press_challenge

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

struct press_info {
  char               pi_button;
  int                pi_count;
  struct press_info *pi_next;
};

int
get_press_info(const char *string, struct press_info **press_info)
{
  struct press_info start = {0};
  struct press_info *it   = &start;

  while (*string) {
    it->pi_next = malloc(sizeof(struct press_info));
    if (!it->pi_next) return -1;

    it            = it->pi_next;
    it->pi_button = *string;
    it->pi_count  = 0;
    it->pi_next   = NULL;
    while (*string == it->pi_button) {
      string++;
      it->pi_count++;
    }
  }

  *press_info = start.pi_next;
  return 0;
}

int
press_info_to_char(const struct press_info *pi,
                   const struct press_info *previous)
{
  const char lookup_table[12][5] = {"1", "2abc", "3def", "4ghi",
                                    "5jkl", "6mno", "7pqrs", "8tuv",
                                    "9wxyz", "*^", "#.,", "0+ "};
  const int data_sizes[] = {1, 4, 4, 4, 4, 4, 5, 4, 5, 2, 2, 3};

  if (pi->pi_button == '0') {
    if (previous &&
        previous->pi_button != '1' &&
        pi->pi_count == 1 &&
        pi->pi_next &&
        pi->pi_next->pi_button == previous->pi_button)
      return 0;

    return lookup_table[11][pi->pi_count % data_sizes[11]];
  }

  for (int i = 0; i < 11; i++)
    if (lookup_table[i][0] == pi->pi_button)
      return lookup_table[i][pi->pi_count % data_sizes[i]];

  return -1;
}

int
main(int argc, char **argv)
{
  if (argc != 2) errx(1, "usage: %s [keypad sequence]", *argv);

  int    button;
  struct press_info *pi = NULL, *prev = NULL;

  if (get_press_info(argv[1], &pi))
    errx(1, "unable to read input");

  while (pi) {
    if ((button = press_info_to_char(pi, prev)) == -1)
      errx(1, "unable to parse character: %c", pi->pi_button);
    else if (button)
      putchar(button);

    prev = pi;
    pi = pi->pi_next;
  }

  putchar('\n');
}