[Golang] Number letter counts - Problem 17 - Project Euler

Problem: [1]

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?

NOTE: Do not count spaces or hyphens. For example, 342 (three hundred and forty-two) contains 23 letters and 115 (one hundred and fifteen) contains 20 letters. The use of "and" when writing out numbers is in compliance with British usage.

Solution:

21124

First we convert numbers to words [2], and then count the letters.

Run Code on Go Playground

package main

import (
      "fmt"
)

var NumberToWord = map[int]string{
      1:  "one",
      2:  "two",
      3:  "three",
      4:  "four",
      5:  "five",
      6:  "six",
      7:  "seven",
      8:  "eight",
      9:  "nine",
      10: "ten",
      11: "eleven",
      12: "twelve",
      13: "thirteen",
      14: "fourteen",
      15: "fifteen",
      16: "sixteen",
      17: "seventeen",
      18: "eighteen",
      19: "nineteen",
      20: "twenty",
      30: "thirty",
      40: "forty",
      50: "fifty",
      60: "sixty",
      70: "seventy",
      80: "eighty",
      90: "ninety",
}

func convert1to99(n int) (w string) {
      if n < 20 {
              w = NumberToWord[n]
              return
      }

      r := n % 10
      if r == 0 {
              w = NumberToWord[n]
      } else {
              w = NumberToWord[n-r] + "-" + NumberToWord[r]
      }
      return
}

func convert100to999(n int) (w string) {
      q := n / 100
      r := n % 100
      w = NumberToWord[q] + " " + "hundred"
      if r == 0 {
              return
      } else {
              w = w + " and " + convert1to99(r)
      }
      return
}

func Convert1to1000(n int) (w string) {
      if n > 1000 || n < 1 {
              panic("func Convert1to1000: n > 1000 or n < 1")
      }

      if n < 100 {
              w = convert1to99(n)
              return
      }
      if n == 1000 {
              w = "one thousand"
              return
      }
      w = convert100to999(n)
      return
}

func CountLetter(s string) (n int) {
      for _, r := range s {
              if r == ' ' {
                      continue
              }
              if r == '-' {
                      continue
              }
              n++
      }
      return
}

func main() {
      totalLetters := 0
      for i := 1; i <= 1000; i++ {
              totalLetters += CountLetter(Convert1to1000(i))
      }
      fmt.Println(totalLetters)
}

Tested on: Go Playground

References:

[1]	Number letter counts - Problem 17 - Project Euler

[2]	[Golang] Convert Numbers to Words From 1 to 1000