Function for calculating dice odds when rolling multiple die

Question

I'm working on a way to display dice statistics like you can see at the site https://anydice.com/ , I've spent a bit looking through the source code but it's pretty thick alltogether so i decided to ask here. Basically, all i need help with is making a function that:

• Lets you pick a dice with a certain amount sides, for example: 6.
• Lets you pick how many times you want to roll this dice.
• Returns the percentages of rolling each possible number with said dice.

I know the question might be a bit shitty, but this is kind of my last resort.

So far, I've tried finding the functions and stumbled upon this medium blog however I was wondering if it could maybe be done with percentages.

Answer 1

Here's a way.

// Two 6-sided dice, one 8-sided
const dice = [
  [1, 2, 3, 4, 5, 6],
  [1, 2, 3, 4, 5, 6],
  [1, 2, 3, 4, 5, 6, 7, 8],
];

// Function to get all combinations of two arrays
function cartesianProduct(a, b) {
  return a.flatMap(c => b.map(d => [c, d].flat()));
}

// Function to get sum of two numbers
function sum(a, b) { return a + b; }

// All combinations of all dice
const allPossibleRolls = dice.reduce(cartesianProduct);

// Sum for each set of rolls
const sums = allPossibleRolls.map(rolls => rolls.reduce(sum));

// Count how many times each sum appears
const counts = sums.reduce((acc, n) => Object.assign(acc, {
  [n]: (acc[n] || 0) + 1
}), {});

// Convert each count into a percent by dividing by length of allPossibleRolls
const percents = Object.fromEntries(Object.entries(counts).map(
  ([sum, count]) => [sum, count / allPossibleRolls.length]));

Object.entries(percents).forEach(([k,v]) => {
  console.log(`${k} = ${(v * 100).toFixed(5)}%`);
});

Doesn't dedupe equivalent rolls like the Medium post you linked to mentions, so the rolls [1, 2] and [2, 1] and treated as separate possibilites. Not sure if that throws off the math. But this returns the same answer as AnyDice.