convert number to its english presentation.

numtext.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef unsigned long number_t;
const char *SMALL_NUMBERS[] = {"zero",    "one",       "two",      "three",
                               "four",    "five",      "six",      "seven",
                               "eight",   "nine",      "ten",      "eleven",
                               "twelve",  "thirteen",  "fourteen", "fifteen",
                               "sixteen", "seventeen", "eighteen", "nineteen"};
const char *TEN_POWERS[] = {"",      "",      "twenty",  "thirty", "forty",
                            "fifty", "sixty", "seventy", "eighty", "ninety"};
// 可以往上加 trillion 等，同时 SCALE 也要再乘 1000
const char *THOUSAND_POWERS[] = {"billion", "million", "thousand", ""};
// 默认十亿 (1 billion)
const number_t SCALE = 1000 * 1000 * 1000;

/// 将正整数转换成其英文文本表示
///
/// @param str 用于存放结果的字符串，需要提前准备好足够的空间
/// @param n $n \in N_+$
void numtext(char *str, number_t n);
/// @param n 0 < n <= 1000
void spell_hundreds(char *str, number_t n);

void spell_hundreds(char *str, number_t n) {
  if (n == 1000) {
    strcat(str, "one thousand");
    return;
  }
  if (n >= 100) {
    strcat(str, SMALL_NUMBERS[n / 100]);
    strcat(str, " hundred");
    n %= 100;
    if (n > 0) {
      strcat(str, " and ");
    }
  }
  if (n >= 20) {
    strcat(str, TEN_POWERS[n / 10]);
    n %= 10;
    if (n > 0) {
      strcat(str, " ");
    }
  }
  if (0 < n && n < 20) {
    strcat(str, SMALL_NUMBERS[n]);
  }
}

void numtext(char *str, number_t n) {
  if (n < 20) {
    strcat(str, SMALL_NUMBERS[n]);
    return;
  }
  const char **scale_level = THOUSAND_POWERS;
  number_t scale = SCALE;
  while (scale > 0) {
    if (n >= scale) {
      number_t h = n / scale;
      spell_hundreds(str, h);
      strcat(str, " ");
      strcat(str, *scale_level);
      n %= scale;
      if (n > 0) {
        strcat(str, " and ");
      }
    }
    scale /= 1000;
    scale_level += 1;
  }
}

void display(number_t n) {
  char *str = (char *)calloc(4096, sizeof(char));
  numtext(str, n);
  printf("%s\n", str);
}

int main() {
  display(0);
  display(1);
  display(10);
  display(19);
  display(20);
  display(99);
  display(109);
  display(999);
  display(1000);
  display(100032);
  display(999999);
  display(1000001);
  display(9999999);
  display(1000000000);
  display(1000000001);
  display(9999999999);
  display(1000000000000);
  return 0;
}