#include "library.h"

// version 1. This function is a bit boring, but...

void speak(int n)
{ // if ever speak(n) is called with n between 0 and 20
  // it can not fail, under all circumstances
  if (n==0)
    print("zero");
  else if (n==1)
    print("one");
  else if (n==2)
    print("two");
  else if (n==3)
    print("three");
  else if (n==4)
    print("four");
  else if (n==5)
    print("five");
  else if (n==6)
    print("six");
  else if (n==7)
    print("seven");
  else if (n==8)
    print("eight");
  else if (n==9)
    print("nine");
  else if (n==10)
    print("ten");
  else if (n==11)
    print("eleven");
  else if (n==12)
    print("twelve");
  else if (n==13)
    print("thirteen");
  else if (n==14)
    print("fourteen");
  else if (n==15)
    print("fifteen");
  else if (n==16)
    print("sixteen");
  else if (n==17)
    print("seventeen");
  else if (n==18)
    print("eighteen");
  else if (n==19)
    print("nineteen");
  else if (n==20)
    print("twenty");
  else
  { print("I don't know how to say "); print(n); } }

void main()
{ speak(7); }

// we can even make it test itself, so there can be no question.

void test_from_to(int from, int to)
{ if (from<to)
    test_from_to(from, to-1);
  print(to);
  print(" ");
  speak(to);
  new_line(); }

void main()
{ test_from_to(1, 20); }

// version 2, a slight modification taking advantage of a small
// bit of logic in the names of numbers

#include "library.h"

void speak(int n)
{ // if ever speak(n) is called with n between 0 and 20
  // it can not fail, under all circumstances
  if (n==0)
    print("zero");
  else if (n==1)
    print("one");
  else if (n==2)
    print("two");
  else if (n==3)
    print("three");
  else if (n==4)
    print("four");
  else if (n==5)
    print("five");
  else if (n==6)
    print("six");
  else if (n==7)
    print("seven");
  else if (n==8)
    print("eight");
  else if (n==9)
    print("nine");
  else if (n==10)
    print("ten");
  else if (n==11)
    print("eleven");
  else if (n==12)
    print("twelve");
  else if (n==13)
    print("thirteen");
  else if (n==14 || n==16 || n==17 || n==19)
  {   // n-10 must be either 4, 6, 7, or 9.
      // speak(4), speak(6), speak(7), and speak(9) are absolutely
      // guaranteed to work. Functions work the same way regardless
      // of their circumstances. So speak(n-10) is guaranteed to
      // do exactly the right thing: print "four", "six", "seven",
      // or "nine".
    speak(n-10);
    print("teen"); }
  else if (n==15)
    print("fifteen");
  else if (n==18)
    print("eighteen");
  else if (n==20)
    print("twenty");
  else
  { print("I don't know how to say "); print(n); } }

// Note that the function is just one big conditional
// if ... else if ... else if ... else if ... else ...
// no matter what I do after the last "else", nothing
// can alter the fact that it must work correctly
// whenever n is between 0 and 20.

// version 3. Taking advantahe of that to go further

#include "library.h"

void speak(int n)
{ // if ever speak(n) is called with n between 0 and 29
  // it can not fail, under all circumstances
  if (n==0)
    print("zero");
  else if (n==1)
    print("one");
  else if (n==2)
    print("two");
  else if (n==3)
    print("three");
  else if (n==4)
    print("four");
  else if (n==5)
    print("five");
  else if (n==6)
    print("six");
  else if (n==7)
    print("seven");
  else if (n==8)
    print("eight");
  else if (n==9)
    print("nine");
  else if (n==10)
    print("ten");
  else if (n==11)
    print("eleven");
  else if (n==12)
    print("twelve");
  else if (n==13)
    print("thirteen");
  else if (n==14 || n==16 || n==17 || n==19)
  { speak(n-10);
    print("teen"); }
  else if (n==15)
    print("fifteen");
  else if (n==18)
    print("eighteen");
  else if (n==20)
    print("twenty");
  else if (n>=21 && n<=29)
  {    // we already know that speak(20) is guaranteed to work
    speak(20);
    print(" ");
       // n is between 21 and 29 therefore n-20 is between 1 and 9
       // therefore speak(n-20) is guarateed to do the right thing.
    speak(n-20); }
  else
  { print("I don't know how to say "); print(n); } }

// version 4. Using exactly the same pattern to go a bit further.

