calculate entropy of file

shent.c

/*
 Shannon entropy calculation
 $ cc -Wall shent.c -o shent -lm
 $ ./shent shent.c
*/
#include <stdio.h>
#include <stdint.h>
#include <math.h>

int
main(int argc, char *argv[])
{
	uint64_t map[256];
	size_t i;
	FILE *f;
	long int flen;
	double info = 0.0;

	if (argc < 2) {
		fprintf(stderr, "Usage: %s file\n", argv[0]);
		return 1;
	}

	f = fopen(argv[1], "r");
	if (!f) {
		fprintf(stderr, "Can't open %s\n", argv[1]);
		return 1;
	}

	for (i=0; i<(sizeof(map)/sizeof(map[0])); i++) {
		map[i] = 0;
	}

	while (!feof(f)) {
		char buf[1024*8];
		size_t r;

		r = fread(buf, 1, sizeof(buf), f);
		if (r == 0) break;
		for (i=0; i<r; i++) {
			size_t index = buf[i];
			map[index]++;
		}
	}
	flen = ftell(f);
	fclose(f);

	for (i=0; i<(sizeof(map)/sizeof(map[0])); i++) {
		double freq;

		if (map[i] == 0) continue;
		freq = (double)map[i] / flen;
		info += freq * log2(freq);
	}

	info = -info;

	printf("%f\n", info);

	return 0;
}