Beginner Exercism • javascript

Largest Series Product

Challenge Overview

Your task is to look for patterns in the long sequence of digits in the encrypted signal.

Introduction

You work for a government agency that has intercepted a series of encrypted communication signals from a group of bank robbers. The signals contain a long sequence of digits. Your team needs to use various digital signal processing techniques to analyze the signals and identify any patterns that may indicate the planning of a heist.

Instructions

Your task is to look for patterns in the long sequence of digits in the encrypted signal.

The technique you’re going to use here is called the largest series product.

Let’s define a few terms, first.

input: the sequence of digits that you need to analyze
series: a sequence of adjacent digits (those that are next to each other) that is contained within the input
span: how many digits long each series is
product: what you get when you multiply numbers together

Let’s work through an example, with the input "63915".

To form a series, take adjacent digits in the original input.
If you are working with a span of 3, there will be three possible series:
- "639"
- "391"
- "915"
Then we need to calculate the product of each series:
- The product of the series "639" is 162 (6 × 3 × 9 = 162)
- The product of the series "391" is 27 (3 × 9 × 1 = 27)
- The product of the series "915" is 45 (9 × 1 × 5 = 45)
162 is bigger than both 27 and 45, so the largest series product of "63915" is from the series "639". So the answer is 162.

Source: Exercism javascript/largest-series-product

💡 View Solution

export const largestProduct = (digits, span) => {
  if (span < 0) throw new Error('Span must be greater than zero');
  if (span > digits.length) throw new Error('Span must be smaller than string length');
  if (/[^0-9]/.test(digits)) throw new Error('Digits input must only contain digits');
  if (span === 0) return 1;

  let maxProduct = 0;
  for (let i = 0; i <= digits.length - span; i++) {
    const series = digits.substring(i, i + span);
    const product = series.split('').reduce((acc, digit) => acc * Number(digit), 1);
    if (product > maxProduct) maxProduct = product;
  }
  return maxProduct;
};