feat: Project Euler Problem 17 (#1168)

Project-Euler/Problem017.js

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+/**
+ * Problem 17 - Number letter counts
+ *
+ * @see {@link https://projecteuler.net/problem=17}
+ *
+ * If the numbers 1 to 5 are written out in words: one, two, three, four, five, then there are 3 + 3 + 5 + 4 + 4 = 19 letters used in total.
+ * If all the numbers from 1 to 1000 (one thousand) inclusive were written out in words, how many letters would be used?
+ *
+ * @author Chetan07j
+ */
+
+// Array of number word length from 0 -> 19
+const ones = [4, 3, 3, 5, 4, 4, 3, 5, 5, 4, 3, 6, 6, 8, 8, 7, 7, 9, 8, 8]
+
+// Array for tens from 20, 30, 40, 50, 60, 70, 80, 90 in words length
+const tens = [6, 6, 5, 5, 5, 7, 6, 6]
+
+// Word length for words thousand, hundred, and
+const thousandLength = 8
+const hundredLength = 7
+const andLength = 3
+
+/**
+ * Function to convert number to word
+ *
+ * This function is called recursively to handle thousand and its sub part
+ */
+const numberToWordLength = (n) => {
+  let count = 0
+
+  // If number is < 20 then return its corresponding value from ones
+  if (n < 20) {
+    return ones[n]
+  }
+
+  /**
+   * To calculate tens value "n / 10 - 2" is performed, which might return decimal value
+   * to extract proper integer value Math.floor is added
+   * Here  "-2" is performed as our tens array start with index 0
+   * To get appropriate value from that for our number it is required
+   * e.g.,
+   * -> 34 -> 34/10= 3.4 -> Math.floor(3.4) = 3
+   * -> ones[3] = 5 // this is wrong
+   * -> 3 - 2 = 1 -> ones[1] = 6
+   *
+   * To find ones part, unit is identified by n % 10
+   * If > 0 then ones word is appended to tens word otherwise nothing
+   * e.g.,
+   * 1. 34 -> 10
+   * 2. 30 -> 6
+   */
+  if (n >= 20 && n < 100) {
+    const unit = n % 10
+    return tens[Math.floor(n / 10 - 2)] + ((unit !== 0) ? ones[unit] : 0)
+  }
+
+  // Find thousand, hundred and sub part
+  const hundred = Math.floor(n / 100) % 10
+  const thousand = Math.floor(n / 1000)
+  const sub = n % 100
+
+  // Find ones for thousand part number
+  // e.g., thousand = 2 => inWord = twothousand
+  if (n > 999) {
+    count += numberToWordLength(thousand) + thousandLength
+  }
+
+  // Find ones for hundred part number
+  // e.g., hundred = 1 => inWord = onehundred
+  if (hundred !== 0) {
+    count += ones[hundred] + hundredLength
+  }
+
+  // Find and part of number
+  // e.g., 922 => ninehundred"andtwentytwo"
+  if (sub !== 0) {
+    count += andLength + numberToWordLength(sub)
+  }
+
+  // return number in word format
+  return count
+}
+
+/**
+ * Function responsible for calculating total number word length
+ * for provided input number
+ * Validation is performed for input
+ * Loop is executed to find total word length for given number range
+ * starting from 1
+ *
+ *
+ * @param {number} number
+ * @returns {number}
+ */
+const countNumberWordLength = (number) => {
+  let count = 0
+
+  // Not a number check
+  if (Number.isNaN(parseInt(number))) {
+    throw new Error('Invalid input, please provide valid number')
+  }
+
+  // Number should be greater than 1
+  if (number < 1) {
+    throw new Error('Please provide number greater that 1')
+  }
+
+  // Loop to calculate word length by calling {@link numberToWord}
+  for (let i = 1; i <= number; i++) {
+    count += numberToWordLength(i)
+  }
+
+  // return final count for number word length
+  return count
+}
+
+export { countNumberWordLength }

Project-Euler/test/Problem017.test.js

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+import { countNumberWordLength } from '../Problem017.js'
+
+describe('Number letter count', () => {
+  test.each([[5, 19], [100, 864], [1000, 21124]])('Number letter count from 1 to %i', (n, expected) => {
+    expect(countNumberWordLength(n)).toBe(expected)
+  })
+
+  test.each([
+    ['test', 'Invalid input, please provide valid number'],
+    [0, 'Please provide number greater that 1']
+  ])('Should throw an error for input %i', (n, expected) => {
+    expect(() => countNumberWordLength(n)).toThrowError(expected)
+  })
+})