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

#define LOG(x) 3.32 * log10(x)
#define ENTROPY(x) -(x*LOG(x))

FILE *in;
unsigned int table[256];

void read_input(void);
double analyze(void);
void usage(void);

main (argc, argv)
char **argv;
{
	double result;
	if (argc==1) usage();
	
	in = fopen(*++argv,"r");
	if (!in) printf("\nCouldn't open input file.");
	if (!in) exit(-1);
	
	printf("\n   ** Reading file ... **");
	read_input();
	printf("\n   ** Calculating  ... **\n");
	result = analyze();
	
	printf("\n   The file \"%s\" has zero-order",*argv);
	printf("\n   entropy of %1u.%2u bits per byte.\n",(int)(result),(int)(100*result-100*(int)(result)));
	printf("\n   Approximate shrinkage potential");
	printf("\n   using Huffman techniques : ");
	printf(" %3u%%\n\n\n",(int)(100-(result*100)/8));
	
	fclose(in);
	return(0L);
}

void read_input()
{
	int ch;
	
	while ((ch=getc(in)) != EOF)
		table[ch]++;
}

double analyze()
{
	double accum = 0.0;
	double freq;
	long fsize = 0L;
	register int z;
	
	fsize = ftell(in);
	for (z=0; z<256; z++)
		if (table[z])
		{
			freq = (double) table[z]/fsize;
			accum += (double) ENTROPY(freq);
		}
	return accum;
}

void usage()
{
	printf("\n\n");
	printf("        Entropy v1.00 by Kas Thomas. Public Domain.\n\n");
	printf("        Syntax:   ENTROPY   {filename}   [Enter]\n\n");
	printf("        Entropy is a measure of information storage efficiency.\n");
	printf("        This program calculates a file's entropy, hence its\n");
	printf("        compressibility, using the entropy equation of Shannon.\n");
	printf("        (see \"Information Theory : Symbols, Signals, & Noise,\"\n");
	printf("        by John Pierce, Dover, 1981).\n\n");
	exit(0L);
}