// Or
const randomColor = () => `#${(~~(Math.random()*(1<<24))).toString(16)}`;
console.log(randomColor);

//Retries maxRetries number of times, meaning that if you have 0, then it'll run the function once.
async function retryPromise(promiseFn, maxRetries = 3, delayOffset = 0, delayRandomRange = 0) {
	return new Promise((resolve, reject) => {
		promiseFn()
			.then(resolve, (error) => { //On error
				if (maxRetries <= 0) {
					return reject(error);
				} else {
					if(delayRandomRange * Math.random() + delayOffset < 1.0){
						return retryPromise(promiseFn, maxRetries - 1, delayOffset, delayRandomRange).then(resolve, reject);
					}else{
						return new Promise((resolveTwo, rejectTwo) => {
							setTimeout(() => {
								return retryPromise(promiseFn, maxRetries - 1, delayOffset, delayRandomRange).then(resolveTwo, rejectTwo);
							}, delayRandomRange * Math.random() + delayOffset);
						}).then(resolve, reject);
					}
				}
			});
	});
}

//Tests for that function.

//Returns a function that will fail numTimes times, then will return aValue
function functionThatReturnsAFunctionThatFailsACoupleTimes(numTimes, aValue){
	return async function(){
		if(numTimes == 0){
			return aValue;
		}
		numTimes--;
		throw false;
	};
}

const SMALL_NUMBER = 10;

function testTest(failNumberOfTimes){

	let functionThatFailsACoupleTimes = functionThatReturnsAFunctionThatFailsACoupleTimes(failNumberOfTimes, true);

	//Test my test
	for(let i = 0; i < (failNumberOfTimes * 2); ++i){
		function evalTest(val){
			let testTestFailedReason = "";
			if(val === undefined){
				testTestFailedReason = "Test test returned undefined for some reason.";
			}
			if((i + 1) > failNumberOfTimes){
				if(val === true){
					//We're good
				}else{
					testTestFailedReason = "Test test didn't return true when it should have.";
				}
			}else{
				if(val === false){
					//We're good
				}else{
					testTestFailedReason = "Test test didn't return false when it should have.";
				}
			}
			testTestFailedReason = testTestFailedReason || "Test test passed test case";
			console.log(testTestFailedReason, "at index", i, "where the function returned", val);
		}
		functionThatFailsACoupleTimes().then(evalTest, evalTest)
	};
}

testTest(SMALL_NUMBER);

let testCaseCounter = 1;

const throwsNoError = [
	(val)=>{
		if(val == true){
			console.log("Passed test case " + (testCaseCounter++));
		}else{
			console.error("Unexpected return value", val);
		}
	},
	()=>{
		console.error("It wasn't supposed to fail!")
	}
]

const throwsError = [
	(val)=>{
		console.error("It wasn't supposed to succeed!", val);
	},
	(val)=>{
		if(val == false){
			console.log("Passed test case " + (testCaseCounter++));
		}else{
			console.error("Unexpected return value", val);
		}
	}
];

//Runs SMALL_NUMBER times, because SMALL_NUMBER - 1 is the number of retries after the first one
let functionThatFailsACoupleTimes = functionThatReturnsAFunctionThatFailsACoupleTimes(SMALL_NUMBER - 1, true);

retryPromise(functionThatFailsACoupleTimes, SMALL_NUMBER - 1).then(...throwsNoError)

functionThatFailsACoupleTimes = functionThatReturnsAFunctionThatFailsACoupleTimes(SMALL_NUMBER - 1, true);

//Runs SMALL_NUMBER - 1 times, because SMALL_NUMBER - 2 is the number of retries after the first one
retryPromise(functionThatFailsACoupleTimes, SMALL_NUMBER - 2).then(...throwsError)

//Testing the delay. You'll have to wait a bit too.

functionThatFailsACoupleTimes = functionThatReturnsAFunctionThatFailsACoupleTimes(SMALL_NUMBER - 1, true);

//Runs SMALL_NUMBER times, because SMALL_NUMBER - 1 is the number of retries after the first one
retryPromise(functionThatFailsACoupleTimes, SMALL_NUMBER - 1, 500, 500).then(...throwsNoError)

functionThatFailsACoupleTimes = functionThatReturnsAFunctionThatFailsACoupleTimes(SMALL_NUMBER - 1, true);

//Runs SMALL_NUMBER - 1 times, because SMALL_NUMBER - 2 is the number of retries after the first one
retryPromise(functionThatFailsACoupleTimes, SMALL_NUMBER - 2, 500, 500).then(...throwsError)

const deepMerge = (a, b, fn) =>
  [...new Set([...Object.keys(a), ...Object.keys(b)])].reduce(
    (acc, key) => ({ ...acc, [key]: fn(key, a[key], b[key]) }),
    {}
  );
  
deepMerge(
  { a: true, b: { c: [1, 2, 3] } },
  { a: false, b: { d: [1, 2, 3] } },
  (key, a, b) => (key === 'a' ? a && b : Object.assign({}, a, b))
);
// { a: false, b: { c: [ 1, 2, 3 ], d: [ 1, 2, 3 ] } }

function formatDate(date) {
  return date.toLocaleDateString();
}

function Comment(props) {
  return (
    <div className="Comment">
      <div className="UserInfo">
        <img
          className="Avatar"
          src={props.author.avatarUrl}
          alt={props.author.name}
        />
        <div className="UserInfo-name">
          {props.author.name}
        </div>
      </div>
      <div className="Comment-text">{props.text}</div>
      <div className="Comment-date">
        {formatDate(props.date)}
      </div>
    </div>
  );
}

const comment = {
  date: new Date(),
  text: 'I hope you enjoy learning React!',
  author: {
    name: 'Hello Kitty',
    avatarUrl: 'https://placekitten.com/g/64/64',
  },
};
ReactDOM.render(
  <Comment
    date={comment.date}
    text={comment.text}
    author={comment.author}
  />,
  document.getElementById('root')
);

// `date` is a `Date` object
const formatYmd = date => date.toISOString().slice(0, 10);

// Example
formatYmd(new Date());      // 2020-05-06

const kNearestNeighbors = (data, labels, point, k = 3) => {
  const kNearest = data
    .map((el, i) => ({
      dist: Math.hypot(...Object.keys(el).map(key => point[key] - el[key])),
      label: labels[i]
    }))
    .sort((a, b) => a.dist - b.dist)
    .slice(0, k);

  return kNearest.reduce(
    (acc, { label }, i) => {
      acc.classCounts[label] =
        Object.keys(acc.classCounts).indexOf(label) !== -1
          ? acc.classCounts[label] + 1
          : 1;
      if (acc.classCounts[label] > acc.topClassCount) {
        acc.topClassCount = acc.classCounts[label];
        acc.topClass = label;
      }
      return acc;
    },
    {
      classCounts: {},
      topClass: kNearest[0].label,
      topClassCount: 0
    }
  ).topClass;
};
const data = [[0, 0], [0, 1], [1, 3], [2, 0]];
const labels = [0, 1, 1, 0];

kNearestNeighbors(data, labels, [1, 2], 2); // 1
kNearestNeighbors(data, labels, [1, 0], 2); // 0