package engine

import "math"

// Shannon computes the Shannon entropy of a string in bits per character.
// Returns 0.0 for empty strings.
// A value >= 3.5 indicates high randomness, consistent with real API keys.
func Shannon(s string) float64 {
	if len(s) == 0 {
		return 0.0
	}
	freq := make(map[rune]float64)
	for _, c := range s {
		freq[c]++
	}
	n := float64(len([]rune(s)))
	var entropy float64
	for _, count := range freq {
		p := count / n
		entropy -= p * math.Log2(p)
	}
	return entropy
}