Shannon Entropy example from a post on Functionally Imperative found here: https://functionallyimperative.com/t/entropy

entropy.go

// Shannon Entropy example from a post on Functionally Imperative
// https://functionallyimperative.com/t/entropy

package main

import (
	"crypto/rand"
	"fmt"
	"math"
)

func main() {

	cases := []struct {
		input       []byte
		description string
	}{
		{[]byte("     "), "five spaces '     '"},
		{[]byte{0, 0, 0, 0, 0}, "five zeros [ 0, 0, 0, 0, 0 ]"},
		{[]byte("hello, world"), "the string 'hello, world'"},
		{randBytes(16), "  16 random bytes"},
		{randBytes(32), "  32 random bytes"},
		{randBytes(64), "  64 random bytes"},
		{randBytes(128), " 128 random bytes"},
		{randBytes(256), " 256 random bytes"},
		{randBytes(512), " 512 random bytes"},
		{randBytes(1024), "1024 random bytes"},
		{randBytes(1_000_000), "One million random bytes"},
	}

	for i := range cases {
		c := cases[i]
		fmt.Printf("entropy: %.10f for: %v\n", entropy(c.input), c.description)
	}

}

func randBytes(len int) []byte {
	r := make([]byte, len)
	rand.Read(r)
	return r
}

func entropy(b []byte) float64 {
	var score float64
	var freqArray [256]float64
	for i := 0; i < len(b); i++ {
		freqArray[b[i]]++
	}
	len := float64(len(b))
	for i := 0; i < 256; i++ {
		if freqArray[i] != 0 {
			freq := freqArray[i] / len
			score -= freq * math.Log2(freq)
		}
	}
	return score / 8
}

// entropy: 0.0000000000 for: five spaces '     '
// entropy: 0.0000000000 for: five zeros [ 0, 0, 0, 0, 0 ]
// entropy: 0.3777569011 for: the string 'hello, world'
// entropy: 0.5000000000 for:   16 random bytes
// entropy: 0.6171875000 for:   32 random bytes
// entropy: 0.7211818603 for:   64 random bytes
// entropy: 0.8232230957 for:  128 random bytes
// entropy: 0.9021275742 for:  256 random bytes
// entropy: 0.9485006656 for:  512 random bytes
// entropy: 0.9782479345 for: 1024 random bytes
// entropy: 0.9999804355 for: One million random bytes