telephone

telephone.c

#include <assert.h>
#include <stdbool.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
  
static char keypad[10][5]={"0","1","ABC","DEF","GHI","JKL","MNO","PQRS","TUV","WXYZ"};
static char options[10]  ={ 1 , 1 ,  3  ,  3  ,  3  ,  3  ,  3  ,   4  ,  3  ,   4  }; 

static inline bool
is_digit (char c)
{
  return '0' <= c && c <= '9';
}

static char *
digits_to_letters (char *t)
{
  assert (t!=NULL);
  unsigned i;
  for (i=0; t[i]!='\0'; i++)
    if (is_digit (t[i]))
      t[i]=keypad[t[i]-'0'][0];
  return t;
}

static bool
next_letter (char *t, unsigned i)
{
  switch (t[i])
    {
      // Do nothing,
      case '0': t[i]='0'; return true;
      case '1': t[i]='1'; return true;
      case '9': t[i]='9'; return true;
      // Wrap,
      case 'C': t[i]='A'; return true;
      case 'F': t[i]='D'; return true;
      case 'I': t[i]='G'; return true;
      case 'L': t[i]='J'; return true;
      case 'O': t[i]='M'; return true;
      case 'S': t[i]='P'; return true;
      case 'V': t[i]='T'; return true;
      case 'Z': t[i]='W'; return true;
      // Or increment.
      default: t[i]++; return false;
    }
}

static inline char
letter_to_digit (char c)
{
  // Map letters to digits,
  if      ('A' <= c && c <= 'C') c='2';
  else if ('D' <= c && c <= 'F') c='3';
  else if ('G' <= c && c <= 'I') c='4';
  else if ('J' <= c && c <= 'L') c='5';
  else if ('M' <= c && c <= 'O') c='6';
  else if ('P' <= c && c <= 'S') c='7';
  else if ('T' <= c && c <= 'V') c='8';
  else if ('W' <= c && c <= 'Z') c='9';
  // Except for '0' and '1'.
  return c;
}

static char *
letters_to_digits (char *t)
{
  assert (t!=NULL);
  unsigned i;
  for (i=0; t[i]!='\0'; i++)
    t[i]=letter_to_digit (t[i]);
  return t;
}

static inline bool
is_letter (char c)
{
  return 'A' <= c && c <= 'Z';
}

static char *
next_telephone_number (char *t, unsigned l, unsigned h)
{
  assert (t!=NULL);
  assert (l <= h);
  if (is_letter (t[l]))
    {
      if (next_letter (t, l) && l<h)
        next_telephone_number (t, l+1, h);
    }
  else
    {
      if (l<h)
        next_telephone_number (t, l+1, h);
    }
  return t;
}

void
map_telephone_numbers (char *t, void (*func)(char *))
{
  assert (t!=NULL);
  unsigned n=strlen (t), i;
  long long combinations=1;
  for (i=0; i<n; i++)
    if (is_digit (t[i]))
      combinations *= options[t[i]-'0'];
    else if (is_letter (t[i]))
      combinations *= options[letter_to_digit (t[i])-'0'];
  digits_to_letters (t);
  while (combinations>0)
    {
      func (t);
      combinations--;
      next_telephone_number (t, 0, n-1);
    }
  letters_to_digits (t);
}

void
print_number (char *t)
{
  printf ("%s\n", t);
}

int
main (int argc, char **argv)
{
  if (argc!=2)
    return -1;
  char *t=strdup (argv[1]);
  if (t==NULL)
    return -1;
  map_telephone_numbers (t, print_number);
  /* For inputs with no capital letters in them, the following is true:
       assert (strcmp (t, argv[1])==0); */
  free (t);
  return 0;
}