map((x) => Object.values(x));

  //And build an array of arrays
  let final_data = [ headers, ...data ];

  //Convert the array of arrays to a csv string
  return Array.toCSVString(final_data);
}