Add basic entropy function.

Add a metric entropy function check, but do not configure it to execute.
I need to look over how this is structured to determine how I want to
handle new checks.

passfilt-server/checkpass.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"crypto/sha1"
 	"fmt"
+	"math"
 	"net/http"
 	"strings"
 	"sync"
@@ -50,6 +51,32 @@ func isPwnd(p string) bool {
 	return false
 }
 
+// metricEntropy tries to measure the 'randomness' of a password
+// it returns the shannon entropy divided by the length of the
+// password.
+func metricEntropy(pass string) float64 {
+	m := map[rune]float64{}
+	for _, r := range pass {
+		m[r]++
+	}
+
+	var hx float64
+	for _, val := range m {
+		hx += val * math.Log2(val)
+	}
+
+	l := float64(len(pass))
+	hx = (math.Log2(l) - (hx / l))
+
+	mEntropy := hx / l
+
+	if math.IsNaN(mEntropy) {
+		return 0.0
+	}
+
+	return mEntropy
+}
+
 func checkpass(user string, pass string, banlist *sync.Map) bool {
 	// by default, passwords are considered OK
 	passOk := true

passfilt-server/checkpass_test.go

@@ -26,3 +26,16 @@ func TestIsPwnd(t *testing.T) {
 		t.Error("Random password detected as pwnd.")
 	}
 }
+
+func TestMetricEntropy(t *testing.T) {
+	s := "abbcccdddd"
+
+	if metricEntropy(s) != 0.18464393446710153 {
+		t.Error("Failed to calculate correct entropy.", metricEntropy(s))
+	}
+
+	s = ""
+	if metricEntropy(s) != 0 {
+		t.Error("Failed to handle empty string.", metricEntropy(s))
+	}
+}