#include "library.h"

void speak(int n)
{ // if ever speak(n) is called with n between 0 and 59
  // it can not fail, under all circumstances
  if (n==0)
    print("zero");
  else if (n==1)
    print("one");
  else if (n==2)
    print("two");
  else if (n==3)
    print("three");
  else if (n==4)
    print("four");
  else if (n==5)
    print("five");
  else if (n==6)
    print("six");
  else if (n==7)
    print("seven");
  else if (n==8)
    print("eight");
  else if (n==9)
    print("nine");
  else if (n==10)
    print("ten");
  else if (n==11)
    print("eleven");
  else if (n==12)
    print("twelve");
  else if (n==13)
    print("thirteen");
  else if (n==14 || n==16 || n==17 || n==19)
  { speak(n-10);
    print("teen"); }
  else if (n==15)
    print("fifteen");
  else if (n==18)
    print("eighteen");
  else if (n==20)
    print("twenty");
  else if (n>=21 && n<=29)
  { speak(20);
    print(" ");
    speak(n-20); }
  else if (n==30)
    print("thirty");
  else if (n>=31 && n<=39)
  { speak(30);
    print(" ");
    speak(n-30); }
  else if (n==40)
    print("forty");
  else if (n>=41 && n<=49)
  { speak(40);
    print(" ");
    speak(n-40); }
  else if (n==50)
    print("fifty");
  else if (n>=51 && n<=59)
  { speak(50);
    print(" ");
    speak(n-50); }
  else
  { print("I don't know how to say "); print(n); } }

// we notice that the cases for 21 to 29, 31 to 39, 41 to 49
// and 51 to 59 are identical but for the appearances of the
// numbers 20, 30, 40 and 50.
// with a bit of thinking, we discovered that the formula
// n - n%10  just removes the last digit from a number, so
// it is exactly what we want to make a generalisation

// we also added 60, 70, 80, 90 to the functions speaking
// repertoire.

#include "library.h"

void speak(int n)
{ // if ever speak(n) is called with n between 0 and 99
  // it can not fail, under all circumstances
  if (n==0)
    print("zero");
  else if (n==1)
    print("one");
  else if (n==2)
    print("two");
  else if (n==3)
    print("three");
  else if (n==4)
    print("four");
  else if (n==5)
    print("five");
  else if (n==6)
    print("six");
  else if (n==7)
    print("seven");
  else if (n==8)
    print("eight");
  else if (n==9)
    print("nine");
  else if (n==10)
    print("ten");
  else if (n==11)
    print("eleven");
  else if (n==12)
    print("twelve");
  else if (n==13)
    print("thirteen");
  else if (n==14 || n==16 || n==17 || n==19)
  { speak(n-10);
    print("teen"); }
  else if (n==15)
    print("fifteen");
  else if (n==18)
    print("eighteen");
  else if (n==20)
    print("twenty");
  else if (n==30)
    print("thirty");
  else if (n==40)
    print("forty");
  else if (n==50)
    print("fifty");
  else if (n==80)
    print("eighty");
  else if (n==60 || n==70 || n==90)
  { speak(n/10);
    print("ty"); }
  else if (n>=21 && n<=99)
  { const int ty = n - n%10;
     // this just removes last digit, e.g. 37 -> 30
    speak(ty);
    print(" ");
    speak(n-ty); }
  else
  { print("I don't know how to say "); print(n); } }


// this is where we were when time ran out.


#include "library.h"

void speak(int n)
{ // if ever speak(n) is called with n between 0 and 99
  // it can not fail, under all circumstances
  if (n==0)
    print("zero");
  else if (n==1)
    print("one");
  else if (n==2)
    print("two");
  else if (n==3)
    print("three");
  else if (n==4)
    print("four");
  else if (n==5)
    print("five");
  else if (n==6)
    print("six");
  else if (n==7)
    print("seven");
  else if (n==8)
    print("eight");
  else if (n==9)
    print("nine");
  else if (n==10)
    print("ten");
  else if (n==11)
    print("eleven");
  else if (n==12)
    print("twelve");
  else if (n==13)
    print("thirteen");
  else if (n==14 || n==16 || n==17 || n==19)
  { speak(n-10);
    print("teen"); }
  else if (n==15)
    print("fifteen");
  else if (n==18)
    print("eighteen");
  else if (n==20)
    print("twenty");
  else if (n==30)
    print("thirty");
  else if (n==40)
    print("forty");
  else if (n==50)
    print("fifty");
  else if (n==80)
    print("eighty");
  else if (n==60 || n==70 || n==90)
  { speak(n/10);
    print("ty"); }
  else if (n>=21 && n<=99)
  { const int ty = n - n%10;
     // this just removes last digit, e.g. 37 -> 30
    speak(ty);
    print(" ");
    speak(n-ty); }
  else if (n>=100 && n<=999)
  { speak(n/100);
    print(" hundred and ");
    speak(n%100); }
  else
  { print("I don't know how to say "); print(n); } }

void test_from_to(int from, int to)
{ if (from<to)
    test_from_to(from, to-1);
  print(to);
  print(" ");
  speak(to);
  new_line(); }

void main()
{ test_from_to(0, 999); }