Added distribution test

hash_test.go

@@ -2,6 +2,7 @@ package hash_test
 
 import (
 	"bytes"
+	"math"
 	"testing"
 
 	"git.akyoto.dev/go/hash"
@@ -57,3 +58,46 @@ func TestSameByte(t *testing.T) {
 		}
 	}
 }
+
+func TestDistribution(t *testing.T) {
+	const (
+		maxSingleDiff  = 0.03
+		maxAverageDiff = 0.01
+	)
+
+	totalDiff := 0.0
+	on := [64]int{}
+	off := [64]int{}
+
+	for sum := range hashes {
+		for bit := 0; bit < 64; bit++ {
+			if (sum & (1 << bit)) != 0 {
+				on[bit]++
+			} else {
+				off[bit]++
+			}
+		}
+	}
+
+	for bit := 0; bit < 64; bit++ {
+		total := on[bit] + off[bit]
+
+		if total == 0 {
+			continue
+		}
+
+		probability := float64(on[bit]) / float64(total)
+		diff := probability - 0.5
+		totalDiff += math.Abs(diff)
+
+		if math.Abs(diff) > maxSingleDiff {
+			t.Logf("Bit %d: %.1f%% (should be closer to 50%%)", bit+1, probability*100)
+		}
+	}
+
+	averageDiff := totalDiff / 64
+
+	if averageDiff > maxAverageDiff {
+		t.Fatalf("Average difference too high (> %.2f): %f", maxAverageDiff, averageDiff)
+	}
+}