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// Birthday problem
// https://en.wikipedia.org/wiki/Birthday_problem

// given n possible values and a set size of k,
// determine the probability of a collision
function odds( n, k ) {
  return 1 - Math.exp( -0.5 * k * ( k - 1 ) / n );
}

// given n possible values and a probability of p,

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function memoize( fn ) {
  const map = new Map();
  const key = Symbol('key');

  return function() {
    let item = map;
    let cached = false;

    for ( let i = 0; i < arguments.length; ++i ) {
      let cache = item.get( arguments[ i ] );

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function compose() {
  var fns = Array.prototype.slice.call( arguments );
  return function( arg ) {
    return fns.reduceRight(function( result, fn ) {
      return fn( result );
    }, arg );
  };
}

function double( n ) {

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map([ 'a', 'b', 'c' ], function( value ) {
  return value.toUpperCase();
}); // [ 'A', 'B', 'C' ]

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function map( arr, fn ) {
  if ( arr.length === 0 ) {
    return [];
  }
  return [ fn( arr[ 0 ] ) ].concat( map( arr.slice( 1 ), fn ) );
}

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function reverse( str ) {
  if ( str.length <= 1 ) {
    return str;
  }
  return reverse( str.substr( 1 ) ) + str[ 0 ];
}

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function factorial( n ) {
  if ( n === 1 ) {
    return 1;
  }
  return n * factorial( n - 1 );
}

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function curry( fn ) {
  var arity = fn.length;

  return (function resolver() {
    var mem = Array.prototype.slice.call( arguments );
    return function() {
      var args = mem.slice();
      Array.prototype.push.apply( args, arguments );
      return ( args.length >= arity ? fn : resolver ).apply( null, args );
    };

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var EventEmitter = (function() {
 
  var cache = new WeakMap();
 
  function EventEmitter( obj ) {
    cache.set( this, {} );
  }
 
  EventEmitter.prototype = {
    

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var EventEmitter = (function() {

  var cache = new WeakMap();

  function EventEmitter() {
    cache.set( this, {} );
  }

  EventEmitter.prototype = {