/* entropy — calculate shannon entropy of input */

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

int main(int argc, char *argv[])
{
	FILE *fp;
	unsigned long freq[256] = {0};
	unsigned long total = 0;
	int c;
	double entropy = 0.0;

	fp = (argc > 1) ? fopen(argv[1], "rb") : stdin;
	if (!fp) {
		perror(argv[1]);
		return 1;
	}

	while ((c = fgetc(fp)) != EOF) {
		freq[c]++;
		total++;
	}

	if (fp != stdin)
		fclose(fp);

	if (total == 0) {
		printf("0.000000\n");
		return 0;
	}

	for (int i = 0; i < 256; i++) {
		if (freq[i] == 0)
			continue;
		double p = (double)freq[i] / total;
		entropy -= p * log2(p);
	}

	printf("%.6f\n", entropy);
	return 0;
}