A command night bot to output a random value but weight those values so some are rarer

nightbot-weighted-random.js

/** @author=TheNerdMan */

/**
 * Creates an array of shape [{outputStr: string, weight: number}]
 *
 * @param stringArray An array of strings, strings should be formated as such "\<String To Ouput\>, \<weight\>"
 * an example is ["Foo, 0.5", "Bar, 0.25", "FooBar, 0.25"].
 *
 * Weights must add up to 1.0.
 */
function getWeights(stringArray /*: string[]*/) {
	var recordArray = stringArray.map((c) => {
		return { outputStr: c.split(",")[0]?.trim(), weight: c.split(",")[1]?.trim() }
	});
	validateWeights(recordArray);
	return recordArray;
}

/**
 * Makes sure weights add up to 1, throws a js error if not.
 *
 * @param mappedArray An array of shape [{outputStr: string, weight: number}], 
 */
function validateWeights(mappedArray /*: [{outputStr: string, weight: number}]*/) {
	// validate weights add up to 1.0
	var weightSum = mappedArray
		.map((c) => parseFloat(c.weight))
		.reduce((a, b) => a + b, 0)
	weightSum = parseFloat(weightSum.toFixed(1));
	// throw error if weights don't add up to 1.0
	if (weightSum !== 1.0) throw new Error("Weights must add up to 1.0");

}

/**
  * Creates a "flat" array of ints that map to a index of a value from the weighted array.
  * 
  * @param mappedArray An array of shape [{outputStr: string, weight: number}]
  * @example
  *		[{ outputStr: "Foo", weight: 0.5 }, { outputStr: "Bar", weight: 0.25 }, { outputStr: "FooBar", weight: 0.25 }] 
  *		outputs 
  * 	[1,1,1,1,2,2,3,3]  
  */
function createFlatWeightedArray(mappedArray /*: [{outputStr: string, weight: number}]*/) {
	// create a "flat" array of ints that map to a index of a value from the weighted array.
	var flatWeightArray = [];
	mappedArray.forEach((c) => {
		var weight = parseFloat(c.weight);
		var weightCount = Math.floor(weight * 10);
		for (var i = 0; i < weightCount; i++) {
			flatWeightArray.push(mappedArray.indexOf(c));
		}
	});
	return flatWeightArray;
}

/**
  * Gets the list from paste bin using api.allorigins.win to get around cors errors.
  * If that service goes down then uh oh
  */
function retrieveListFromPasteBin(url /*: string*/) {
	const request = new XMLHttpRequest();
	request.open("GET", `https://api.allorigins.win/raw?url=${encodeURIComponent(url)}`, false); // `false` makes the request synchronous
	request.send(null);

	if (request.status === 200) {
		console.log(request.responseText);
		return request.responseText;
	}
	throw new Error(`Request Error: ${request.status}`);
}

function main(pasteBinUrl /*: string*/) {
	if (!pasteBinUrl) throw new Error("Paste Bin Url missing");

	// get the text
	var pasteBinText = retrieveListFromPasteBin(pasteBinUrl);
	// Remove line endings
	var rawArray = pasteBinText.replace(/(\r\n|\n|\r)/gm, "").split(";");
	// Drop the last item if user has put ; on every line
	if (!rawArray[rawArray.length - 1]) rawArray.pop();

	// Get the weights in a nice object
	var weightedArray = getWeights(rawArray);
	// now we do some maths to create an array of numbers, counting up to the amount of items we have
	// but we add more numbers to make choosing a random number weighted to users needs.
	// e.g. 
	// 		[{ outputStr: "Foo", weight: 0.5 }, { outputStr: "Bar", weight: 0.25 }, { outputStr: "FooBar", weight: 0.25 }] 
	//		becomes 
	// 		[1,1,1,1,2,2,3,3] 
	var flatWeightArray = createFlatWeightedArray(weightedArray);

	// now pick from that array, the answer will map to a value in the weighted array;
	var weightedRandomIndex = flatWeightArray[Math.floor(Math.random() * flatWeightArray.length)]

	var result = weightedArray[weightedRandomIndex];

	return result.outputStr;
}

main(q)