z1.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <dirent.h>
#include <math.h>

// we need it as globals to use in comparing function
int symbols[256], stats[256];

// Analizes input file and prints result in output file
void analize(char *input_name, char *output_name);

// Converts input filename to output filename
void convert_filename(char* name, char* result);

// comparing function
int compare_frequency(const void * a, const void * b);

// Main program
int main() {
  DIR *dp;
  struct dirent *ep;
  dp = opendir("./input");

  if (dp != NULL) {
    while((ep = readdir(dp)))
      if(ep->d_name[0] != '.') {
        // analizes each file in input directory
        char input[100], output[100];
        strcpy(input, ep->d_name);
        convert_filename(input, output);
        analize(input, output);
      }
    closedir(dp);
  }

  return 0;
}

void analize(char *input_name, char *output_name) {
  // Initializing
  FILE *input, *output;
  int i, count = 0, c;
  double entropy = 0;
  for(i = 0; i < 256; i++) {
    stats[i] = 0;
    symbols[i] = i;
  }
  char input_path[100], output_path[100];
  strcpy(input_path, "./input/");
  strcat(input_path, input_name);
  strcpy(output_path, "./output/");
  strcat(output_path, output_name);

  // opens input file for binary reading, and output for writing
  input = fopen(input_path, "rb");
  output = fopen(output_path, "w");
  if(input == NULL) printf("!!!!\n");

  // collects data from input file
  while((c = fgetc(input)) != EOF) {
    stats[c]++;
    count++;
  }

  // sorts symbols by frequency
  qsort(symbols, 256, sizeof(int), compare_frequency);

  // writes results into output file
  fprintf(output, "%s\n", input_name);
  fprintf(output, "%i\n\n", count);
  for(i = 0; i < 256; i++) {
    double p = (double) stats[symbols[i]] / count;
    // ignore absent symbols
    if(p != 0) {
      entropy -= p * log2(p);
      fprintf(output, "%i\t%e\n", symbols[i], p);
    }
  }
  fprintf(output, "\n%e\n", entropy);

  // closes files
  fclose(input);
  fclose(output);
}

void convert_filename(char* name, char* result) {
  char variant[100], extension[100];
  sscanf(name, "%*[^123456780]%[^.].%s", variant, extension);
  sprintf(result, "%s%s.tab", extension, variant);
}

int compare_frequency(const void * a, const void * b) {
  return ( stats[*(int*)b] - stats[*(int*)a] );
}