compress.c

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

typedef struct token_t
{
  char token;
  int  count;
}

char *mk_string(token_t token)
{
  int len = (int) floor(log10(token.count)) + 3;
  
  char *new_s = malloc(sizeof(char) * len);
  sprintf(new_s, "%c%d", token.token, token.count);
  
  return new_s;
}

char *compress(char *s)
{
  token_t tokens[strlen(s)];
  
  int i, len;
  int pos   = 0;  // Position in new_s
  int count = 0;
  int prev  = -1; // Remember previous character
  for (i = 0, len = strlen(s) + 1; i < len; i++) {
    if (s[i] == prev) {
      count++;
    } else if (prev != -1) {
      // Write char + count to file, exclude initial case
      tokens[pos].token = prev;
      tokens[pos].count = count;
      pos++;
      
      // Start seq for this char
      prev  = s[i];
      count = 1;
    }
  }
  
  // Max size is 2 * strlen(s) + 1 (for \0)
  char *new_s = malloc(sizeof(char) * strlen(s) * 2 + 1);
  for (int i = 0, len = strlen(s) + 1; i < len; i++) {
    char *tmp_s = mk_string(tokens[i]);
    strcat(new_s, tmp_s);
    free(tmp_s);
  }
  
  if (strlen(new_s) > strlen(s)) {
    free(new_s);
    return s;
  } else {
    return new_s;
  }
}